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Frequently Asked Questions (FAQS);faqs.104



Chapter 10 is on user interfaces: it describes and illustrates
the Smalltalk MVC paradigm (also: InterViews)

--
Raimund K. Ege                             School of Computer Science
                                             Florida Int'l University
[e-mail]           [phone]              University Park
[e-mail]     FAX [phone]              Miami, FL 33199

**

From: [e-mail] (Nils Erik Asmundvaag)
Newsgroups: comp.lang.smalltalk
Subject: Re: MVC -- good introductions?
Date: 11 Mar 92 10:56:38 GMT
Organization: Glasgow University Computing Science Dept.

The book

    Smalltalk-80: A Practical Introduction
    (ISBN 0-273-03105-8)

    by Philip D. Gray & Ramzan Mohamed, 1990
    and published by Pitman (at least in the UK)

contains two chapters on interactive applications and the MVC.

I found it very helpful when first learning about the MVC.

Nils E. Asmundvaag

--
-------------------------------------------------------------------------------
Nils Erik Asmundvaag
University of Glasgow, Scotland
[e-mail]      [e-mail]


**

From: [e-mail] (Bruce Samuelson)
Newsgroups: comp.lang.smalltalk
Subject: Re: how are st80 views and controllers used?
Date: 12 Mar 92 14:12:48 GMT
Organization: UTexas at Arlington, Linguistics

There are two papers on MVC that provide an introductory overview of
the pre-version 4.0 ST80 scheme.  Much of what's in them also applies
to version 4.0.  I understand that one of PPS's priorities in the
forthcoming version 4.1 will be improved documentation.  We'll see how
well they explain the new windowing scheme launched in version 4.0.
It would certainly be helpful to get an overview of what's going on
before plunging into the source code of the myriad new classes.

(1) A Cookbook for using the Model-View-Controller User Interface
Paradigm in Smalltalk-80 by Glenn E. Krasner and Stephen T. Pope,
ParcPlace Systems, copyright 1988.

(2) Applications Programming in Smalltalk-80: How to Use
Model-View-Controller (MVC) by Steve Burbeck, Softsmarts, Inc.,
copyright 1987.

The first paper may still be in print.  Try [e-mail].

The second paper is probably no longer available.  I think Softsmarts
is the company that used to sell a version of Smalltalk for 80286
machines but went out of business some years ago.  The phone number on
the paper is listed as 415-327-8100 (Palo Alto, California).  You may
try asking ParcPlace (or, less likely, Digitalk) if they have copies
of Burbeck's paper.

--
**********************************************************
* Bruce Samuelson    Department of Linguistics     *
* [e-mail]    University of Texas at Arlington *
**********************************************************


---

3.2)        Is there a Smalltalk bibliography?

Answer:

    There are many... here is one:

From: [e-mail] (Ronald Schultz)
Newsgroups: comp.lang.smalltalk
Subject: Smalltalk Relevant Texts
Date: 10 Jan 92 16:08:05 GMT
Organization: Grebyn Timesharing


A list of Smalltalk-relevant texts.  Retrieved from the Digitalk
forum on Compuserve.  If you know of any additional texts, please
let me know.  Thanx.


==========================================================================
Ron Schultz
Berard Software Engineering, Inc.
Columbus Ohio Office                           Headquarters
5634 Claire Court                              301 Lakeforest Drive
Dublin, Ohio 43017                             Gaithersburg, Md. 20877
Phone  [phone]                          [phone]
FAX    [phone]                          [phone]
=========================================================================

Smalltalk 80 The Language, Adele Goldberg & David Robson
        Addison-Wesley 1989             ISBN 0-201-13688-0

Smalltalk 80 The Interactive Programming Environment, Adele Goldberg
        Addison Wesley 1984             ISBN 0-201-11372-4

Smalltalk 80 Bits of History, Words of Advice , Glenn Krasner
        Addison Wesley 1984             ISBN 0-201-11669-3

Inside Smalltalk Volume I, Wilf Lalonde & John Pugh
        Prentice Hall 1991              ISBN 0-13-468414-1

Inside Smalltalk Volume II, Wilf Lalonde & John Pugh
        Prentice Hall 1991              ISBN 0-13-465964-3

