Turnbull China Bikeride/Turnbull China Bikeride - Disc 2.iso/STUTTGART/LANG/GNUST/Docs/tutorial

Text File | 1994-06-24 | 135KB | 3,697 lines

                          A Tutorial for GNU Smalltalk

                                Andrew Valencia
                              Valencia Consulting




          Overview

          What this manual presents
               This  document  provides a tutorial introduction to the
          Smalltalk language in general, and the GNU Smalltalk  imple-
          mentation  in  particular.   It  does not provide exhaustive
          coverage of every feature of the language and its libraries;
          instead,  it  attempts to introduce a critical mass of ideas
          and techniques to get the Smalltalk  novice  moving  in  the
          right direction.

          Who this manual is written for
               This  manual assumes that the reader is acquainted with
          the basics of computer science, and  has  reasonable  profi-
          ciency  with  a  procedural  language  such  as  C.  It also
          assumes that the reader is already familiar with  the  usual
          janitorial tasks associated with programming-editing, moving
          files, and so forth.

          1.  Getting started

          1.1.  Starting up Smalltalk
               Assuming that GNU Smalltalk has been installed on  your
          system, starting it is as simple as:
               % mst
          the system loads in Smalltalk, and displays a startup banner
          like:
               Smalltalk 1.1.1 Ready

               st>
          You are now ready to try your hand  at  Smalltalk!   By  the
          way, when you're ready to quit, you exit Smalltalk by typing
          control-D on an empty line.

          1.2.  Saying hello
               An initial exercise is to make Smalltalk say "hello" to
          you.   Type in the following line ("printNl" is a upper case
          N and a lower case L):
                'Hello, world' printNl !
          The system then prints back 'Hello, world' to you.1












                                       -2-


          1.3.  What actually happened
               The  front-line  Smalltalk interpreter gathers all text
          until a '!'  character  and  executes  it.   So  the  actual
          Smalltalk code executed was:
                'Hello, world' printNl
          This  code  does two things.  First, it creates an object of
          type "String" which contains the characters "Hello,  world".
          Second,  it sends the message named "printNl" to the object.
          When the object is done processing the message, the code  is
          done and we get our prompt back.
               You'll  notice that we didn't say anything about print-
          ing the string, even though that's in  fact  what  happened.
          This  was  very much on purpose-the code we typed in doesn't
          know anything about printing strings.  It knew how to get  a
          string  object,  and  it  knew how to send a message to that
          object.  That's the end of the story for the code we  wrote.
          But  for  fun,  let's  take a look at what happened when the
          string object received the "printNl" message.

          1.4.  What a string does with a "printNl" message
               The string object containing "Hello,  world"  was  sent
          the message "printNl".  It then goes to a table2 which lists



            1 It  also  prints  out  a  lot  of  statistics.


          Ignore these; they provide information on the per-


          formance of the underlying Smalltalk engine.   You


          can inhibit them by starting Smalltalk as:


               % mst -q


            2 Which  table?   This is determined by the type


          of the object.  An object has a type, known as the


          class to which it belongs.  Each class has a table


          of methods.  For the  object  we  created,  it  is


          known as a member of the "String" class.  So we go










                                       -3-


          the  messages  which  strings  can receive, and what code to
          execute.  It  finds  that  there  is  indeed  an  entry  for
          "printNl"  and runs this code.  This code then walks through
          its  characters, printing each of them out to the terminal.3
          The central point  is  that  an  object  is  entirely  self-
          contained;  only  the  object  knew how to print itself out.
          When we want an object to  print  out,  we  ask  the  object
          itself to do the printing.

          1.5.  Doing math
               A similar piece of code prints numbers:
               1234 printNl !
          Notice  how  we used the same message, but have sent it to a
          new type of object-an integer (from class  "Integer").   The
          way  in  which  an integer is printed is much different from
          the way a string is printed on the inside,  but  because  we
          are  just  sending  a message, we do not have to be aware of
          this.  We tell it to "printNl", and it prints itself out.
               As a user of an object, we can thus usually send a par-
          ticular message and expect basically the same kind of behav-
          ior,  regardless  of  object's   internal   structure   (for
          instance,  we  have seen that sending "printNl" to an object
          makes the object print itself).  In later chapters  we  will
          see  a  wide range of types of objects.  Yet all of them can
          be printed out the same way-with "printNl".
               White space is ignored, except as it  separates  words.
          This example could also have looked like:
                          1234
                printNl          !
               An integer can be sent a number of messages in addition
          to just printing itself.  An important set of  messages  for
          integers are the ones which do math:



          to the table associated with the String class.


            3 Actually, the message "printNl" was  inherited


          from  Object.   It  sent  a  "print" message, also


          inherited by Object, which then sent "printOn:" to


          the  object, specifying that it print to "stdout".


          The String class then prints its characters to the


          standard output.









                                       -4-


               (9 + 7) printNl !
          Answers  (correctly!)  the  value  16.  The way that it does
          this, however, is a significant departure from a  procedural
          language.

          1.6.  Math in Smalltalk
               In this case, what happened was that the object "9" (an
          Integer), received a "+" message with  an  argument  of  "7"
          (also an Integer).  The "+" message for integers then caused
          Smalltalk to create a new object "16" and return it  as  the
          resultant  object.   This  "16"  object  was  then given the
          "printNl" message, and printed "16" on the terminal.
               Thus, math is not a special case in  Smalltalk;  it  is
          done  exactly  like everything else-by creating objects, and
          sending them messages.  This may seem odd to  the  Smalltalk
          novice,  but  this  regularity turns out to be quite a boon-
          once you've mastered just a few paradigms, all of  the  lan-
          guage  "falls  into  place."   Before  you go on to the next
          chapter, make sure you try math involving  "*"  (multiplica-
          tion),  "-"  (subtraction),  and "/" (division) also.  These
          examples should get you started:
               (8 * (4 / 2)) printNl !
               (8 - (4 + 1)) printNl !
               (5 + 4) printNl !
               (2/3 + 7) printNl !
               (2 + 3 * 4) printNl !
               (2 + (3 * 4)) printNl !




































                                       -5-


          2.  Using some of the Smalltalk classes
               This chapter has examples which need a  place  to  hold
          the  objects they create.  The following line creates such a
          place; for now, treat it as magic.  At the end of the  chap-
          ter we will revisit it with an explanation.  Type in:
               Smalltalk at: #x put: 0 !
          Now let's create some new objects.

          2.1.  An array in Smalltalk
               An  array  in  Smalltalk  is similar to an array in any
          other language, although the syntax  may  seem  peculiar  at
          first.  To create an array with room for 20 elements, do:
               x := Array new: 20 !
          The  "Array new: 20" creates the array; the "x :=" part con-
          nects the name "x" with the object.  Until you assign  some-
          thing  else  to "x", you can refer to this array by the name
          "x".
               Changing elements of the array is not  done  using  the
          ":="  operator;  this operator is used only to bind names to
          objects.   In  fact,  you  never  modify  data   structures;
          instead,  you send a message to the object, and it will mod-
          ify itself.  For instance:
               (x at: 1) printNl !
          which prints:
               nil
          The slots of an array are initially set to "nothing"  (which
          Smalltalk  calls  "nil").   Let's  set the first slot to the
          number 99:
               x at: 1 put: 99 !
          and now make sure the 99 is actually there:
               (x at: 1) printNl !
          which then prints out:
               99
          These examples show how to manipulate an array.   They  also
          show  the  standard  way  in which messages are passed argu-
          ments.  In most cases, if a message takes an  argument,  its
          name  will  end with ":".4 So when we said "x at: 1" we were
          sending a message to whatever object was currently bound  to
          "x"  with  an  argument of 1.  For an array, this results in
          the first slot of the array being returned.
               The second operation, "x at: 1 put: 99"  is  a  message
          with  two arguments.  It tells the array to place the second
          argument (99) in the slot specified by the first (1).  Thus,
          when  we  re-examine  the  first  slot,  it  does indeed now



            4 Alert  readers  will  remember  that  the math


          examples of the  previous  chapter  deviated  from


          this.









                                       -6-


          contain 99.
               There is a shorthand for describing  the  messages  you
          send  to  objects.  You just run the message names together.
          So we would say that our array accepts both  the  "at:"  and
          "at:put:" messages.
               There  is  quite a bit of sanity checking built into an
          array.  The request
               6 at: 1
          fails with an error; 6 is an integer, and can't be  indexed.
          Further,
               x at: 21
          fails  with  an error, because the array we created only has
          room  for  20 objects.5 Finally, note that the object stored
          in an array is just like any other  object,  so  we  can  do
          things like:
               ((x at: 1) + 1) printNl !
          which  (assuming  you've  been  typing in the examples) will
          print 100.

          2.2.  A set in Smalltalk
               We're done with the array we've been  using,  so  we'll
          assign  something  new  to  our  "x" variable.  Note that we
          don't need to do anything special about  the  old  array-the
          fact  that nobody is using it any more will be automatically
          detected, and the memory reclaimed.6  So,  to  get  our  new
          object, simply do:
               x := Set new !
          Which creates an empty set.  To view its contents, do:
               x printNl !
          the  kind of object is printed out (i.e., Set), and then the
          members are listed within parenthesis.  Since it's empty, we
          see:
               Set ()
          Now  let's toss some stuff into it.  We'll add the numbers 5
          and 7, plus the string 'foo'.  We could type:






            5 As of release  1.1,  GNU  Smalltalk  does  not


          actually catch this error.


            6 This is known as "garbage collection."  It  is


          generally  done  when  Smalltalk  finds that it is


          running low on memory.









                                       -7-


               x add: 5 !
               x add: 7 !
               x add: 'foo' !
          But let's save a little typing by using a  Smalltalk  short-
          hand:
               x add: 5; add: 7; add: 'foo' !
          This  line  does  exactly  what the previous example's three
          lines did.  The trick is that the semicolon operator  causes
          the  message  to be sent to the same object as the last mes-
          sage sent.  So saying "; add: 7" is the same  as  saying  "x
          add:  7",  because "x" was the last thing a message was sent
          to.  This may not seem like such a big savings, but  compare
          the ease when your variable is named "aVeryLongVariableName"
          instead of just "x"!  We'll  revisit  some  other  occasions
          where ";" saves you trouble, but for now let's continue with
          our set.  Type either version of the example, and make  sure
          that we've added 5, 7, and "foo":
               x printNl !
          we'll see that it now contains our data:
               Set (5 'foo' 7)
          What if we add something twice?  No problem-it just stays in
          the set.  So a set is like a big  checklist-either  it's  in
          there, or it isn't.  To wit:
               x add:5; add: 5; add: 5; add: 5 !
               x printNl !
          We've  added  "5" several times, but when we printed our set
          back out, we just see:
               Set (5 'foo' 7)
               What you put into a set with "add:", you can  take  out
          with "remove:".  Try:
               x remove: 5 !
               x printNl !
          The set now prints as:
               Set ('foo' 7)
          The "5" is indeed gone from the set.
               We'll  finish  up  with one more of the many things you
          can do with a set- checking for membership.  Try:
               (x includes: 7) printNl !
               (x includes: 5) printNl !
          From which we see that x does indeed contain 7, but  not  5.
          Notice  that  the  answer  is  printed as "true" or "false".
          Once again, the thing returned is an object-in this case, an
          object  known  as  a  boolean.   We'll  look  at  the use of
          booleans later, but for now we'll just say that booleans are
          nothing  more  than objects which can only either be true or
          false-nothing else.  So they're very useful for  answers  to
          yes  or  no  questions,  like the ones we just posed.  Let's
          take a look at just one more kind of data structure:

          2.3.  Dictionaries
               A dictionary is a special kind of collection.   With  a
          regular  array,  you must index it with integers.  With dic-
          tionaries, you can index it with any object  at  all.   Dic-
          tionaries  thus  provide  a very powerful way of correlating









                                       -8-


          one piece of information to another.  Their only downside is
          that  they  are  somewhat less efficient than simple arrays.
          Try the following:
               x := Dictionary new.
               x at: 'One' put: 1 !
               x at: 'Two' put: 2 !
               x at: 1 put: 'One' !
               x at: 2 put: 'Two' !
          This fills our dictionary in with some data.   The  data  is
          actually  stored  in pairs of key and value (the key is what
          you give to at:-it specifies a slot; the value  is  what  is
          actually  stored  at that slot).  Notice how we were able to
          specify not only integers but also strings as both  the  key
          and  the  value.   In fact, we can use any kind of object we
          want as either-the dictionary doesn't care.
               Now we can map each key to a value:
               (x at: 1) printNl !
               (x at: 'Two') printNl !
          which prints respectively:
                'One'
                2
          We can also ask a dictionary to print itself:
               x printNl !
          which prints:
               Dictionary (1,'One' 2,'Two' 'One',1 'Two',2 )
          where the first member of each pair is the key, and the sec-
          ond the value.

