Future Workshop/Future Workshop.iso/utility/cenvid/cmm_vs_c.doc

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                    CEnvi Shareware Manual, Chapter 3:
                      Cmm versus C, for C Programmers

                     CEnvi unregistered version 1.009
                             29 November 1994

                       CEnvi Shareware User's Manual

          Copyright 1993, Nombas, All Rights Reserved.
          Published by Nombas, PO BOX 875, MEDFORD MA 02155-0007 USA
          [phone]

          Thank you for trying this shareware version of CEnvi from Nombas,
          a member of the Association of Shareware Professionals (ASP).

3.  Cmm versus C: The Cmm language for C programmers


          This chapter is for those who already know how to program in the
          C language.  This chapter describes only those elements of Cmm
          that differ from standard C, and so if you don't already
          understand C, then this shouldn't have much meaning for you.
          Non-C programmers should instead look at the previous chapter.

          Since it is assumed that readers of this chapter are already
          knowledgeable in C, only those aspects of Cmm that differ from C
          are described here.  If it's not mentioned here, then assume that
          Cmm behavior will be standard C.

          Deviations from C are a result of these two harmonious Cmm
          directives: Convenience and Safety.  Cmm is different from C
          where the change makes Cmm more convenient for small programs,
          command-line code, or scripting files, or if unaltered C rules
          encourage coding that is potentially unsafe.

3.1.  C Minus Minus

          Cmm is "C minus minus" where the minuses are Type Declarations
          and Pointers.  If you already know C and can remember to forget
          these two aspects of C (I repeat, no Type Declarations and no
          Pointers) then you know Cmm.  If you were to take C code, and
          delete all the lines, code-words, and symbols that either declare
          data types or explicitly point to data, then you would be left
          with Cmm code; and although you would be removing bytes of source
          code, you would not be removing capabilities.

          All of the details below that compare Cmm against C follow from
          the general rule:
            *Cmm is C minus Type Declarations and minus Pointers.

3.2.  Data Types

          The only recognized data types are Float, Long, and Byte.  The
          words "Float", "Long", and "Byte" do not appear in Cmm source
          code; instead, the data types are determined by context.  For
          instance 6 is a Long, 6.6 is a Float, and '6' is a Byte.  Byte is
          unsigned, and the other types are signed.

3.3.  Automatic Type Declaration

          There are no type declarators and no type casting.  Types are
          determined from context.  If the code says "i=6" then i is a
          Long, unless a previous statement has indicated otherwise.

          For instance, this C code:
              int Max(int a,int b)
              {
                int result;
                result = ( a < b ) ? b : a ;
                return result;
              }
          could become this Cmm code:
              Max(a,b)
              {
                result = ( a < b ) ? b : a ;
                return result;
              }

3.4.  Array Representation

          Arrays are used in Cmm much like they are in C, except that they
          are stored differently: a first-order array (e.g., a string) is
          stored in consecutive bytes in memory, but arrays of arrays are
          not in consecutive memory locations.  The C declaration "char
          c[3][3];" would state that there are nine consecutive bytes in
          memory.  In Cmm a similar statement such as "c[2][2] = 'A'" would
          tell you that there are (at least) three arrays of characters,
          and the third array of arrays has (at least) three characters in
          it; and although the characters in c[0] are in consecutive bytes,
          and the characters in c[1] are in consecutive bytes, the two
          arrays c[0] and c[1] are not necessarily adjacent in memory.

3.4.1   Array Arithmetic

          When one array is assigned to the other, as in:
              foo = "cat";
              goo = foo;
          then both variables define the same array and start at the same
          offset 0.  In this case, if foo[2] is changed then you will find
          that goo[2] has also been changed.

          Integer addition and subtraction can also be performed on arrays.
          Array addition or subtraction sets where the array is based.  By
          altering the previous code segment to:
              foo = "cat";
              goo = foo + 1;
          goo and foo would now be arrays containing the same data, except
          that now goo is based one element further, and foo[2] is now the
          same data as goo[1].