Object-Oriented Graphics, P. Wisskirchen
    Springer-Verlag 1990        ISBN 3-540-52859-8

Practical Smalltalk: Using Smalltalk/V, Dan Shafer and Dean A. Ritz.
        Springer-Verlag                 ISBN 0-387-97394-X

Rapid Prototyping for Object Oriented Systems, Mark Mullen
        Addison Wesley 1990             ISBN 0-201-55024-5

Object-Oriented Design, Peter Coad and Ed Yourdon
    Yourdon Press 1991        ISBN 0-13-630070-7

Object Oriented Programming for Artificial Intelligence, Ernest Tello
        Addison Wesley 1989             ISBN 0-201-09228-x

The Well Tempered Object, Stephen Travis Pope
        MIT Press 1991                  ISBN 0-262-16126-5

RefTalk/Vwin, David Carl O'Neal
        NuVista Press 1991              ISBN pending

Human-Computer Interface Design Guidelines, C. Marlin Brown
        Ablex Publishing 1989           ISBN 0-89391-332-4

Designing Object-Oriented Software,
                Rebecca Wirfs-Brock, Brian Wilkerson, and Lauren Wiener
        Prentice-Hall 1990              ISBN 0-13-629825-7

Object Oriented Programming with Smalltalk/V, Dusko Savic
        Ellis Horwood 1990              ISBN 0-13-040692-9

An Introduction to Object Oriented Programming & Smalltalk
        Lewis Pinson & Richard Wiener
        Addison Wesley 1988             ISBN 0-201-19127-x

SAA Common User Access Advanced Interface Design Guide
        IBM 1989                        IBM Document # SC26-4582-0

IBM Red Books----(available from your IBM representative
                  contact your local office of IBM and request
                  the placing of an IBM Red Book Order.  If you
                  are an IBM customer, the books are free.  If you
                  are not an IBM customer, the books may have a nominal
                  fee.)----

A Practical Introduction to Object Oriented Programming
        IBM 1990                        IBM Document # GG24-3641

Object Oriented Design - A preliminary Approach
        IBM 1990                        IBM Document # GG24-3647

Developing a CUA Workplace Application
        IBM 1990                        IBM Document # GG24-3580-00

Managing the Development of Object Oriented Applications
        IBM 1990                        IBM Document # GG24-3581-00

Object Oriented Analysis of the ITSO Common Scenario
        IBM 1990                        IBM Document # GG24-3566

CUA Evaluation
        IBM 1990                        IBM Document # GG24-3456

SAA CUA '91 Guide
        IBM 1991                        IBM Document # SC34-4289

SAA CUA '91 Reference
        IBM 1991                        IBM Document # SC34-4290

SAA - A Guide for Evaluating Applications
        IBM 1991                        IBM Document # G320-9803


---

3.3)+        What are the "blue book", "purple book", etc?

Answer:

Date: Wed, 11 Nov 92 12:52:39 PST
From: [e-mail] (Mike Khaw)


blue
        Goldberg, Adele, and David Robson, _Smalltalk-80: The Language
        and Its Implementation_, Addison-Wesley, 1983. ISBN
    0-201-11371-6. *Out of print*

orange
        Goldberg, Adele, _Smalltalk-80: the Interactive Programming
        Environment_, Addison-Wesley, 1984. ISBN 0-201-11372-4.

green
        Krasner, Glenn, ed., _Smalltalk-80: Bits of History, Words of
        Advice_, Addison-Wesley, 1983, ISBN 0-201-11669-3

purple
        Goldberg, Adele, and David Robson, _Smalltalk-80: The Language_,
        Addison-Wesley, 1989, ISBN 0-201-13688-0

The books are actually cream or tan. The color referred to is the color
used as the background of the illustration on the front cover (as well
as for the Addison-Wesley logo on the spine).

The purple book is an update/revision of the blue book, with the
section on the abstract bytecode machine omitted (because it was out
of date, according to Adele).
----------

Mike


---

4.0)+    [Programming issues]

---

4.1)+        What are some "classic Smalltalk bugs", both in the
            system and programmer domains?