          2.4.  Smalltalk dictionary
               If  you'll  remember from the beginning of the chapter,
          we started out by saying:
               Smalltalk at: #x put: 0 !
          This code should look familiar-the at:put:  message  is  how
          we've  been  storing  information in our own arrays and dic-
          tionaries.  In a Smalltalk environment the name  "Smalltalk"
          has been preset to point to a dictionary7 which both you and
          Smalltalk can use.  To see how  this  sharing  works,  we'll
          first  try  to  use  a variable which Smalltalk doesn't know
          about:
               y := 0 !
          Smalltalk complains because  "y"  is  an  unknown  variable.
          Using our knowledge of dictionaries, and taking advantage of
          our access to Smalltalk's dictionary, we  can  add  it  our-
          selves:



            7 Actually,  a SystemDictionary, which is just a


          Dictionary with some extra  hooks  to  run  things


          when Smalltalk first starts









                                       -9-


               Smalltalk at: #y put: 0 !
          The only mystery left is why we're using "#y" instead of our
          usual quoted string.  This is one of those simple  questions
          whose  answer  runs  surprisingly deep.  The quick answer is
          that "#y" and "'y'" are pretty much the  same,  except  that
          the  former will always be the same object each time you use
          it, whereas the latter can be a new string each time you  do
          so.8
               Now that we've added "y" to Smalltalk's dictionary,  we
          try again:
               y := 1 !
          It  works!  Because you've added an entry for "y", Smalltalk
          is now perfectly happy to let you use this new variable.
               If you have some spare time,  you  can  print  out  the
          entire Smalltalk dictionary with:
               Smalltalk printNl !
          As you might suspect, this will print out quite a large list
          of names!  If you get tired of watching Smalltalk  grind  it
          out,  use  your  interrupt key (control-C, usually) to bring
          Smalltalk back to interactive mode.

          2.5.  Closing thoughts
               You've seen how Smalltalk provides you with  some  very
          powerful  data  structures.   You've also seen how Smalltalk
          itself uses these same facilities to implement the language.
          But  this  is  only the tip of the iceberg-Smalltalk is much
          more than a collection of "neat" facilities to use.
               The objects and methods which are automatically  avail-
          able  are  only the beginning of the foundation on which you
          build your programs-Smalltalk allows you  to  add  your  own
          objects and methods into the system, and then use them along
          with everything else.  The art of programming  in  Smalltalk
          is  the art of looking at your problems in terms of objects,
          using the existing object types to good effect, and  enhanc-
          ing  Smalltalk  with  new types of objects.  Now that you've
          been exposed to the basics of Smalltalk manipulation, we can
          begin  to look at this object-oriented technique of program-
          ming.






            8 For  more detail, please feel free to skip out


          to chapter 12 and read the section "Two Flavors of


          Equality"  and the following section "Checking for


          the Two Types of Equality."









                                      -10-


          3.  The Smalltalk class hierarchy
               When programming in Smalltalk, you  sometimes  need  to
          create  new  kinds  of objects, and define what various mes-
          sages will do to these objects.  In the next chapter we will
          create some new classes, but first we need to understand how
          Smalltalk organizes  the  types  and  objects  it  contains.
          Because this is a pure "concept" chapter, without any actual
          Smalltalk code to run, we will keep  it  short  and  to  the
          point.

          3.1.  Class Object
               Smalltalk  organizes all of its classes as a tree hier-
          archy.  At the very top of this hierarchy is class "Object".
          Following somewhere below it are more specific classes, such
          as the ones we've worked with-strings, integers, arrays, and
          so forth.  They are grouped together based on their similar-
          ities-for instance, types of objects which may  be  compared
          as  greater or less than each other fall under a class known
          as "Magnitude".
               One of the first tasks when creating a new object is to
          figure  out  where  within this hierarchy your object falls.
          Coming up with an answer to this problem is at least as much
          art as science, and there are no hard-and-fast rules to nail
          it down.  We'll take a look at three  kinds  of  objects  to
          give you a feel for how this organization matters.

          3.2.  Animals
               Imagine that we have three kinds of objects, represent-
          ing "Animals", "Parrots", and "Pigs".  Our messages will  be
          "eat",  "sing",  and  "snort".   Our first pass at inserting
          these objects into the Smalltalk  hierarchy  would  organize
          them like:
               Object
                    Animals
                    Parrots
                    Pigs
          This  means  that  Animals, Parrots, and Pigs are all direct
          descendants of "Object", and are  not  descendants  of  each
          other.
               Now  we  must  define  how each animal responds to each
          kind of message.
                    Animals
                         eat--Say "I have now eaten"
                         sing--Error
                         snort--Error
                    Parrots
                         eat--Say "I have now eaten"
                         sing--Say "Tweet"
                         snort--Error
                    Pigs
                         eat--Say "I have now eaten"
                         sing--Error
                         snort--Say "Oink"
          Notice how we kept having to indicate an action  for  "eat".









                                      -11-


          An  experienced  object designer would immediately recognize
          this as a clue that we haven't set  up  our  hierarchy  cor-
          rectly.  Let's try a different organization:
                    Animals
                         Parrots
                              Pigs
          That  is,  Parrots  inherit from Animals, and Pigs from Par-
          rots.  Now Parrots inherit all of the actions from  Animals,
          and  Pigs  from  both  Parrots and Animals.  Because of this
          inheritance, we may now define a new set  of  actions  which
          spares us the redundancy of the previous set:
                    Animals
                         eat--Say "I have now eaten"
                         sing--Error
                         snort--Error
                    Parrots
                         sing--Say "Tweet"
                    Pigs
                         snort--Say "Oink"
          Because  Parrots and Pigs both inherit from Animals, we have
          only had to define the "eat" action once.  However, we  have
          made  one  mistake  in  our class setup-what happens when we
          tell a Pig to "sing"?  It says "Tweet", because we have  put
          Pigs  as an inheritor of Parrots.  Let's try one final orga-
          nization:
                    Animals
                         Parrots
                         Pigs
          Now Parrots and Pigs inherit from Animals, but not from each
          other.  Let's also define one final pithy set of actions:
                    Animals
                         eat--Say "I have eaten"
                    Parrots
                         sing--Say "Tweet"
                    Pigs
                         snort--Say "Oink"
          The  change is just to leave out messages which are inappro-
          priate.  If Smalltalk detects that a message is not known by
          an  object  or  any  of its ancestors, it will automatically
          give an error-so you don't have to do  this  sort  of  thing
          yourself.   Notice  that  now  sending  "sing" to a Pig does
          indeed not say  "Tweet"-it  will  cause  a  Smalltalk  error
          instead.

          3.3.  The bottom line of the class hierarchy
               The  goal  of  the  class  hierarchy is to allow you to
          organize objects into a relationship which allows a particu-
          lar  object  to inherit the code of its ancestors.  Once you
          have identified an  effective  organization  of  types,  you
          should  find that a particular technique need only be imple-
          mented once, then inherited by  the  children  below.   This
          keeps  your  code  smaller, and allows you to fix a bug in a
          particular algorithm in only once place-then have all  users
          of it just inherit the fix.









                                      -12-


               You  will find your decisions for adding objects change
          as you gain experience.  As you become  more  familiar  with
          the  existing  set  of objects and messages, your selections
          will increasingly "fit in" with the existing ones.  But even
          a  Smalltalk "pro" stops and thinks carefully at this stage-
          so don't be daunted if your first choices seem difficult and
          error-prone.
























































                                      -13-


          4.  Creating a new class of objects
               With  the  basic  techniques presented in the preceding
          chapters, we're ready do our first real  Smalltalk  program.
          In this chapter we will construct three new types of objects
          (known as  "classes"),  using  the  Smalltalk  technique  of
          inheritance  to tie the classes together, create new objects
          belonging to these classes (known as creating  instances  of
          the class), and send messages to these objects.
               We'll  exercise  all  this  by implementing a toy home-
          finance accounting system.  We will keep track of our  over-
          all  cash,  and  will have special handling for our checking
          and savings accounts.  From this point on, we will be defin-
          ing  classes  which  will be used in future chapters.  Since
          you will probably not be running this whole tutorial in  one
          Smalltalk session, it would be nice to save off the state of
          Smalltalk and resume it without having  to  retype  all  the
          previous  examples.   To  save  the  current  state  of  GNU
          Smalltalk, type:
               Smalltalk snapshot: 'myimage.img' !
          and from your shell, to later restart  Smalltalk  from  this
          "snapshot":
               % mst -I myimage.img
          Such  a  snapshot  currently takes a little over 300K bytes,
          and contains all variables,  classes,  and  definitions  you
          have added.

          4.1.  Creating a new class
               Guess  how you create a new class?  This should be get-
          ting monotonous by now-by sending a message  to  an  object.
          The way we create our first "custom" class is by sending the
          following message:
               Object subclass: #Account
                    instanceVariableNames: 'balance'
                    classVariableNames: ''
                    poolDictionaries: ''
                    category: nil !
          Quite a mouthful, isn't it?  Most people end up  customizing
          their editor to pop this up at a push of a button.  But con-
          ceptually, it isn't really that bad.  The Smalltalk variable
          "Object"  is  bound to the grand-daddy of all classes on the
          system.  What we're doing here is telling the "Object" class
          that  we  want  to  add to it a subclass known as "Account".
          The  other  parts  of  the  message  can  be  ignored,   but
          "instanceVariableNames: 'balance'" tells it that each object
          in  this  subclass  will  have  a  hidden   variable   named
          "balance".9



            9 In  case  you're having a hard time making out


          the font, the "''"'s after classVariableNames: and










                                      -14-


          4.2.  Documenting the class
               The next step is to associate a  description  with  the
          class.  You do this by sending a message to the new class:
               Account comment: 'I represent a place to deposit and withdraw money' !
          A  description is associated with every Smalltalk class, and
          it's considered good form to add a description to  each  new
          class you define.  To get the description for a given class:
               (Account comment) printNl !
          And your string is printed back to you.  Try this with class
          Integer, too:
               (Integer comment) printNl !

          4.3.  Defining a method for the class
               We  have  created a class, but it isn't ready to do any
          work for us-we have to define some messages which the  class
          can process first.  We'll start at the beginning by defining
          methods for instance creation:
               !Account class methodsFor: 'instance creation'!

               new
                    | r |

                    r := super new.
                    r init.
                    ^r
               !!
          Again, programming your editor to do  this  is  recommended.
          The  important  points about this are "Account class", which
          means that we are defining messages which are to be sent  to
          the Account class itself.  "methodsFor: 'instance creation'"
          is more documentation support; it says that all of the meth-
          ods  defined  will  be  to  support creating objects of type
          Account.  Finally, the text starting with "new"  and  ending
          with "!!" defined what action to take for the message "new".
          When you enter this definition, GNU  Smalltalk  will  simply
          give  you  another  prompt.  You method has been compiled in
          and is ready for use.  GNU Smalltalk is pretty quiet on suc-
          cessful  method  definitions--but you'll get plenty of error
          messages if there's a problem!
               This is also the first example where we've had  to  use
          more  than  one  statement, and thus a good place to present
          the statement separator-the ".".  Like Pascal, and unlike C,
          statements  are  separated rather than terminated.  Thus you
          need only use the "." when you have finished  one  statement
          and  are  starting another.  This is why our last statement,
          "^r", does not have a "." following.




          poolDictionaries:  are  a pair of single quotes-an


          empty string.