          To demonstrate:
              foo = "cat";  // foo[0] is 'c', foo[1] = 'a'
              goo = foo + 1;// goo[0] is 'a', goo[-1] = 'c'
              goo[0] = 'u'; // goo[0] is 'u', foo[1] = 'u', foo is "cut"
              goo++;        // goo[0] is 't', goo[-2] = 'c'
              goo[-2] = 'b' // goo[0] is 't', foo[0] = 'b', foo is "but"

3.4.2   Automatic Array Allocation

          Arrays are dynamic, and any index, (positive or negative) into an
          array is always valid.  If an element of an array is referred to,
          then the Cmm must see to it that such an element will exist.  For
          instance if the first statement in the Cmm source code is "foo[4]
          = 7;" then the Cmm interpreter will make an array of 5 integers
          referred to by the variable foo.  If a statement further on
          refers to "foo[6]" then the Cmm interpreter will grow foo, if it
          has to, to ensure that the element foo[6] exists.  This works
          with negative indices as well.  When you refer to foo[-10], then
          foo is grown in the other direction if it needs to be, but foo[4]
          will still refer to that "7" you put there earlier.  Arrays can
          reach any dimension order, so that foo[6][7][34][-1][4] is a
          valid value.

3.5.  Structures

          Structures are created dynamically, and their elements are not
          necessarily contiguous in memory.  When CEnvi comes across the
          statement 'foo.animal = "dog"' it creates a structure element of
          foo that is referred to as "animal" and is an array of
          characters, and this "animal" variable is thereafter carried
          around with "foo" (much like a stem variable in REXX).  The
          resulting code looks very much like regular C code, except that
          there is not a separate structure definition anywhere.









          This C code:

              struct Point {
                int Row;
                int Column;
              };

              struct Square {
                struct Point BottomLeft;
                struct Point TopRight;
              };

              void main()
              {
                struct Square sq;
                int Area;
                sq.BottomLeft.Row = 1;
                sq.BottomLeft.Column = 15;
                sq.TopRight.Row = 82;
                sq.TopRight.Column = 120;
                Area = AreaOfASquare(sq);
              }

              int AreaOfASquare(struct Square s)
              {
                int width, height;
                width = s.TopRight.Column - s.BottomLeft.Column + 1;
                height = s.TopRight.Row - s.BottomLeft.Row + 1;
                return( length * height );
              }

          can be changed into the equivalent Cmm code simply be removing
          declaration lines, resulting in:

              main()
              {
                sq.BottomLeft.Row = 1;
                sq.BottomLeft.Column = 15;
                sq.TopRight.Row = 82;
                sq.TopRight.Column = 120;
                Area = AreaOfASquare(sq);
              }

              int AreaOfASquare(s)
              {
                width = s.TopRight.Column - s.BottomLeft.Column + 1;
                height = s.TopRight.Row - s.BottomLeft.Row + 1;
                return( length * height );
              }

          Structures can be passed, returned, and modified just as any
          other variable.  Of course structures and arrays are independent,
          so you could very well have the statement "foo[8].animal.forge[3]
          = bil.bo".

3.6.  Passing Variables by Reference

          By default, LValues in Cmm are passed to functions by reference,
          and so if the function alters a variable then the variable in the
          calling function is altered as well IF IT IS AN LVALUE.  So
          instead of this C code:

              main() {
                .
                .
                .
                CQuadrupleInPlace(&i);
                .
                .
                .
              }

              void CQuadrupleInPlace(int *j)
              {
                *j *= 4;
              }

          the Cmm version would be:

              main() {
                .
                .
                .
                QuadrupleInPlace(i);
                .
                .
                .
              }

              void QuadrupleInPlace(j)
              {
                j *= 4;
              }

          If the rare circumstance arises that you want to pass a copy of
          an LValue to a function, instead of passing the variable by
          reference, then you can use the Cmm "copy-of" operator "=".
          foo(=i) can be interpreted as saying "pass a value equal to i,
          but not i itself"; so that "foo(=i) ... foo(j) { j *= 4; }" would
          not change the value at i.

          Note that for this Cmm version, the following calls to
          QuadrupleInPlace() would be valid, but no value will have changed
          upon return from QuadrupleInPlace() because an LValue is not
          being passed:
              QuadrupleInPlace(8);
              QuadrupleInPlace(i+1);
              QuadrupleInPlace(=1);

3.7.  Data Pointers(*) and Addresses(&)

          No pointers.  None.  The "*" symbol NEVER means "pointer" and the
          "&" symbol never means "address".  This may at first cause
          seasoned C programmers to gasp in disbelief, but it turns out to
          be not such a big deal, and these two operators are seldom
          missed, after considering these two rules:
              1) "*var" can be replaced in all instances by "var[0]"
              2) variables (if LValues) are passed by reference

3.8.  Case Statements

          Case statements in a switch statement may be a constant, a
          variable, or any other statement that can be evaluated to a
          variable.  So you might see this Cmm code:

              switch(i) {
                case 4:
                case foe():
                case sqrt(foe()):
                case (PILLBOX * 3 - 2):
                default:
              }

3.9.  Initialization: Code external to functions

          All code that is not inside any function block is interpreted
          before main() is called.  So the following Cmm code:

              printf("hey there ");

              main()
              {
                printf("ho there ");
              }

              printf("hi there ");

          would result in the output "hey there hi there ho there ".