Answer:

Newsgroups: comp.lang.smalltalk
From: [e-mail] (Ralph Johnson)
Subject: catalog of classic bugs
Summary: version 1 of the catalog of classic Smalltalk bugs
Organization: University of Illinois, Dept. of Comp. Sci., Urbana, IL
Date: Wed, 12 Aug 1992 22:29:16 GMT

Classic Smalltalk Bugs

compiled by Ralph Johnson -- University of Illinois at Urbana-Champaign

Every programming system is prone to certain kinds of bugs.  A good
programmer learns these bugs, and how to avoid them.  Smalltalk is
no different.  Smalltalk eliminates lots of bugs that are common in
other languages, such as bugs in linear search algorithms.  (Just use do:)
However, it has its own set of classic bugs, which every new Smalltalk
programmer runs into.

There are several reasons to collect classic bugs.  The first is that
it will help experienced programmers test and debug programs, and can
help us design better programs in the first place.  Second, if we
teach these bugs up front then people should learn to be good
programmers faster.  Third, perhaps we can redesign the system to
eliminate some of these bugs, or we can write checking tools to
spot them automatically.

Bug 1: Variable-sized classes

Set, Dictionary, and OrderedCollection are variable-sized classes
that grow.  They grow by making a copy of themselves and "becoming"
the copy.  If you add new instance variables to a subclass then
you have to make sure that these instance variables get copied, too,
or else you will mysteriously lose the values of the instance
variables at random points in time.

Smalltalk-80 R4.0 (and probably some earlier versions) has a
copyEmpty: method in Collection that you are supposed to override
if you make a subclass of Collection that adds instance varaibles.
The solution to this bug is to write a version of copyEmpty: for
your class.

It has been suggested that it would be easy to write a tool that
checked that every new subclass of Collection that added instance
variables also defined a method for copyEmpty:.

Collection bugs

Bug 2: add: returns its argument

For every collection that grows, add: returns its argument,
not the receiver, and people usually assume that it returns
its receiver.  Thus, they write "(c add: x) add: y" when they should
really write "c add: x; add: y" or else "c add: x. c add: y".
Note that this is one of the good uses for "yourself", you can write
(Set new
    add: x;
    add: y;
    ...;
    yourself)
to make sure that you have the new Set.

Note that there are good reasons why add: returns its arguments,
and even if there weren't, it is a very, very bad mistake to
implement add: so that it returns the receiver, and so confuse
every other Smalltalk programmer on the planet.
Making add: return its argument often keeps you from resorting
to temps, because you can create the argument to add: on the
fly, and then do other things with it after the add:.  If you
want to access the collection, you can do it with yourself or
cascaded messages, as described above.

Bug 3: changing collection while iterating over it

You should never, never, never iterate over a collection which
the iteration loop somehow modifies. Elements of the collection
will be moved during the iteration, and elements might be missed
or handled twice.  Instead, make a copy of the collection you
are iterating over, i.e.
   aCollection copy do: [:each | aCollection remove: each]
is a good program, but if you leave out the copy then it isn't.

Mario Wolczko suggested a solution that catches this problem the
instance it occurs (at some performance penalty of course).  The
solution is to change the collection classes.  Each iteration method
enters that collection into a set of collections being iterated over
(IteratedCollections), executes the block, then removes the collection
from the set.  Collections are usually (only?) modified using at:put:
or basicAt:put:, so these are overriden to check that the collection
is not in IteratedCollections.  If it is, an error is signalled.  You
can either use this technique all the time, or you can just install
these classes when you are testing and debugging your program.  These
changes are packaged in a file Iterator-check.st that is available on
the Manchester and Illinois servers.  On the Illinois server, it is
in /pub/MANCHESTER/manchester/4.0/Iterator-check.st.

Bug 4: modifying copies of collections

It is common for an object to have an accessing method that returns a
collection of objects that you can modify.  However, sometimes
an object will return a copy of this collection to keep you from
modifying it.  Instead, you are probably supposed to use messages
that will change the collection for you.  The problem is that this
is often poorly documented, and a person who likes to modify collections
directly will run into problems.  See  "ScheduledControllers
scheduledControllers" for an example.

The solution is to either provide better documentation, to claim
that nobody is allowed to modify copies of collections returned
from other objects, or to have objects that don't want their
collections modified to return immutable versions of the collections
that will give an error if you try to modify them.