                                      -15-


               The best way to describe how this method  works  is  to
          step through it.  Imagine we sent a message to the new class
          Account with the command line:
               Account new !
               "Account" receives the message "new" and looks  up  how
          to  process  this message.  It finds our new definition, and
          starts running it.  The first line, "| r |", creates a local
          variable  named  "r"  which can be used as a placeholder for
          the objects we create.  "r" will go away as soon as the mes-
          sage is done being processed.
               The  first  real step is to actually create the object.
          The line "r := super new" does this  using  a  fancy  trick.
          The word "super" stands for the same object that the message
          "new" was  originally  sent  to  (remember-it's  "Account"),
          except  that  when Smalltalk goes to search for the methods,
          he starts one level higher up in the hierarchy than the cur-
          rent  level.   So for a method in the Account class, this is
          the Object class (because the class Account inherits from is
          Object-go  back  and  look  at  how  we  created the Account
          class), and the Object class' methods then execute some code
          in  response  to the "new" message.  As it turns out, Object
          will do the actual creation of the object when sent a  "new"
          message.
               One  more time in slow motion: the Account method "new"
          wants to do some fiddling about when new  objects  are  cre-
          ated,  but he also wants to let his parent do some work with
          a method of the same name.  By saying "r := super new" he is
          letting his parent create the object, and then he is attach-
          ing it to the variable "r".  So after this line of code exe-
          cutes,  we  have a brand new object of type Account, and "r"
          is bound to it.  You will understand  this  better  as  time
          goes  on, but for now scratch your head once, accept it as a
          recipe, and keep going.
               We have the new object, but we haven't set it  up  cor-
          rectly.  Remember the hidden variable "balance" which we saw
          in the beginning of this chapter?  "super new" gives us  the
          object  with  the "balance" field containing nothing-we want
          our  balance field to start at 0.10 So what we need to do is



            10 And unlike C, Smalltalk draws  a  distinction


          between  0  and nil.  nil is the "nothing" object,


          and you will receive an error if you  try  to  do,


          say,  math  on  it.  It really does matter that we


          initialize our instance variable to the  number  0









                                      -16-


          ask the object to set itself up.  By saying "r init", we are
          sending  the "init" message to our new Account. We'll define
          this method in the next section-for  now  just  assume  that
          sending the "init" message will get our Account set up.
               Finally, we say "^r".  In English, this is "return what
          r is attached to".  This means that whoever  sent  "Account"
          the  "new" message will get back this brand new account.  At
          the same time, our temporary variable "r" ceases to exist.

          4.4.  Defining an instance method
               We need to define the "init"  method  for  our  Account
          objects,  so  that our "new" method defined above will work.
          Here's the Smalltalk code:
               !Account methodsFor: 'instance initialization'!
               init
                    balance := 0
               !!
          It looks quite a bit like the  previous  method  definition,
          except  that  the  first  one  said  "Account class methods-
          For:...", and ours says "Account methodsFor:...".  The  dif-
          ference  is that the first one defined a method for messages
          sent directly to "Account", but the second one is  for  mes-
          sages  which  are sent to Account objects once they are cre-
          ated.
               The method named "init" has only one line, "balance  :=
          0".   This  initializes the hidden variable "balance" (actu-
          ally called an instance variable  )  to  zero,  which  makes
          sense  for  an  account  balance.   Notice  that  the method
          doesn't end  with  "^r"  or  anything  like  it-this  method
          doesn't  return  a value to the message sender.  When you do
          not specify a return value, Smalltalk  defaults  the  return
          value  to  the  object  currently executing.  For clarity of
          programming, you might consider explicitly returning  "self"
          in cases where you intend the return value to be used.11




          if we wish to do math on it in the future.


            11 And  why  didn't  the  designers  default the


          return value to nil?  Perhaps they didn't appreci-


          ate  the  value  of void functions.  After all, at


          the time Smalltalk was being  designed,  C  didn't


          even have a void data type.









                                      -17-


          4.5.  Looking at our Account
               Let's create an instance of class Account:
               Smalltalk at: #a put: (Account new) !
          Can  you  guess  what this does?  The "Smalltalk at: #a put:
          <something>"  hearkens  back  to  chapter  2-it  creates   a
          Smalltalk  variable.   And  the  "Account new" creates a new
          Account, and returns it.  So this line creates  a  Smalltalk
          variable  named "a", and attaches it to a new Account-all in
          one line.
               Let's take a look at the Account object  we  just  cre-
          ated:
               a printNl !
          It prints:
               an Account
          Hmmm... not very informative.  The problem is that we didn't
          tell our Account how to print itself, so we're just  getting
          the  default  system  "printNl"  method-which tells what the
          object is, but not what it contains.  So clearly we must add
          such a method:
               !Account methodsFor: 'printing'!
               printOn: stream
                    super printOn: stream.
                    ' with balance: ' printOn: stream.
                    balance printOn: stream
               !!
          Now give it a try again:
               a printNl !
          which prints:
               an Account with balance: 0
          This  may  seem  a  little  strange.  We added a new method,
          printOn:, and our printNl message  starts  behaving  differ-
          ently.   It  turns out that the printOn: message is the cen-
          tral printing function-once you've defined it,  all  of  the
          other printing methods end up calling it.  Its argument is a
          place to print to-quite often it is the  variable  "stdout".
          This  variable  is usually hooked to your terminal, and thus
          you get the printout to your screen.
               The "super printOn: stream" lets our parent do what  it
          did  before-print  out  what  our type is.  The "an Account"
          part of the printout came from this.   "'  with  balance:  '
          printOn:  stream"  creates the string " with balance: ", and
          prints  it  out  to  the  stream,  too.   Finally,  "balance
          printOn: stream" asks whatever object is hooked to the "bal-
          ance" variable to print itself to the stream.  We set  "bal-
          ance" to 0, so the 0 gets printed out.

          4.6.  Moving money around
               We  can  now  create accounts, and look at them.  As it
          stands, though, our balance will always be 0-what a tragedy!
          Our  final  methods  will  let  us  deposit and spend money.
          They're very simple:












                                      -18-


               !Account methodsFor: 'moving money'!
               spend: amount
                    balance := balance - amount
               !
               deposit: amount
                    balance := balance + amount
               !!
          With these methods you can now deposit and spend amounts  of
          money.  Try these operations:
               a deposit: 125!
               a deposit: 20!
               a printNl!
               a spend: 10!
               a printNl!

          4.7.  Specialized objects
               We  now have a generic concept, "Account".  We can cre-
          ate them, check their balance, and move money in and out  of
          them.   They provide a good foundation, but leave out impor-
          tant information that particular  types  of  accounts  might
          want.  In the next chapter, we'll take a look at fixing this
          problem using subclasses.









































                                      -19-


          5.  Two Subclasses for the Account Class
               This chapter continues from  the  previous  chapter  in
          demonstrating  how  one  creates  classes  and subclasses in
          Smalltalk.  In this chapter we will create two special  sub-
          classes  of Account, known as Checking and Savings.  We will
          continue to inherit the capabilities of  Account,  but  will
          tailor  the two kinds of objects to better manage particular
          kinds of accounts.

          5.1.  The Savings class
               We create the Savings class as a subclass  of  Account.
          It  holds money, just like an Account, but has an additional
          property that we will model: it is paid  interest  based  on
          its  balance.   We create the class Savings as a subclass of
          Account:
               Account subclass: #Savings
                    instanceVariableNames: 'interest'
                    classVariableNames: ''
                    poolDictionaries: ''
                    category: nil !
          The instance variable "interest"  will  accumulate  interest
          paid.  Thus, in addition to the "spend:" and "deposit:" mes-
          sages which we inherit from our  parent,  Account,  we  will
          need  to  define a method to add in interest deposits, and a
          way to clear  the  interest  variable  (which  we  would  do
          yearly, after we have paid taxes).  We first define a method
          for allocating a new account-we need to make sure  that  the
          interest field starts at 0.
               !Savings methodsFor: 'initialization'!
               init
                    interest := 0.
                    ^ super init
               !!
          Recall  that  the parent took care of the "new" message, and
          created a new object of the appropriate  size.   After  cre-
          ation,  the  parent  also  sent an "init" message to the new
          object.  As a subclass  of  Account,  the  new  object  will
          receive  the  "init"  message  first;  it  sets  up  its own
          instance variable, and then passes the "init" message up the
          chain  to  let  its parent take care of its part of the ini-
          tialization.
               With our new "Savings" account created, we  can  define
          two methods for dealing specially with such an account:




















                                      -20-


               !Savings methodsFor: 'interest'!
               interest: amount
                    interest := interest + amount.
                    self deposit: amount
               !
               clearInterest
                    | oldinterest |

                    oldinterest := interest.
                    interest := 0.
                    ^oldinterest
               !!
          The  first  method says that we add the "amount" to our run-
          ning total of interest.  The  line  "self  deposit:  amount"
          tells  Smalltalk  to  send ourselves a message, in this case
          "deposit: amount".  This then causes Smalltalk  to  look  up
          the  method  for  "deposit:",  which it finds in our parent,
          Account.  Executing this method  then  updates  our  overall
          balance.12
               One  may wonder why we don't just replace this with the
          simpler "balance := balance + amount".  The answer  lies  in
          one of the philosophies of object-oriented languages in gen-
          eral, and Smalltalk in particular.  Our goal is to encode  a
          technique  for  doing something once only, and then re-using
          that technique when needed.   If  we  had  directly  encoded
          "balance  :=  balance  + amount" here, there would have been
          two places that knew  how  to  update  the  balance  from  a
          deposit.  This may seem like a useless difference.  But con-
          sider if later we decided to start counting  the  number  of
          deposits  made.   If  we  had  encoded "balance := balance +
          amount" in each place that needed to update the balance,  we
          would  have to hunt each of them down in order to update the



            12 "self"  is  much  like  "super",  except that


          "self" will start looking for a method at the bot-


          tom  of  the  type  hierarchy  for the object, but


          "super" starts looking one level up from the  cur-


          rent  level.   Thus,  using "super" forces inheri-


          tance, but "self" will find the  first  definition


          of the message which it can.









                                      -21-


          count  of  deposits.   By   sending   "self"   the   message
          "deposit:",  we  need  only  update  this  method once; each
          sender of this message would then automatically get the cor-
          rect up-to-date technique for updating the balance.
               The  second  method,  "clearInterest",  is simpler.  We
          create a temporary variable "oldinterest" to hold  the  cur-
          rent  amount  of interest.  We then zero out our interest to
          start the year afresh.  Finally, we return the old  interest
          as  our  result, so that our year-end accountant can see how
          much we made.13

          5.2.  The Checking class
               Our  second  subclass  of Account represents a checking
          account.  We will keep track of two facets:
                    - What check number we are on
                    - How many checks we have left in our checkbook
          We will define this as another subclass of Account:
               Account subclass: #Checking
                    instanceVariableNames: 'checknum checksleft'
                    classVariableNames: ''
                    poolDictionaries: ''
                    category: nil !
          We have two instance variables, but we really only  need  to
          initialize one of them-if there are no checks left, the cur-
          rent check number can't matter.  Remember, our parent  class
          Account  will send us the "init" message.  We don't need our
          own class-specific "new" function, since our  parent's  will
          provide everything we need.
               !Checking methodsFor: 'Initialization'!
               init
                    checksleft := 0.
                    ^super init
               !!
          As  in Savings, we inherit most of abilities from our super-
          class, Account.  For  initialization,  we  leave  "checknum"



            13 Of course, in a  real  accounting  system  we


          would never discard such information-we'd probably


          throw it into a Dictionary object, indexed by  the


          year  that  we're  finishing.  The ambitious might


          want to try their hand  at  implementing  such  an


          enhancement.









                                      -22-


          alone,  but  set  the  number  of checks in our checkbook to
          zero.  We finish by letting our parent class do its own ini-
          tialization.

          5.3.  Writing checks
               We  will  finish  this  chapter  by adding a method for
          spending money through our checkbook.  The mechanics of tak-
          ing a message and updating variables should be familiar:
               !Checking methodsFor: 'spending'!
               newChecks: number count: checkcount
                    checknum := number.
                    checksleft := checkcount
               !

               writeCheck: amount
                    | num |

                    num := checknum.
                    checknum := checknum + 1.
                    checksleft := checksleft - 1.
                    self spend: amount.
                    ^ num
               !!
          "newChecks:"  fills  our  checkbook  with checks.  We record
          what check number we're starting with, and update the  count
          of the number of checks in the checkbook.
               "writeCheck:"  merely notes the next check number, then
          bumps up the check number, and down the  check  count.   The
          message "self spend: amount" resends the message "spend:" to
          our own object.  This causes its method to be looked  up  by
          Smalltalk.   The  method  is then found in our parent class,
          Account, and our balance is  then  updated  to  reflect  our
          spending.
               You can try the following examples:
               Smalltalk at: #c put: (Checking new) !
               c printNl !
               c deposit: 250 !
               c printNl !
               c newChecks: 100 count: 50 !
               c printNl !
               (c writeCheck: 32) printNl !
               c printNl !
          For  amusement,  you might want to add a printOn: message to
          the checking class so  you  can  see  the  checking-specific
          information.
               In this chapter, you have seen how to create subclasses
          of your own classes.  You have added new methods, and inher-
          ited methods from the parent classes.  These techniques pro-
          vide the majority of the structure for building solutions to
          problems.   In  the following chapters we will be filling in
          details on further language mechanisms and types,  and  pro-
          viding   details   on  how  to  debug  software  written  in
          Smalltalk.