3.10. Unnecessary tokens

          If symbols are redundant, then they are usually unnecessary in
          Cmm.  This allows for more flexibility in the non-C-trained and
          also lets more code get in the tiny space available on the
          command line.  Besides, I got tired of my compiler saying
          "missing semi-colon"; What good is a semi-colon if it doesn't
          tell the compiler anything new?  So you can have the statement
          "foo()" as well as "foo();".  It certainly doesn't hurt to have
          the semi-colon there, especially when it can clarify a "return;"
          statement, for example, but it isn't necessary.  Similarly, "("
          and ")" are often unnecessary, so you may have "while a < b a++"
          as a complete statement.

3.11. #include

          The #include statement has been enhanced for reading source
          snippets from within other types of files.  So we have

              #Include <filespec,Extension,Prefix,HeaderLine,FooterLine>

          where filespec is the same as in C's #include <filespec>
          statement, Extension is a file extension (such as BAT) that may
          be added to the filespec (so batch files can say #include
          <%0,bat>", Prefix specifies that only those lines in filespec
          that begin with Prefix will be source, and HeaderLine and
          FooterLine specify that source will be read only from sections of
          filespec between HeaderLine and FooterLine.  If a full path is
          not specified then CEnvi searches for the file in various paths
          in this order:
            *Search the current directory.
            *If the code is run from a *.cmm source file, then search in
              the source directory for the *.cmm file.
            *If this is the Windows version of CEnvi, searches all the
              files in the CMMPATH profile value (from WIN.INI in the
              [CEnvi] section).
            *Search all directories in the CMMPATH environment variable.
            *Search the directory that CEnvi.exe is executed from.
            *Search all directories from the PATH environment variable.

          In CEnvi a file will not be included more than once, and so if it
          has already been included, a second (or third) #include statement
          will have no effect.

3.12. Macros

          Function macros are not supported.  Since speed is not of primary
          importance, a macro gains little over a function call, and so any
          macros may simply become functions.

          Token replacement macros ("#define NULL 0") are supported in Cmm.

3.13. Back-quote strings

          The back-quote character (`), also known as a "back-tick" or
          "grave accent", can be used in Cmm in place of a double quote (")
          to specify a string where escape sequences are not to be
          translated.  So, for example, here are two ways to describe the
          same file name:
              "c:\\autoexec.bat"  // traditional method
              `c:\autoexec.bat`   // alternative method

3.14. Converting existing C code to Cmm

          Converting existing C code to Cmm, should you choose to do so, is
          mostly a process of deleting unnecessary text.  You search on
          type declarations, such as "int", "float", "struct", "char",
          "[]", etc... and then delete these declaration strings.  For
          instance, these instances of C code (or C++ code) on the left can
          be replaced by the Cmm code on the right:

                   C                               Cmm
              ----------                      -------------

              int i;    ................... i (or nothing at all)

              int foo = 3; ................ foo = 3;

              struct {    ................... /* nothing */
                int row;
                int col;
              }

              char name[] = "George"; ..... name = "George";

              int go(int a,char *s,int &c)    go(a,s,c)
              int zoo[] = { 1, 2, 3 }; .... zoo = { 1, 2, 3 };

          The next step in converting C to Cmm is to search for the address
          and pointer operators ("*", "&").  If the '&' and '*' work
          together so that the address of a variable is passed to a
          function, then both of these operators become unnecessary because
          Cmm passes lvars by reference.  If there are still "*" found then
          they are usually referring to the zeroth value of a pointer
          address, and so can be replaced with [0], as in "*foo = 4"
          replaced by "foo[0] = 4".  Finally, the "->" operator for
          structures must be replaced by "." either because the structure
          is now being passed by referenced or because the element of the
          structure is being referred to by its array index: "foo->row" may
          need to become "foo[0].row".