Bug 5: Missing ^

It is very easy to leave off a return caret on an expression.
If there is no return at the end of a method, Smalltalk returns
the receiver of the method.  It only takes one missing return
to mess up a long chain of method invocations.

Bug 6: Class instance creation methods

Writing a correct instance create method is apparently non-trivial.
The correct way to do it is to have something like
new
  ^super new init
where you redefine init in each class to initialize that class's
instance variables.  In turn, init is defined as a class method
init
  super init "to initialize inherited instance variables"
  "initialize variables that I define"

There are lots of ways to do this wrong.  Perhaps the most common
is to forget the return, i.e. to write
   super new init
The result is that you have the class where you want the instance of
the class.  This is a special case of bug #?.

Another error is to make an infinite loop by writing
  ^self new init

If Smalltalk doesn't respond when you think it should, press ^C to
get the debugger.  If the debugger shows a stack of "new" messages
then you know you made this mistake.

Finally, you should only define "new" once for each class hierarchy
and let subclasses inherit the method.  If you redefine it in each
class then you will reinitialize the new object many times, wasting
time and perhaps memory.

One way to keep this from happening is to make the "new" method in
Object send init, and have the "init" method in Object do nothing.
Of course, sometimes the version of init that you define has arguments,
and this wouldn't help those cases.  It is probably better to rely
on education to eliminate this kind of error.

Bug 7: Recompiling bugs in Smalltalk/V

It is easy to have references to obsolete objects in Smalltalk/V
if you change code without cleaning things up carefully. For example,
the associations whose keys are the referenced names in the Pool
Dictionary are stored in the CompiledMethods at compile time. If you
create a new version of the Pool Dictionary and install it by simple
assignment then the compiled methods still refer to the old associations.

If you substitute a new instance of Dictionary or replace, rather than
update an association in a pool dictionary, you have to recompile all
methods using variables scoped to that Pool.

This is is also annoying when using ENVY, where the methods are under
strict control. Perhaps Pool Dictionaries should be be first-class
versioned pre-requisites of Classes, just like the class definition.

BTW we are using/VPM 1.4 with ENVY 1.3

1. If you prune & graft a subtree of your class structure you have to
make sure that all referencing methods are recompiled. Otherwise you
will run (or your customer, because this is only detected at run time)
into an Deleted class error message.
Thomas Muhr posted a "Bite" a while ago to handle this problem for
Smalltalk/V 286.

Bug 8: Opening windows

Neither Smalltalk-80 nor Smalltalk-V return to the sender when a
new window is opened in a standard fashion. Thus, any code after
a message to open a window will never be executed.  This is the
cause of much frustration.  For example, if you try to open two
windows at once, i.e.

TextPane new open.
TextPane new open

is Smalltalk-V and

aScheduledWindow1 open.
aScheduledWindow2 open

in Smalltalk-80, then you will get only one open window,
and one forgotten piece of code.

I don't know the fix for Smalltalk-V.  The fix for Smalltalk-80 is
to use the openNoTerminate method to open the window, which does
not transfer control to it.  A useful trick is to store the new
window in a global variable so you can test it.  In earlier
versions of Smalltalk-80, open would obliterate the current
process, not just never return to it.  In Version 4.1 that
never happens.

Bug 9: blocks

Blocks are very powerful, and it isn't hard for programmers to get
into trouble trying to be too tricky.  To compound problems, the
two versions of Smalltalk have slightly different semantics for
blocks, and one of them often leads to problems.

Originally blocks did not have truly local variables.  The block
parameters were really local variables in the enclosing method.
Thus,

| x y |
  x := 0.
  (1 to: 100) do: [:z | x := x + z]

actually had three temporaries, x, y, and z.  This leads to bugs
like the following

someMethod

 | a b  |
 a := #(4 3 2 1).
 b := SortedCollection sortBlock: [:a :b | a someOperation: b].
 b addAll: a.
 Transcript show: a.

When elements are added to b, the sortBlock is used to tell where
to put them, but this will change a and b.  addAll: is OK, but
the "a" that gets displayed on the transcript will be an integer,
not an array.

Early versions of Smalltalk-80 (2.4 and before?) implemented blocks
like this, and Smalltalk/V still does.  However, in current PPS
implementations, blocks are close to being closures. You can declare
variables local to a block, and the names of the block parameters are
local to the block. Most people agree that this is a much better
definition of blocks than the original one.  Nevertheless, people
planning to use Smalltalk/V should realise that it has a different
semantics for blocks.