                                      -23-


          6.  Code blocks
               The Account/Saving/Checking example from the last chap-
          ter  has  several  deficiencies.   It  has  no record of the
          checks and their values.  Worse, it allows you  to  write  a
          check  when  there  are no more checks-the Integer value for
          the number of checks will just calmly go negative!   To  fix
          these  problems we will need to introduce more sophisticated
          control structures.

          6.1.  Conditions and decision making
               Let's first add some code to keep you from writing  too
          many  checks.   We will simply update our current method for
          the Checking class; if you have entered the methods from the
          previous  chapters, the old definition will be overridden by
          this new one.
               !Checking methodsFor: 'spending'!
               writeCheck: amount
                    | num |

                    (checksleft < 1)
                         ifTrue: [ ^self error: 'Out of checks' ].
                    num := checknum.
                    checknum := checknum + 1.
                    checksleft := checksleft - 1.
                    self spend: amount
                    ^ num
               !!
          The two new lines are:
               (checksleft < 1)
                    ifTrue: [ ^self error: 'Out of checks' ].
          At first glance, this appears to be a completely new  struc-
          ture.   Look  again!   The  only new construct is the square
          brackets.
               The first line is a simple boolean expression.  "check-
          sleft" is our integer, as initialized by our Checking class.
          It is sent the message "<", and the argument 1.  The current
          number  bound to "checksleft" compares itself against 1, and
          returns a boolean object telling whether it is less than  1.
               Now this boolean-being either true or false-is sent the
          message "IfTrue:", with an argument which is called  a  code
          block.  A code block is an object, just like any other.  But
          instead of holding a number, or a Set, it  holds  executable
          statements.
               So what does a boolean do with a code block which is an
          argument to a ifTrue: message?  It depends on which boolean!
          If  the  object  is  the "true" object, it executes the code
          block it has been handed.  If it is the "false"  object,  it
          returns  without  executing  the  code block.  So the tradi-
          tional  "conditional  construct"  has   been   replaced   in
          Smalltalk  with  boolean objects which execute the indicated
          code block or not, depending on their truth-value.14












                                      -24-


               In the case of our example, the actual code within  the
          block  sends an error message to the current object.  error:
          is handled by the parent class Object, and will  pop  up  an
          appropriate  complaint when the user tries to write too many
          checks.  In general, the way you handle  a  fatal  error  in
          Smalltalk  is  to send an error message to yourself (through
          the "self" pseudo-variable),  and  let  the  error  handling
          mechanisms inherited from the Object class take over.
               As  you  might guess, there is also an ifFalse: message
          which booleans  accept.   It  works  exactly  like  ifTrue:,
          except  that  the logic has been reversed; a boolean "false"
          will execute the codeblock, and a boolean "true" will not.
               You should take a little time to play with this  method
          of  representing  conditionals.  You can run your checkbook,
          but can also invoke the conditional functions directly:
               true ifTrue: [ 'Hello, world!' printNl ] !
               false ifTrue: [ 'Hello, world!' printNl ] !
               true ifFalse: [ 'Hello, world!' printNl ] !
               false ifFalse: [ 'Hello, world!' printNl ] !

          6.2.  Iteration and collections
               Now that we have some  sanity  checking  in  place,  it
          remains  for  us  to  keep a log of the checks we write.  We
          will do so by adding a Dictionary  object  to  our  Checking
          class,  logging  checks into it, and providing some messages
          for querying our check-writing history.  But  this  enhance-
          ment  brings  up  a very interesting question-when we change
          the "shape" of an object (in this  case,  by  adding  a  new
          instance  variable  to  the  Checking class-our dictionary),
          what happens to the existing class, and its objects?
               The answer is that the old objects  continue  to  exist
          with their current shape.15 New objects will  have  the  new



            14 It is interesting to note that because of the


          way  conditionals are done, conditional constructs


          are not part of the  Smalltalk  language-they  are


          merely a defined behavior for the Boolean class of


          objects.


            15 This  is  the  case  in GNU Smalltalk.  Other


          implementations will refuse to redefine the  class









                                      -25-


          shape.   As  this  can lead to very puzzling behavior, it is
          usually best to eradicate all of the old objects,  and  then
          implement your changes.  If this were more than a toy object
          accounting system, this would  probably  entail  saving  the
          objects  off,  converting  to the new class, and reading the
          objects back into the  new  format.   For  now,  we'll  just
          ignore  what's currently there, and define our latest Check-
          ing class.
               Account subclass: #Checking
                    instanceVariableNames: 'checknum checksleft history'
                    classVariableNames: ''
                    poolDictionaries: ''
                    category: nil !
               This is the same syntax as the last time we  defined  a
          checking  account,  except that we have three instance vari-
          ables-the "checknum" and "checksleft" which have always been
          there,  and our new "history" variable.  We must now feed in
          our definitions for each of the messages our object can han-
          dle,  since  we  are basically defining a new class under an
          old name.16 Go ahead and do  this  now-the  methods  are  in
          chapter 5.  We are using the same Account class, so you only
          need to do the Checking methods.
               With our new Checking instance variable, we are all set
          to  start  recording our checking history.  Our first change
          will be in the "init" message handling:
               !Checking methodsFor: 'initialization'!
               init
                    checksleft := 0.
                    history := Dictionary new.
                    ^ super init
               !!
          This provides us with a Dictionary, and hooks it to our  new



          until  all  of its instances have been hunted down


          and eradicated!


            16 Technically, GNU Smalltalk has associated  us


          with  the  existing  class methods, but with a new


          set of instance methods.  It is often  simpler  to


          work  as  if  you had to define everything, rather


          than trying to take advantage of this.









                                      -26-


          "history" variable.
               Our  next  method  records  each check as it's written.
          The method is a little more involved, as  we've  added  some
          more sanity checks to the writing of checks.
               !Checking methodsFor: 'spending'!
               writeCheck: amount
                    | num |

                    "Sanity check that we have checks left in our checkbook"
                    (checksleft < 1)
                         ifTrue: [ ^self error: 'Out of checks' ].

                    "Make sure we've never used this check number before"
                    num := checknum.
                    (history includesKey: num)
                         ifTrue: [ ^self error: 'Duplicate check number' ].

                    "Record the check number and amount"
                    history at: num put: amount.

                    "Update our next checknumber, checks left, and balance"
                    checknum := checknum + 1.
                    checksleft := checksleft - 1.
                    self spend: amount.
                    ^ num
               !!
               We  have  added  three  things to our latest version of
          writeCheck:.  First, since our routine has  become  somewhat
          involved,  we  have  added  comments.   In Smalltalk, single
          quotes are used for strings; double quotes enclose comments.
          We have added comments before each section of code.
               Second,  we have added a sanity check on the check num-
          ber we propose to use.  Dictionary objects  respond  to  the
          includesKey:  message  with  a boolean, depending on whether
          something is currently stored under the  given  key  in  the
          dictionary.  If the check number is already used, the error:
          message is sent to our object, aborting the operation.
               Finally, we add a new entry to the dictionary.  We have
          already  seen  the  at:put: message from chapter 2.  Our use
          here simply associates a check  number  with  an  amount  of
          money  spent.17  With  this,  we now have a working Checking



            17 You  might  start to wonder what one would do


          if you wished to associate two pieces of  informa-


          tion  under  one  key.  Say, the value and who the


          check was written to.  There are several ways; the









                                      -27-


          class, with reasonable sanity checks and per-check  informa-
          tion.
               Let  us  finish the chapter by enhancing our ability to
          get access to all this information.  We will start with some
          simple print-out functions.
               !Checking methodsFor: 'printing'!
               printOn: stream
                    super printOn: stream.
                    ', checks left: ' printOn: stream.
                    checksleft printOn: stream.
                    ', checks written: ' printOn: stream.
                    (history size) printOn: stream.
               !
               check: num
                    | c |
                    c := history at: num ifAbsent: [ ^self error: 'No such check #' ].
                    ^c
               !!
          There  should  be  very  few  surprises here.  We format and
          print our information, while letting our parent classes han-
          dle  their  own  share of the work.  When looking up a check
          number, we once again take advantage of the fact that blocks
          of executable statements are an object; in this case, we are
          using the at:ifAbsent: message supported by  the  Dictionary
          class.   If the requested key value is not found in the dic-
          tionary, the code block is executed.  This allows us to cus-
          tomize  our  error handling, as the generic error would only
          tell the user "key not found".
               While we can look up a check if we know its number,  we
          have  not  yet written a way to "riffle through" our collec-
          tion of checks.   The  following  function  loops  over  the
          checks,  printing  them  out one per line.  Because there is



          best  would  probably  be  to create a new, custom


          object which contained this information, and  then


          store  this  object  under the check number key in


          the dictionary.  It would also  be  valid  (though


          probably  over-kill)  to store a dictionary as the


          value-and then store as many pieces of information


          as you'd like under each slot!









                                      -28-


          currently only a single numeric value under each  key,  this
          might seem wasteful.  But we have already considered storing
          multiple values under each check number, so it  is  best  to
          leave  some  room for each item.  And, of course, because we
          are simply sending a printing message to an object, we  will
          not  have to come back and re-write this code so long as the
          object in the dictionary honors  our  printNl/printOn:  mes-
          sages.
               !Checking methodsFor: 'printing'!
               printChecks
                    history associationsDo: [:assoc|
                         (assoc key) print.
                         ' - ' print.
                         (assoc value) printNl.
                    ]
               !!
          We  still  see  a code block object being passed to the dic-
          tionary, but ":assoc|" is something new.  A code  block  can
          optionally receive arguments.  In this case, the argument is
          the key/value pair, known in Smalltalk  as  an  Association.
          This  is  the  way  that  a  dictionary  object  stores  its
          key/value pairs internally.   In  fact,  when  you  sent  an
          at:put:  message  to a dictionary object, the first thing it
          does is pack them into a new  object  from  the  Association
          class.  If you only wanted the value portion, you could call
          history with a do: message instead.
               Our code merely uses the "key" and "value" messages  to
          ask  the association for the two values.  We then invoke our
          printing interface upon them.   While  the  printNl  message
          implicitly  uses "stdout", we don't want a newline until the
          end, so the "print" message is used instead.  It  is  pretty
          much the same as "printNl", except it doesn't add a newline.
               It is important that you be clear on  the  relationship
          between an Association and the argument to a code block.  In
          this example, we passed a associationsDo: message to a  dic-
          tionary.  A dictionary invokes the passed code block with an
          Association when processing an associationsDo: message.  But
          code  blocks  can  receive  any type of argument-the type is
          determined by the code which invokes the  code  block;  Dic-
          tionary,  in  this case.  In the next chapter we'll see more
          on how code blocks are used; we'll also look at how you  can
          invoke code blocks in your own code.





















                                      -29-


          7.  Code blocks, part two
               In the last chapter, we looked at how code blocks could
          be used to build conditional expressions, and how you  could
          iterate across all entries in a collection.18 We  built  our
          own  code  blocks,  and  handed  them  off for use by system
          objects.  But there is nothing  magic  about  invoking  code
          blocks;  your own code will often need to do so.  This chap-
          ter  will  shows  some  examples  of  loop  construction  in
          Smalltalk,  and  then demonstrate how you invoke code blocks
          for yourself.

          7.1.  Integer loops
               Integer loops are constructed by telling  a  number  to
          drive the loop.  Try this example to count from 1 to 20:
               1 to: 20 do: [:x| x printNl ] !
          There's also a way to count up by more than one:
               1 to: 20 by: 2 do: [:x| x printNl ] !
          Finally, counting down is done with a negative interval:
               20 to: 1 by: -1 do: [:x| x printNl ] !

          7.2.  Intervals
               It  is also possible to represent a range of numbers as
          a standalone object.  This allows you to represent  a  range
          of  numbers  as  a single object, which can be passed around
          the system.
               Smalltalk at: #i put: (Interval from: 5 to: 10) !
               i printNl !
               i do: [:x| x printNl] !
          As with the integer  loops,  the  Interval  class  can  also



            18 The  do:  message is understood by most types


          of Smalltalk collections.  It works for  the  Dic-


          tionary  class,  as well as sets, arrays, strings,


          intervals, linked lists, bags, and  streams.   The


          associationsDo: message works only with dictionar-


          ies.  The difference is that do: passes  only  the


          value  portion,  while  associationsDo: passes the


          entire key/value pair in an Association object.









                                      -30-


          represent steps greater than 1.  It is done much like it was
          for our numeric loop above:
               i := (Interval from: 5 to: 10 by: 2)
               i printNl !
               i do: [:x| x printNl] !

          7.3.  Invoking code blocks
               Let us revisit the checking example and  add  a  method
          for  scanning only checks over a certain amount.  This would
          allow our user to find "big" checks, by passing in  a  value
          below  which  we  will  not  invoke their function.  We will
          invoke their code block with the check number  as  an  argu-
          ment;  they  can  use our existing check: message to get the
          amount.
               !Checking methodsFor: 'scanning'!
               checksOver: amount do: aBlock
                    history associationsDo: [:assoc|
                         ((assoc value) > amount)
                              ifTrue: [aBlock value: (assoc key)]
                    ]
               !!
          The structure of this loop is much like our printChecks mes-
          sage from chapter 6.  However, in this case we consider each
          entry, and only invoke the supplied  block  if  the  check's
          value is greater than the specified amount.  The line:
               ifTrue: [aBlock value: (assoc key)]
          invokes  the user-supplied block, passing as an argument the
          association's key, which is the check  number.   The  value:
          message,  when  received  by  a  code block, causes the code
          block to  execute.   Code  blocks  take  "value",  "value:",
          "value:value:",  and  "value:value:value:"  messages, so you
          can pass from 0 to 3 arguments to a code block.19 You  might
          find  it  puzzling  that an association takes a "value" mes-
          sage, and so does a code block.  Remember, each  object  can
          do its own thing with a message.  A code block gets run when
          it  receives  a  "value"  message.   An  association  merely
          returns the value part of its key/value pair.  The fact that
          both take the same message is, in this case, coincidence.
               Let's quickly set up a new checking account  with  $250
          (wouldn't  this  be  nice  in real life?) and write a couple
          checks.  Then we'll see if our new method does the job  cor-
          rectly:





            19 There is also a  valueWithArguments:  message


          which  accepts  an array holding as many arguments


          as you would like.









                                      -31-


               Smalltalk at: #mycheck put: (Checking new) !
               mycheck deposit: 250 !
               mycheck newChecks: 100 count: 40 !
               mycheck writeCheck: 10 !
               mycheck writeCheck: 52 !
               mycheck writeCheck: 15 !
               mycheck checksOver: 1 do: [:x| printNl] !
               mycheck checksOver: 17 do: [:x| printNl] !
               mycheck checksOver: 200 do: [:x| printNl] !
               We  will finish this chapter with an alternative way of
          writing our checksOver: code.  In this example, we will  use
          the  message  select:  to  pick  the checks which exceed our
          value, instead of doing the comparison  ourselves.   We  can
          then  invoke the new resulting collection against the user's
          code   block.    Unlike   our   previous    definition    of
          checksOver:do:,  this  one  passes the user's code block the
          association, not just a check number.  How could  this  code
          be rewritten to remedy this, while still using select:?
               !Checking methodsFor: 'scanning'!
               checksOver: amount do: aBlock
                    | chosen |
                    chosen := history select: [:amt| amt > amount].
                    chosen associationsDo: aBlock
               !!
               You  can  use  the same set of tests that we ran above.
          Notice that our code block:
               [:x| x printNl]
          now prints out an  Association.   This  has  the  very  nice
          effect--with  our  old method, we were told which check num-
          bers were above a given amount.  With this  new  method,  we
          get  the  check number and amount in the form of an Associa-
          tion.  When we print an association, since the  key  is  the
          check  number  and  the  value is the check amount, we get a
          list of checks over the amount in the format:
               CheckNum -> CheckVal




























                                      -32-


          8.  When Things Go Bad
               So far we've been working with examples which work  the
          first time.  If you didn't type them in correctly, you prob-
          ably received a flood  of  unintelligible  complaints.   You
          probably  ignored  the  complaints,  and  typed  the example
          again.
               When developing your own Smalltalk code, however, these
          messages  are the way you find out what went wrong.  Because
          your objects, their methods, the error  printout,  and  your
          interactive  environment  are  all contained within the same
          Smalltalk session, you can use these error messages to debug
          your code using very powerful techniques.

          8.1.  A Simple Error
               First, let's take a look at a typical error.  Type:
               7 plus: 1 !
          This will print out:
               7 did not understand selector 'plus:'

               UndefinedObject>>#executeStatements
               UndefinedObject>>nil
          The  first  line  is pretty simple; we sent a message to the
          "7" object which was not understood;  not  surprising  since
          the  "plus" operation should have been "+".  The two remain-
          ing lines reflect the way the  GNU  Smalltalk  invokes  code
          which we type to our command prompt; it generates a block of
          code which is invoked via an  internal  function  "executeS-
          tatements".   Thus,  this output tells you that you directly
          typed a line which  sent  an  invalid  message  to  the  "7"
          object.
               The  last  two lines of the error output are actually a
          stack backtrace.  The most recent call is the one nearer the
          top  of  the  screen.  In the next example, we will cause an
          error which happens deeper within an object.

          8.2.  Nested Calls
               Type the following lines:
               Smalltalk at: #x put: (Dictionary new) !
               x at: 1 !
          The error you receive will look like:
               Dictionary new: 32 "<0x33788>" error: key not found

               MethodContext>>#value
               Dictionary>>#at:ifAbsent:
               Dictionary>>#at
               UndefinedObject>>#executeStatements
               UndefinedObject>>nil
               The error itself is pretty clear; we  asked  for  some-
          thing  within the Dictionary which wasn't there.  The object
          which had the error is identified as "Dictionary  new:  32".
          A  Dictionary's default size is 32; thus, this is the object
          we created with "Dictionary new".
               The stack backtrace shows us the inner structure of how
          a  Dictionary responds to the at: message.  Our hand-entered









                                      -33-


          command causes the usual two entries for  "UndefinedObject".
          Then  we see a Dictionary object responding to an "at:" mes-
          sage (the "Dictionary>>#at" line).   This  code  called  the
          object  with  an "at:ifAbsent:" message.  All of a sudden, a
          different object receives a "value" message,  and  then  the
          error happened.
               This  isn't  quite true; the error happened in the Dic-
          tionary object.  The mystery is where  this  "MethodContext"
          came  from.   Fortunately,  it isn't much of a mystery.  The
          answer lies in what we covered in  the  last  two  chapters:
          code blocks.
               A  very  common way to handle errors in Smalltalk is to
          hand down a block of code which will be called when an error
          occurs.   For  the Dictionary code, the "at:" message passes
          in a block of code to the at:ifAbsent:  code  to  be  called
          when "at:ifAbsent:" can't find the given key.  Thus, without
          even looking at the code for Dictionary itself, we can guess
          that  the  Dictionary "at:" message handling looks something
          like:
               at: key ifAbsent: errCodeBlock
                    ...look for key...
                    (keyNotFound) ifTrue: [ ^(errCodeBlock value) ]
                    ...

               at: key
                    ^self at: key ifAbsent: [^self error: 'key not found']
          The  key  is  that  we  see  in  the  stack  backtrace  that
          at:ifAbsent:  is  called  from  at:,  and a MethodContext is
          called to give the error.  Once we realize that a MethodCon-
          text  is  just  a  fancy name for a code block, we can guess
          that at: handed in a code  block  to  print  an  error,  and
          at:ifAbsent:  used  the  standard  "value" message to invoke
          this code block when it  realized  that  the  requested  key
          couldn't be found in the Dictionary.
               It  would  be nice if each entry on the stack backtrace
          included source line numbers.  Unfortunately, at this  point
          GNU  Smalltalk doesn't provide this feature.  Of course, you
          have the source code available....

          8.3.  Some Shortcomings in GNU Smalltalk
               Unfortunately,  there  are  some   errors   which   GNU
          Smalltalk  is  not very helpful in detecting.  This informa-
          tion applies to the latest version  currently  available-GNU
          Smalltalk  1.1.1.   Try  indexing  something which isn't any
          sort of Collection:
               (7 at: 99) printNl !
          One  would  expect  to  receive  an error,20 but instead GNU



            20 And in fact, you will  receive  an  error  in


          most Smalltalk systems.









                                      -34-


          Smalltalk simply returns the receiving object-7.  Similarly,
          one would expect Array bounds to be checked:
               Smalltalk at: #x put: (Array new: 10) !
               (x at: 7 put: 123) printNl !
               (x at: 11 put: 1234) printNl !
               (x at: 7) printNl !
               (x at: 11) printNl !
          But this example returns no error in GNU  Smalltalk.21  When
          an  assignment  to  an  array  slot is correct, the returned
          value is the value assigned-123  in  this  case.   When  you
          assign  outside the array bounds, you will receive the array
          object itself as the  result!   Thus,  if  you  accidentally
          index  your  array  incorrectly, you will have to figure out
          what happened from a place further in  your  code  where  an
          error  crops up because your code was trying to operate upon
          an element of the array, but instead is working on the array
          itself.

          8.4.  Looking at Objects
               When  you  are chasing an error, it is often helpful to
          examine the  instance  variables  of  your  objects.   While
          strategic  "printNl"s will no doubt help, you can look at an
          object without having to write all the code  yourself.   The
          "inspect"  message  works  on  any object, and dumps out the
          values of each instance variable within the object.  Thus:
               Smalltalk at: #x put: (Interval from: 1 to: 5) !
               x inspect !
          displays:
               An instance of Interval
                 start: 1
                 stop: 5
                 step: 1
               There's one thing the object inspector doesn't display-
          the  contents  of an indexed class.  Since we haven't looked
          at this kind of object yet, we'll  leave  this  to  its  own
          chapter.
               We'll  finish  this  chapter by emphasizing a technique
          which has already been covered-the use of the "error:"  mes-
          sage  in  your  own objects.  As you saw in the case of Dic-
          tionary, an object can send itself an error: message with  a
          descriptive string to abort execution and dump a stack back-
          trace.  You should plan on using this technique in your  own
          objects.   It  can  be  used  both  for explicit user-caused
          errors, as well as in internal sanity checks.






            21 Again,  it will error in most other Smalltalk


          implementations.









                                      -35-


          9.  Coexisting in the Class Hierarchy
               The early chapters of this paper discussed  classes  in
          one of two ways.  The "toy" classes we developed were rooted
          at Object;  the  system-provided  classes  were  treated  as
          immutable entities.  While one shouldn't modify the behavior
          of the standard classes  lightly,  "plugging  in"  your  own
          classes  in  the  right  place  among  their system-provided
          brethren can provide you powerful new classes with very lit-
          tle effort.
               This  chapter  will  create  two complete classes which
          enhance the existing Smalltalk  hierarchy.   The  discussion
          will  start  with  the  issue  of  where  to connect our new
          classes, and then continue onto implementation.   Like  most
          programming  efforts,  the result will leave many possibili-
          ties for improvements.  The framework, however, should begin
          to  give  you  an  intuition  of  how  to  develop  your own
          Smalltalk classes.

          9.1.  The Existing Class Hierarchy
               To discuss where a new class might go, it is helpful to
          have  a  map  of  the current classes.  The following is the
          class hierarchy of GNU Smalltalk 1.1.1.   Indentation  means
          that  the  line inherits from the earlier line with one less
          level of indentation.22




















            22 This listing is courtesy of the  printHierar-


          chy  method supplied by GNU Smalltalk author Steve


          Byrne.  If you have the GNU Smalltalk source, it's


          in the samples/ directory.