This difference can lead to some amusing problems.  For example, here
is some code written by someone who had obviously learned Scheme.

| anotherArray aBlockArray |

aBlockArray := Array new: 4.
anotherArray := #(1 2 4 8).

1 to: 4 do: [ :anIndex |
 aBlockArray at: anIndex put: [ (anotherArray at: anIndex) * 2 ]].


The programmer expected that each block would be stored in the array
along with its own value of anIndex.  If anIndex were just a local
variable of the method then this will not work.  It assumes that
each execution of the block gets its own version of anIndex, and
Smalltalk/V and old Smalltalk-80 actually make each execution share
the same version.

So, if you are using Smalltalk/V then be careful not to reuse the
names of arguments of blocks unless you know that the blocks are
not going to have their lives overlap.  Thus,

  aCollect do: [:i | ...].
  bCollect do: [:i | ...].

is probably OK because do: does not store its argument, so the
blocks will be garbage by the time the method is finished.
However, if the first block were stored in a variable somewhere
and evaluated during the execution of the second block then
problems would probably occur.

Bug 10: Smalltalk/V class library
Thomas Muhr makes these comments about bugs in the Smalltalk/V
class library that you should know if you want to keep your
programs fast and correct.

2. Never use symbols to label objects if you are dealing with many
objects. This will slow down your system to an almost dead halt. Use
strings instead.
3. Never use Sets when you can otherwise assure the uniqueness. Look
at the implementation of "add:" for Sets and you'll know what I mean:
on every "add:" the new element is compared to all others resulting
into a nonlinear time for adding to Sets.
4. Do not think that if you "collect:" something from a
SortedCollection, that your result will be sorted as the origin,
unless you use the default sortBlock. This is one of the bugs provided
by the language vendor



Many thanks to the many people who contributed bugs or solutions to bugs
to the list.  These include

 [e-mail] (Thomas Muhr)
 [e-mail] (Jan Steinman)
 [e-mail] (Alan Knight)
 [e-mail] (Mario Wolczko)
 [e-mail] (Peter Goodall)
 Aad Nales <[e-mail]>
 [e-mail] (Simon Lewis)
 [e-mail] (Mike Smith)
 [e-mail] (Naci Dai)
 [e-mail] (Dave Robbins)
 [e-mail] (Randy Stafford)
 [e-mail] (Hubert Baumeister)
 [e-mail] (Eliot Miranda)
 [e-mail] (donald)
 [e-mail] (Amir Bakhtiar)
 Kurt Piersol <[e-mail]>
 [e-mail] (Michael Sullivan)
 [e-mail] (Terry)
 [e-mail] (Brent Sterner)
 [e-mail]
 [e-mail] (Nicole Tedesco)
 [e-mail] (Rik Fischer Smoody)
 [e-mail] (Marten Feldtmann)

Comments and additions are welcome.  Please post to comp.lang.smalltalk
or e-mail to [e-mail].

---

End of Smalltalk FAQ

Xref: bloom-picayune.mit.edu alt.privacy:5256 misc.legal:54792 news.answers:4745 alt.society.civil-liberty:7994 comp.society.privacy:855
Path: bloom-picayune.mit.edu!senator-bedfellow.mit.edu!senator-bedfellow.mit.edu!usenet
From: [e-mail] (Chris Hibbert)
Newsgroups: alt.privacy,misc.legal,news.answers,alt.society.civil-liberty,comp.society.privacy
Subject: Social Security Number FAQ
Supersedes: <[e-mail]>
Followup-To: alt.privacy
Date: 22 Dec 1992 06:02:26 GMT
Organization: Massachvsetts Institvte of Technology
Lines: 368
Approved: [e-mail]
Expires: 19 Jan 1993 06:02:09 GMT
Message-ID: <[e-mail]>
NNTP-Posting-Host: pit-manager.mit.edu
Keywords: Social Security Number, SSN, privacy
X-Last-Updated: 1992/12/01
Last-Modified: Nov 30, 1992
Last-Modification: retrieving copies of this posting

Archive-Name: ssn-privacy

[I no longer have a way to read usenet news.  If you have comments on
the following, please send them to [e-mail].]