                                      -36-


               Object
                   Autoload
                   Behavior
                       ClassDescription
                           Class
                           Metaclass
                   BlockContext
                   Boolean
                       False
                       True
                   CFunctionDescriptor
                   CObject
                   Collection
                       Bag
                       MappedCollection
                       SequenceableCollection
                           ArrayedCollection
                               Array
                               ByteArray
                               CompiledMethod
                               String
                                   Symbol
                           Interval
                           LinkedList
                               Semaphore
                           OrderedCollection
                               SortedCollection
                       Set
                           Dictionary
                               IdentityDictionary
                               SystemDictionary
                   Delay
                   FileSegment
                   Link
                       Process
                       SymLink
                   Magnitude
                       Character
                       Date
                       LookupKey
                           Association
                       Number
                           Float
                           Integer
                       Time
                   Memory
                       ByteMemory
                       WordMemory
                   Message
                   MethodContext
                   MethodInfo
                   ProcessorScheduler
                   SharedQueue
                   Stream









                                      -37-


                       PositionableStream
                           ReadStream
                           WriteStream
                               ReadWriteStream
                                   FileStream
                       Random
                       TokenStream
                   UndefinedObject
               While  initially  a  daunting list, you should take the
          time to hunt down the classes we've examined in  this  paper
          so  far.   Notice,  for instance, how an Array is a subclass
          below the "SequenceableCollection" class.  This makes sense;
          you  can  walk  an Array from one end to the other.  By con-
          trast, notice how a Set is at the same  level  as  Sequence-
          ableCollection.   It  doesn't  make sense to walk a Set from
          one end to the other.
               A little puzzling is the relationship of  a  Dictionary
          to  a  Set;  why  is  a Dictionary a subclass of a Set?  The
          answer lies in the basic structure of both a Set and a  Dic-
          tionary.  Both hold an unordered collection of objects.  For
          a set, they're any objects; for a Dictionary, they are Asso-
          ciations.   Thus, Dictionary inherits some of the more basic
          mechanisms for creating itself, and then adds an extra layer
          of interpretation.
               Finally, look at the treatment of numbers-starting with
          the class Magnitude.  While numbers can be ordered by  "less
          than",  "greater  than",  and  so  forth, so can a number of
          other objects.   Each  subclass  of  Magnitude  is  such  an
          object.  So we can compare characters with other characters,
          dates with other dates, and times with other times, as  well
          as numbers with numbers.23

          9.2.  Those Darn Arrays
               Imagine that you're chasing an array problem,  and  the
          lack  of  a  clear  bounds check is making it too hard.  You
          could modify the Smalltalk implementation, but perhaps  it's
          in  somebody  else's directory, so it wouldn't be practical.
          Why not add a subclass, put some sanity checks in the  array
          indexing, and use our superclass to do all the work?











            23 Ignore  LookupKey; its presence appears to be


          historical.









                                      -38-


               Array variableSubclass: #CheckedArray
                    instanceVariableNames: ''
                    classVariableNames: ''
                    poolDictionaries: ''
                    category: nil !

               !CheckedArray methodsFor: 'bounds checking'!
               boundsCheck: index
                    ((index < 1) | (index > (self basicSize))) ifTrue: [
                         ^self illegalIndex
                    ]
               !
               illegalIndex
                    ^self error: 'Illegal index'
               !!

               !CheckedArray methodsFor: 'basic'!
               at: index
                    self boundsCheck: index.
                    ^super at: index

               !
               at: index put: val
                    self boundsCheck: index.
                    ^super at: index put: val
               !!
               Much  of  the  machinery  of  adding  a class should be
          familiar.  Instead of our usual subclass: message, we use  a
          variableSubclass:  message.   This  reflects  the underlying
          structure of an Array object; we'll  delay  discussing  this
          until  the  chapter on the nuts and bolts of arrays.  In any
          case, we inherit all of the actual knowledge of how to  cre-
          ate arrays, reference them, and so forth.  All that we do is
          intercept at: and at:put:  messages,  and  call  our  common
          function to validate the array index.  The way that we coded
          the bounds check bears a little examination.
               Making a first cut at  coding  the  bounds  check,  you
          might  have coded the bounds check in CheckedArray's methods
          twice-once for at:, and again for at:put:.  As always,  it's
          preferable to code things once, and then re-use them.  So we
          instead add a method for bounds checking "boundsCheck:", and
          use  it  for  both  cases.  If we ever wanted to enhance the
          bounds checking (perhaps enhance the error message to  print
          the  offending  index  value?), we only have to change it in
          one place.
               The actual math for calculating whether the bounds have
          been  violated  is  a little interesting.  The first part of
          the expression:
               ((index < 1) | (index > (self basicSize)))
          is true if the index is less than 1, otherwise  it's  false.
          This  part of the expression thus becomes the boolean object
          true or false.  The boolean object then receives the message
          "|",  and  the  argument  "(index > (self basicSize))".  "|"
          means "or"-we want to OR together the two  possible  illegal









                                      -39-


          range checks.  What is the second part of the expression?24
               "index"  is  our  argument, an integer; it receives the
          message ">", and thus will compare itself to the value "self
          basicSize" returns.  While we haven't covered the underlying
          structures Smalltalk uses to build arrays,  we  can  briefly
          say  that the "basicSize" message returns the number of ele-
          ments the Array object can contain.  So the index is checked
          to see if it's less than 1 (the lowest legal Array index) or
          greater than the highest allocated slot in the Array.  If it
          is either (the "|" operator), the expression is true, other-
          wise false.
               From there it's downhill; our boolean  object  receives
          the  ifTrue:  message,  and  a code block which will send an
          error message to the object.  Why do we have a separate mes-
          sage just to print the error?  For purposes of this example,
          it's not needed.  But one could conceive, in general,  of  a
          couple of different sanity checks all sharing the same mech-
          anism for actually printing the  error  message.   So  we'll
          write it this way anyway.




            24 Smalltalk also offers an "or:" message, which


          is different in a subtle way from "|".  or:  takes


          a  code  block, and only invokes the code block if


          it's necessary  to  determine  the  value  of  the


          expression.  This is analogous to the guaranteed C


          semantic that "&&" evaluates left-to-right only as


          far  as needed.  We could have written the expres-


          sions as "((index < 1) or: [index >  (self  basic-


          Size)])".  Since we expect both sides of or: to be


          false most of the time, there isn't much reason to


          delay evaluation of either side.









                                      -40-


          9.3.  Adding a New Kind of Number
               If we were programming an application which did a large
          amount of complex math, we could probably manage it  with  a
          number  of  two-element arrays.  But we'd forever be writing
          in-line code for the math and comparisons; it would be  much
          easier to just implement an object class to support the com-
          plex numeric type.  Where in the class hierarchy would it be
          placed?
               You've probably already guessed-but let's step down the
          hierarchy  anyway.   Everything  inherits  from  Object,  so
          that's  a  safe  starting point.  Complex numbers can not be
          compared with "<" and ">", and yet we strongly suspect that,
          since  they are numbers, we should place them under the Num-
          ber class.  But Number inherits from  Magnitude--how  do  we
          resolve  this conflict?  A subclass can place itself under a
          superclass which allows some operations the subclass doesn't
          wish to allow.  All that you must do is make sure you inter-
          cept these messages and return an error.  So we  will  place
          our  new Complex class under Number, and make sure to disal-
          low comparisons.
               One can reasonably ask whether the real  and  imaginary
          parts  of  our  complex  number  will be integer or floating
          point.  In the grand Smalltalk tradition, we'll  just  leave
          them  as objects, and hope that they respond to numeric mes-
          sages reasonably.  If they don't, the  user  will  doubtless
          receive  errors and be able to track back their mistake with
          little fuss.
               We'll define the four basic math operators, as well  as
          the  (illegal)  relationals.  We'll add printOn: so that the
          printing methods work, and that should give us  our  Complex
          class.   The  class  as  presented suffers some limitations,
          which we'll cover later in the chapter.































                                      -41-


               Number subclass: #Complex
                    instanceVariableNames: 'realpart imagpart'
                    classVariableNames: ''
                    poolDictionaries: ''
                    category: nil !
               !Complex class methodsFor: 'creating'!
               new
                    ^self error: 'use real:imaginary:'
               !
               new: ignore
                    ^self new
               !
               real: r imaginary: i
                    ^(super new) setReal: r setImag: i
               !!

               !Complex methodsFor: 'creating--private'!
               setReal: r setImag: i
                    realpart := r.
                    imagpart := i.
                    ^self
               !!

               !Complex methodsFor: 'basic'!
               real
                    ^realpart
               !
               imaginary
                    ^imagpart
               !!

               !Complex methodsFor: 'math'!
               + val
                    ^Complex real: (realpart + (val real))
                         imaginary: (imagpart + (val imaginary))
               !
               - val
                    ^Complex real: (realpart - (val real))
                         imaginary: (imagpart - (val imaginary))
               !
               * val
                    ^Complex real: ((realpart * (val real)) - (imagpart * (val imaginary)))
                         imaginary: ((realpart * (val imaginary)) +
                              (imagpart * (val real)))
               !
               / val
                    | d r i |
                    d := ((val real) * (val real)) + ((val imaginary) * (val imaginary)).
                    r := ((realpart * (val real)) + (imagpart * (val imaginary))) / d.
                    i := ((imagpart * (val real)) - (realpart * (val imaginary))) / d.
                    ^Complex real: r imaginary: i
               !!

               !Complex methodsFor: 'comparison'!









                                      -42-


               = val
                    ^((realpart = (val real)) & (imagpart = (val imaginary)))
               !
               > val
                    ^self shouldNotImplement
               !
               >= val
                    ^self shouldNotImplement
               !
               < val
                    ^self shouldNotImplement
               !
               <= val
                    ^self shouldNotImplement
               !!

               !Complex methodsFor: 'printing'!
               printOn: aStream
                    aStream nextPut: $(.
                    realpart printOn: aStream.
                    aStream nextPut: $,.
                    imagpart printOn: aStream.
                    aStream nextPut: $)
               !!
               There should be surprisingly little which  is  actually
          new in this example.  The printing method uses both printOn:
          as well as nextPut: to do its printing.   While  we  haven't
          covered  it, it's pretty clear that "$(" generates the ASCII
          character "(" as an object, and nextPut: puts  its  argument
          as the next thing on the stream.
               The  math  operations all generate a new object, calcu-
          lating the real and imaginary parts, and invoking  the  Com-
          plex class to create the new object.  Our creation code is a
          little more compact than earlier examples; instead of  using
          a  local  variable to name the newly-created object, we just
          use the return value and send a message directly to the  new
          object.   Our  initialization  code explicitly returns self;
          what would happen if we left this off?25

          9.4.  Inheritance and Polymorphism
               This is a good time to look at what we've done with the
          two   previous   examples  at  a  higher  level.   With  the
          CheckedArray class, we inherited almost all of the function-
          ality  of  arrays,  with  only a little bit of code added to



            25 Hint: consider what the default return  value


          is  when  no explicit value is provided.  This was


          covered in chapter 4.









                                      -43-


          address our specific needs.  While you may have not  thought
          to try it, all the existing methods for an Array continue to
          work without further effort-you might find it interesting to
          ponder why the following still works:
               Smalltalk at: #a put: (CheckedArray new: 10) !
               a at: 5 put: 1234 !
               a do: [:i| i printNl ] !
          The  strength of inheritance is that you focus on the incre-
          mental changes you make; the things you  don't  change  will
          generally continue to work.
               In  the  Complex  class,  the value of polymorphism was
          exercised.  A Complex number responds to  exactly  the  same
          set of messages as any other number.  If you had handed this
          code to someone, they would know how to do math with Complex
          numbers  without  further instruction.  Compare this with C,
          where a complex number package would  require  the  user  to
          first   find  out  if  the  complex-add  function  was  com-
          plex_plus(), or  perhaps  complex_add(),  or  add_complex(),
          or....

          9.5.  Limitations of the Complex Class
               One glaring deficiency is present in the Complex class-
          what happens if you mix normal numbers with Complex numbers?
          Currently,  the  Complex  class  assumes  that  it will only
          interact with other Complex numbers.  But this is  unrealis-
          tic-mathematically,  a "normal" number is simply one with an
          imaginary part of 0.  Smalltalk was designed to  allow  num-
          bers  to  coerce themselves into a form which will work with
          other numbers.
               The system is clever and  requires  very  little  addi-
          tional  code.   Unfortunately,  it  would  have  tripled the
          amount of explanation required.  If you're interested in how
          coercion  works  in  GNU  Smalltalk,  you  should  find  the
          Smalltalk library source, and trace back  the  execution  of
          the  retry:coercing:  messages.   You  want  to consider the
          value which the "generality" message returns for  each  type
          of  number.   Finally,  you need to examine the coerce: han-
          dling in each numeric class.

























                                      -44-


          10.  Smalltalk Streams
               Our examples have used a  mechanism  extensively,  even
          though  we  haven't discussed it yet.  The Stream class pro-
          vides a framework for a number of data structures, including
          input  and output functionality, queues, and endless sources
          of dynamically-generated data.  A Smalltalk stream is  quite
          similar  to  the  UNIX streams you've used from C.  A stream
          provides a sequential view to an underlying resource; as you
          read  or  write elements, the stream position advances until
          you finally reach the end of the  underlying  medium.   Most
          streams  also allow you to set the current position, provid-
          ing random access to the medium.

          10.1.  The Output Stream
               The examples in this book all work because  they  write
          their  output  to  the stdout stream.  Each class implements
          the printOn: method, and writes its output to  the  supplied
          stream.   The  printNl  method  all objects use is simply to
          send the current object a printOn: message whose argument is
          stdout.  You can invoke the standard output stream directly:
                'Hello, world' printOn: stdout !
                stdout inspect !