       What to do when they ask for your Social Security Number

                           by Chris Hibbert

                        Computer Professionals
                      for Social Responsibility


Many people are concerned about the number of organizations asking for their
Social Security Numbers.  They worry about invasions of privacy and the
oppressive feeling of being treated as just a number.  Unfortunately, I
can't offer any hope about the dehumanizing effects of identifying you with
your numbers.  I *can* try to help you keep your Social Security Number from
being used as a tool in the invasion of your privacy.

Surprisingly, government agencies are reasonably easy to deal with; private
organizations are much more troublesome.  Federal law restricts the agencies
at all levels of government that can demand your number and a fairly
complete disclosure is required even if its use is voluntary.  There are no
comparable Federal laws restricting the uses non-government organizations
can make of it, or compelling them to tell you anything about their plans.
Some states have recently regulations on collection of SSNs by private
entities.  With private institutions, your main recourse is refusing to do
business with anyone whose terms you don't like.


                            Short History

Social Security numbers were introduced by the Social Security Act of 1935.
They were originally intended to be used only by the social security
program, and public assurances were given at the time that use would be
strictly limited.  In 1943 Roosevelt signed Executive Order 9397 which
required federal agencies to use the number when creating new record-keeping
systems.  In 1961 the IRS began to use it as a taxpayer ID number.  The
Privacy Act of 1974 required authorization for government agencies to use
SSNs in their data bases and required disclosures (detailed below) when
government agencies request the number.  Agencies which were already using
SSN as an identifier before January 1, 1975 were allowed to continue using
it.  The Tax Reform Act of 1976 gave authority to state or local tax,
welfare, driver's license, or motor vehicle registration authorities to use
the number in order to establish identities.  The Privacy Protection Study
Commission of 1977 recommended that the Executive Order be repealed after
some agencies referred to it as their authorization to use SSNs.  I don't
know whether it was repealed, but that practice has stopped.

Several states use the SSN as a driver's license number, while others record
it on applications and store it in their database.  Some states that
routinely use it on the license will make up another number if you insist.
According to the terms of the Privacy Act, any that have a space for it on
the application forms should have a disclosure notice.  Many don't, and
until someone takes them to court, they aren't likely to change.  (Though
New York recently agreed to start adding the notice on the basis of a letter
written by a reader of this blurb.)

The Privacy Act of 1974 (5 USC 552a) requires that any federal, state, or
local government agency that requests your Social Security Number has to
tell you four things:

1:  Whether disclosure of your Social Security Number is required or
    optional,

2:  What law authorizes them to ask for your Social Security Number,

3:  How your Social Security Number will be used if you give it to them,
    and

4:  The consequences of failure to provide an SSN.

In addition, the Act says that only Federal law can make use of the Social
Security Number mandatory.  So anytime you're dealing with a government
institution and you're asked for your Social Security Number, just look for
the Privacy Act Statement.  If there isn't one, complain and don't give your
number.  If the statement is present, read it.  If it says giving your
Social Security Number is voluntary, you'll have to decide for yourself
whether to fill in the number.

                        Private Organizations

The guidelines for dealing with non-governmental institutions are much more
tenuous.  Most of the time private organizations that request your Social
Security Number can get by quite well without your number, and if you can
find the right person to negotiate with, they'll willingly admit it.  The
problem is finding that right person.  The person behind the counter is
often told no more than "get the customers to fill out the form completely."

Most of the time, you can convince them to use some other number.  Usually
the simplest way to refuse to give your Social Security Number is simply to
leave the appropriate space blank.  One of the times when this isn't a
strong enough statement of your desire to conceal your number is when
dealing with institutions which have direct contact with your employer.
Most employers have no policy against revealing your Social Security Number;
they apparently believe that it must have been an unintentional slip that
you didn't give out your SSN.

Public utilities (gas, electric, phone, etc.) are considered to be private
organizations under the laws regulating SSNs.  Most of the time they ask for
an SSN, and aren't prohibited from asking for it, but they'll usually relent
if you insist.  Ask to speak to a supervisor, insist that they document a
corporate policy requiring it, ask about alternatives, ask why they need it
and suggest alternatives.


      Lenders and Borrowers (those who send reports to the IRS)