          10.2.  Your Own Stream
               Unlike a pipe you might create  in  C,  the  underlying
          storage  of  a Stream is under your control.  Thus, a Stream
          can provide an anonymous buffer of data,  but  it  can  also
          provide a stream-like interpretation to an existing array of
          data.  Consider this example:
               Smalltalk at: #a put: (Array new: 10) !
               a at: 4 put: 1234 !
               a at: 9 put: 5678 !
               Smalltalk at: #s put: (ReadWriteStream on: a) !
               s inspect !
               s position: 1 !
               s inspect !
               s nextPut: 11; nextPut: 22 !
               (a at: 1) printNl !
               a do: [:x| x printNl] !
               s position: 2 !
               s do: [:x| x printNl] !
               s position: 5 !
               s do: [:x| x printNl] !
               s inspect !
               The key is the on: message; it tells a stream class  to
          create  itself in terms of the existing storage.  Because of
          polymorphism, the object specified by on: does not  have  to
          be  an  Array; any object which responds to numeric at: mes-
          sages can be used.  If you happen to have  the  CheckedArray
          class  still loaded from the previous chapter, you might try
          streaming over that kind of array instead.
               You're wondering if you're stuck with  having  to  know
          how  much  data  will  be queued in a Stream at the time you
          create the stream.  If you use the right  class  of  stream,









                                      -45-


          the answer is no.  A ReadStream provides read-only access to
          an existing collection.  You will receive an  error  if  you
          try  to  write to it.  If you try to read off the end of the
          stream, you will also get an error.
               By contrast, WriteStream and ReadWriteStream  (used  in
          our  example)  will  tell  the underlying collection to grow
          (using the "grow" message) when you write off the end of the
          existing  collection.   Thus,  if  you want to write several
          strings, and don't want to add up their lengths yourself:
                Smalltalk at: #s put: (ReadWriteStream on: (String new: 0)) !
                s inspect !
                'Hello, ' printOn: s !
                s inspect !
                'world' printOn: s !
                s inspect !
                s position: 1 !
                s inspect !
                s do: [:c| c printOn: stdout] !
                (s contents) printNl !
               In this case, we have used a String as  the  collection
          for the Stream.  The printOn: messages add bytes to the ini-
          tially empty string.  Once we've added  the  data,  you  can
          continue  to treat the data as a stream.  Alternatively, you
          can ask the stream to return to you the  underlying  object.
          After  that, you can use the object (a String, in this exam-
          ple) using its own access methods.
               There are many amenities available on a stream  object.
          You  can  ask if there's more to read with "atEnd".  You can
          query the position with "position", and set it  with  "posi-
          tion:".  You can see what will be read next with "peek", and
          you can read the next element with "next".
               In the writing direction, you can write an element with
          "nextPut:".   You  don't need to worry about objects doing a
          printOn: with your stream as a destination;  this  operation
          ends  up as a sequence of nextPut:'s to your stream.  If you
          have  a  collection  of  things  to  write,  you   can   use
          "nextPutAll:"  with the collection as an argument; each mem-
          ber of the collection will be written onto the  stream.   If
          you want to write an object to the stream several times, you
          can use "next:put:":
               Smalltalk at: #s put: (ReadWriteStream on: (Array new: 0)) !
               s next: 4 put: 'Hi!' !
               s position: 1 !
               s do: [:x| x printNl] !

          10.3.  Files
               Streams can also operate on files.  If  you  wanted  to
          dump the file "/etc/passwd" to your terminal, you could cre-
          ate a stream on the file, and then stream over its contents:














                                      -46-


               Smalltalk at: #f put: (FileStream open: '/etc/passwd' mode: 'r') !
               f do: [:c| c printOn: stdout] !
               f position: 30 !
               1 to: 25 do: [(f next) printOn: stdout] !
               f close !
          and, of course, you can load Smalltalk source code:
               FileStream fileIn: '/users/myself/src/source.st' !

          10.4.  Dynamic Strings
               Streams  provide a powerful abstraction for a number of
          data structures.  Concepts like  current  position,  writing
          the  next  position,  and  changing  the way you view a data
          structure when convenient combine to let you write  compact,
          powerful  code.   The  last example is taken from the actual
          Smalltalk source code-it shows a general method  for  making
          an object print itself onto a a string.
               printString
                   | stream |
                   stream := WriteStream on: (String new: 0).
                   self printOn: stream.
                   ^stream contents
               !
               This  method, residing in Object, is inherited by every
          class in Smalltalk.  The first line  creates  a  WriteStream
          which  stores  on  a String whose length is currently 0.  It
          then invokes the  current  object  with  printOn:.   As  the
          object prints itself to "stream", the String grows to accom-
          modate new characters.  When the object  is  done  printing,
          the method simply returns the underlying string.
               As  we've  written  code,  the assumption has been that
          printOn: would go to the terminal.  But replacing  a  stream
          to  a  file  (/dev/tty)  with  a  stream to a data structure
          (String new: 0) works just as well.  The last line tells the
          Stream to return its underlying collection-which will be the
          string which has had all the  printing  added  to  it.   The
          result  is that the printString message returns an object of
          the String class whose contents are the printed  representa-
          tion of the object receiving the printString message.

























                                      -47-


          11.  How Arrays Work
               Smalltalk  provides a very adequate selection of prede-
          fined classes from which to  choose.   Eventually,  however,
          you  will  find the need to code a new basic data structure.
          Because  Smalltalk's  most  fundamental  storage  allocation
          facilities  are  arrays, it is important that you understand
          how to use them to gain efficient access  to  this  kind  of
          storage.

          11.1.  The Array Class
               Our  examples  have  already shown the Array class, and
          its use is fairly obvious.  For many applications,  it  will
          fill  all  your needs-when you need an array in a new class,
          you keep an instance variable,  allocate  a  new  Array  and
          assign  it to the variable, and then send array accesses via
          the instance variable.
               This technique  even  works  for  string-like  objects,
          although  it is wasteful of storage.  An Array object uses a
          Smalltalk pointer for each slot in the array; its exact size
          is  transparent  to  the  programmer,  but you can generally
          guess that it'll be roughly the word size of your machine.26
          For  storing  an  array  of  characters, therefore, an Array
          works but is inefficient.

          11.2.  Arrays at a Lower Level
               So let's step down to a lower level of data  structure.
          a  ByteArray is much like an Array, but each slot holds only
          an integer from 0 to 255-and each slot uses only a  byte  of
          storage.   If  you  only needed to store small quantities in
          each array slot, this would therefore be a much  more  effi-
          cient choice than an Array.  As you might guess, this is the
          type of array which a String uses.
               Aha!  But when you go back to chapter 9 and look at the
          Smalltalk hierarchy, you notice that String does not inherit
          from ByteArray.  To see why, we must delve down yet  another
          level,  and  arrive  at  the  basic  methods for compiling a
          class.
               For most example classes, we've used the message:
                    subclass:
                    instanceVariableNames:
                    classVariableNames:
                    poolDictionaries:
                    category:
          But when we implemented our CheckedArray  example,  we  used
          "variableSubclass:" instead of just "subclass:".  The choice
          of these two kinds of class creation (and a third we'll show
          shortly)  defines  the  fundamental  structure  of Smalltalk
          objects created within a given class.   Let's  consider  the
          differences in the next three sub-sections.




            26 32 bits for most ports of GNU Smalltalk.









                                      -48-


          11.2.1.  subclass:
               This  kind  of  class  creation  specifies the simplest
          Smalltalk object.  The object consists only of  the  storage
          needed  to hold the instance variables.  In C, this would be
          a simple structure with zero or more scalar fields.27

          11.2.2.  variableSubclass:
               This type of class is a superset of a subclass:.  Stor-
          age is still allocated for any instance variables,  but  the
          objects  of  the  class must be created with a new: message.
          The number passed as an argument  to  new:  causes  the  new
          object,  in addition to the space for instance variables, to
          also have that many slots of storage allocated.  The  analog
          in  C  would be to have a structure with some scalar fields,
          followed at its end by an array of  the  requested  size  of
          pointers.

          11.2.3.  variableByteSubclass:
               This  is a special case of variableSubclass:; the stor-
          age allocated as specified by new: is  an  array  of  bytes.
          The  analog  in  C  would be a structure with scalar fields,
          followed by an array of char.

          11.3.  Accessing These New Arrays
               You already know how to  access  instance  variables-by
          name.   But  there  doesn't  seem  to be a name for this new
          storage.  The way an object accesses it is  to  send  itself
          array-type messages-at:, at:put:, and so forth.
               The  problem is when an object wants to add a new level
          of interpretation to the at: and at:put: messages.  Consider
          a  Dictionary-it  is a variableSubclass: type of object, but
          its at: message is in terms of a key, not an  integer  index
          of its storage.  Since it has redefined the at: message, how
          does it access its fundamental storage?
               The answer is that Smalltalk has defined  basicAt:  and
          basicAt:put:,  which will access the basic storage even when
          the at: and at:put: messages have been defined to provide  a
          different abstraction.

          11.4.  An Example
               This can get pretty confusing in the abstract, so let's
          do an example to show how it's pretty  simple  in  practice.
          Smalltalk  arrays  tend to start at 1; let's define an array
          type whose permissible range is arbitrary.






            27 C requires one or more; zero  is  allowed  in


          Smalltalk.









                                      -49-


               ArrayedCollection variableSubclass: 'RangedArray'
                    instanceVariableNames: 'base'
                    classVariableNames: ''
                    poolDictionaries: ''
                    category: nil !
               RangedArray comment: 'I am an Array whose base is arbitrary' !
               !RangedArray class methodsFor: 'creation'!
               new
                    ^self error: 'Use new:base:'
               !
               new: ignore
                    ^self new
               !
               new: size base: b
                    ^(super new: size) init: b
               !!
               !RangedArray methodsFor: 'init'!
               init: b
                    base := (b - 1).    "- 1 because basicAt: works with a 1 base"
                    ^self
               !!
               !RangedArray methodsFor: 'basic'!
               rangeCheck: i
                    ((i <= base) | (i > (base + (self basicSize)))) ifTrue: [
                         'Bad index value: ' printOn: stderr.
                         i printOn: stderr.
                         (Character nl) printOn: stderr.
                         ^self error: 'illegal index'
                    ]
               !
               at: i
                    self rangeCheck: i.
                    ^self basicAt: (i-base)
               !
               at: i put: v
                    self rangeCheck: i.
                    ^self basicAt: (i-base) put: v
               !!
               The code has two parts; an initialization, which simply
          records what index you wish the array to start with, and the
          at: messages, which adjust the requested index so  that  the
          underlying  storage  receives  its  1-based  index  instead.
          We've included a range check  much  like  CheckedArray;  its
          utility will demonstrate itself in a moment:
               Smalltalk at: #a put: (RangedArray new: 10 base: 5) !
               a at: 5 put: 0 !
               a at: 4 put: 1 !
          Since  4 is below our base of 5, a range check error occurs.
          But this check can catch more than just our own misbehavior!
               a do: [:x| x printNl] !
          Our  do:  message  handling  is broken!  The stack backtrace
          pretty much tells the story:











                                      -50-


               RangedArray>>#rangeCheck:
               RangedArray>>#at:
               MethodContext>>#value:
               Integer>>#to:by:do:
               Integer>>#to:do:
               RangedArray>>#do:
          Our code received a do: message.  We didn't define  one,  so
          we  inherited  the  existing  do:  handling.  We see that an
          Integer loop was constructed, that a code block was invoked,
          and  that  our  own  at:  code  was  invoked.  When we range
          checked, we trapped an illegal index.  Just by  coincidence,
          this  version  of  our  range  checking  code also dumps the
          index.  We see that do: has assumed that all arrays start at
          1.
               The immediate fix is obvious; we implement our own do:
               !RangedArray methodsFor: 'basic'!
               do: aBlock
                    1 to: (self basicSize) do: [:x|
                         aBlock value: (self basicAt: x)
                    ]
               !!
          But  the  issues  start  to  run  deep.  If our parent class
          believed that it knew enough to assume a starting  index  of
          1,  why  didn't  it also assume that it could call basicAt:?
          Object-oriented methodology says that one object  should  be
          entirely opaque to another.  But what sort of privacy should
          there be between a higher class  and  its  subclasses?   How
          many  assumption  can  a subclass make about its superclass,
          and how many  can  the  superclass  make  before  it  begins
          infringing  on the sovereignty of its subclasses?  There are
          rarely easy answers.

          11.5.  Basic Allocation
               In this chapter, we've seen the fundamental  mechanisms
          used  to  allocate and index storage.  When the storage need
          not be accessed with peak efficiency, you can use the exist-
          ing  array  classes.   When  every access counts, having the
          storage be an integral part of your own  object  allows  for
          the quickest access.  When you move into this area of object
          development, inheritance and polymorphism  become  trickier;
          each  level  must coordinate its use of the underlying array
          with other levels.





















                                      -51-


          12.  Further Studies
               The question is always how far to go in  one  document.
          At this point, you know how to create classes.  You know how
          to use inheritance, polymorphism, and the basic storage man-
          agement  mechanisms  of  Smalltalk.  You've also seen a sam-
          pling of Smalltalk's powerful classes.   The  rest  of  this
          chapter simply points out areas for further study; perhaps a
          newer version of this document might cover these in  further
          chapters.

          12.1.  Viewing the Smalltalk Source Code
               Depending   on  the  thoroughness  of  the  person  who
          installed GNU Smalltalk, it's possible to  view  the  source
          code  for  a system method.  For instance, to see how a Dic-
          tionary processes a do: message:
               Dictionary edit: #do: !
          The viewer is hard-coded as emacs; this may or may not  work
          at your installation.

          12.2.  Other Ways to Collect Objects
               We've  seen  Array, ByteArray, Dictionary, Set, and the
          various  streams.   You'll  want  to  look   at   the   Bag,
          LinkedList,  and SortedCollection classes.  For special pur-
          poses you'll want to examine ByteMemory and WordMemory.

          12.3.  Flow of Control
               GNU Smalltalk has rudimentary support  for  threads  of
          execution.  The state is embodied in a Process class object;
          you'll also want to look at the ProcessorScheduler class.

          12.4.  Smalltalk Virtual Machine
               GNU Smalltalk is implemented as a  virtual  instruction
          set.   By invoking GNU Smalltalk with the -d option, you can
          view the byte opcodes which are generated as  files  on  the
          command  line  are loaded.  Similarly, running GNU Smalltalk
          with -e will trace the execution  of  instructions  in  your
          methods.
               You  can  look at the GNU Smalltalk source to gain more
          information on the instruction set.  A better first step  if
          you  want  to pursue this subject is to start with "A Little
          Smalltalk"" by Tim Budd.  The source code is  freely  avail-
          able,   and  the  book  provides  a  solid  introduction  to
          Smalltalk-type virtual machines.  The canonical book is from
          the original designers of Smalltalk:
               Smalltalk-80: The Language and its Implementation
                    - Adele Goldberg and David Robson

          12.5.  Two Flavors of Equality
               As  first  seen in chapter two, Smalltalk keys its dic-
          tionary with things like "#word", whereas we  generally  use
          "'word'".   The  former, as it turns out, is from class Sym-
          bol.  The latter is from class String.  What's the real dif-
          ferent  between  a Symbol and a String?  To answer the ques-
          tion, we'll use an analogy from C.









                                      -52-


               In C, if you have a function for comparing strings, you
          might try to write it:
               strcpy(char *p, char *q)
               {
                    return (p == q);
               }
          But  clearly this is wrong!  The reason is that you can have
          two copies of a string-each with the same contents-but  each
          at  its own address.  A correct string compare must walk its
          way through the strings and compare each element.
               In Smalltalk, exactly the same issue  exists,  although
          the  details  of  manipulating storage addresses are hidden.
          If we have two Smalltalk strings, both with  the  same  con-
          tents,  we  don't  necessarily  know  if they're at the same
          storage address.  In  Smalltalk  terms,  we  don't  know  if
          they're the same object.
               The  Smalltalk  dictionary  is searched frequently.  To
          speed the search, it would be nice to not  have  to  compare
          the characters of each element, but only compare the address
          itself.  To do this, you need to have a guarantee  that  all
          strings  with  the  same  contents are the same object.  The
          String class, created like:
               y := 'Hello' !
          does not satisfy this.  Each time you execute this line, you
          may  well  get a new object.  But a very similar class, Sym-
          bol, will always return the same object:
               y := #Hello !
          In general, you can use strings for almost all  your  tasks.
          If  you  ever get into a performance-critical function which
          looks up strings, you can switch to Symbol.  It takes longer
          to  create  a  Symbol,  and the memory for a Symbol is never
          freed (since the class has to keep tabs on  it  indefinitely
          to  guarantee  it continues to return the same object).  You
          can use it, but use it with care.

          12.6.  Checking for the Two Types of Equality
               This paper has generally used the strcmp()-ish kind  of
          checks  for  equality.  If you ever need to ask the question
          "is this the  same  object?",  you  use  the  "=="  operator
          instead of "=":
               Smalltalk at: #x put: 0 !
               Smalltalk at: #y put: 0 !
               x := 'Hello' !
               y := 'Hello' !
               (x = y) printNl !
               (x == y) printNl !
               x := #Hello !
               y := #Hello !
               (x = y) printNl !
               (x == y) printNl !
               Using  C  terms, the former compares contents like str-
          cmp().   The  latter  compares  storage  addresses,  like  a
          pointer comparison.










                                      -53-


          12.7.  Where to get Help
               The  newsgroup comp.lang.smalltalk is read by many peo-
          ple with a great deal of Smalltalk  experience.   There  are
          several  commercial  Smalltalk  implementations; you can buy
          support for these, though  it  isn't  cheap.   For  the  GNU
          Smalltalk  system  in  particular, you can try the newsgroup
          gnu.smalltalk.bug.  If all  else  fails,  you  can  try  the
          author at:
               [e-mail]
          No guarantees, but the author will do his best!

          12.8.  Acknowledgments
               Thanks  to Steve Byrne for writing GNU Smalltalk in the
          first place.  Great thanks to Mark Bush  and  Bob  Roos  for
          their meticulous jobs of proofreading this document, and the
          generous amounts of input they provided  on  refinements  to
          the  contents and structure.  Thanks also to Andrew Berg for
          his comments on the early chapters of the document.
               Any remaining  errors  are  purely  the  fault  of  the
          author.   This document is provided as-is, without warranty,
          but I will happily accept reports of any  errors.   If  time
          permits, I will perhaps even release a corrected revision of
          the document.
               I release this document into  the  public  domain,  and
          simply  request  that  you  acknowledge  me  as the original
          author in any use or derivative work you make of this  docu-
          ment.




                                   Andy Valencia
                                   325 Union Ave #359
                                   Campbell, CA  95008
                                   [e-mail]
                                   November 27, 1992



























                                      -54-


                                   APPENDIX A
                     A Simple Overview of Smalltalk Syntax




               Smalltalk's  power comes from its treatment of objects.
          In this document, we've mostly avoided the issue  of  syntax
          by using strictly parenthesized expressions as needed.  When
          this leads to code which is hard to read due to the  density
          of  parentheses,   a knowledge of Smalltalk's syntax can let
          you simplify expressions.  In general, if it  was  hard  for
          you  to  tell how an expression would parse, it will be hard
          for the next person, too.
               The following presentation presents the grammar a  cou-
          ple  of  related  elements at a time.  We use a BNF style of
          grammar, with some extensions.  The form:
               [ ... ]
          means that "..." can occur zero or one times.
               [ ... ]*
          means zero or more;
               [ ... ]+
          means one or more.
                ... | ... [ | ... ]*
          means that one of the variants must be  chosen.   Characters
          in double quotes refer to the literal characters.  Most ele-
          ments may be separated by white space;  where  this  is  not
          legal,  the  elements  are  presented  without  white  space
          between them.


               methods: "!" id ["class"] "methodsFor:" string "!" [method "!"] "!"
          Methods are introduced by first naming a class (the id  ele-
          ment),  specifying  "class"  if  you're adding class methods
          instead of instance methods, and sending a  string  argument
          to  the methodsFor: message.  Each method is terminated with
          an "!"; two "!"'s in a row signify the end of the new  meth-
          ods.

               method: message [prim] [temps] exprs
               message: id | binsel id | [keysel id]+
               prim: "<" "primitive:" number ">"
               temps: "|" [id]* "|"
          A method definition starts out with a kind of template.  The
          message to be handled is specified with  the  message  names
          spelled  out  and  identifiers in the place of arguments.  A
          special kind of definition is the primitive; it has not been
          covered  in  this  paper;  it  provides  an interface to the
          underlying Smalltalk virtual machine.  temps is the declara-
          tion  of  local variables.  Finally, exprs (covered soon) is
          the actual code for implementing the method.












                                      -55-


               unit: id | literal | block | "(" expr ")"
               unaryexpr: unit [ id ]+
               primary: unit | unaryexpr
          These are the "building blocks" of Smalltalk expressions.  A
          unit  represents  a single Smalltalk value, with the highest
          syntactic precedence.  A unaryexpr is simply  a  unit  which
          receives  a  number  of unary messages.  A unaryexpr has the
          next highest precedence.  A primary is simply  a  convenient
          left-hand-side name for one of the above.

               exprs: [expr "."]* [["^"] expr]
               expr: [id ":="]* expr2
               expr2: primary | msgexpr [ ";" cascade ]*
          A  sequence of expressions is separated by "."'s and can end
          with a returned value ("^").  There can be  leading  assign-
          ments;  unlike  C, assignments apply only to simple variable
          names.  An expression is  either  a  primary  (with  highest
          precedence)  or  a  more  complex message.  cascade does not
          apply to primary constructions, as they are  too  simple  to
          require  the  construct--since  all  primary  construct  are
          unary, you can just add more unary messages:
               1234 printNl printNl printNl !

               msgexpr: unaryexpr | binexpr | keyexpr
          A complex message is either a unary message (which  we  have
          already covered), a binary message ("+", "-", and so forth),
          or a keyword message ("at:", "new:", ....)   Unary  has  the
          highest precedence, followed by binary, and keyword messages
          have the lowest precedence.  Examine the two versions of the
          following  messages.   The second have had parentheses added
          to show the default precedence.
               myvar at: 2 + 3 put: 4
               mybool ifTrue: [ ^ 2 / 4 roundup ]

               (myvar at: (2 + 3) put: (4))
               (mybool ifTrue: ([ ^ (2 / (4 roundup)) ]))

               cascade: id | binmsg | keymsg
          A cascade is used to direct further  messages  to  the  same
          object  which  was last used.  The three types of messages (
          id is how you send a unary message) can thus be sent.

               binexpr: primary binmsg [ binmsg ]*
               binmsg: binsel primary
               binsel: selchar[selchar]
          A binary message is sent to an  object,  which  primary  has
          identified.   Each binary message is a binary selector, con-
          structed from one or two characters, and an  argument  which
          is also provided by a examplesprimary.Some
               1 + 2 - 3 / 4
          which parses as:
               (((1 + 2) - 3) / 4)











                                      -56-


               keyexpr: keyexpr2 keymsg
               keyexpr2: binexpr | primary
               keymsg: [keysel keyw2]+
               keysel: id":"
          Keyword expressions are much like binary expressions, except
          that the selectors are made up of identifiers with  a  colon
          appended.  Where the arguments to a binary function can only
          be from primary, the arguments to a keyword  can  be  binary
          expressions  or primary ones.  This is because keywords have
          the lowest precedence.

               block: "[" [[":" id]* "|" ] exprs "]"
          A code block is  square  brackets  around  a  collection  of
          Smalltalk expressions.  The leading ": id" part is for block
          arguments.

               literal: number | string | charconst | symconst | arrayconst
               arrayconst: "#" array
               array: "(" [number | string | symbol | array | charconst]* ")"
               number: [[dig]+ "r"] ["-"] [hexDig]+ ["." [hexDig]+] ["e"["-"][dig]+].
               string: "'"[char]*"'"
               charconst: "$"char
               symconst: "$"symbol
          We have already shown the use of many  of  these  constants.
          Although  not covered in this paper, numbers can have a base
          specified at their front, and a  trailing  scientific  nota-
          tion.   We have seen examples of character, string, and sym-
          bol constants.  Array  constants  are  simple  enough;  they
          would look like:
               Smalltalk at: #a put: #(1 2 'Hi' $x $Hello 4 5) !

               symbol: id | binsel | keysel[keysel]*
          Symbols  are  mostly used to represent the names of methods.
          Thus, they can hold simple  identifiers,  binary  selectors,
          and keyword selectors:
               $hello
               $+
               $at:put:

               id: letter[letter|dig]*
               selchar: "+" | "-" | "*" | "/" | "~" | "|" | "," |
                    "<" | ">" | "=" | "&"
               hexdig: "0".."9" | "A".."F"
               dig: "0".."9"
          These are the categories of characters and how they are com-
          bined at the most basic level.   selchar  simply  lists  the
          characters which can be combined to name a binary message.