Docu/Tut-B.odc

Appendix B: Differences between Pascal and Component Pascal

Eliminated Features

 Subrange types
Use a standard integer type instead.

 Enumeration types 
Use integer constants instead.

 Arbitrary array ranges
Arrays are always defined over the integer range 0..max-1.

Example
	A = ARRAY 16 OF INTEGER	(* legal indices are in the range 0..15 *)

 No general sets
Type SET denotes the integer set which may include the elements 0..31.

 No explicit DISPOSE
Memory is reclaimed automatically by the garbage collector. Instead of calling DISPOSE, simply set the variable to NIL.
	
 No variant records
Use record extension instead.

 No packed structures
 Use SHORTCHAR or BYTE types for byte-sized values.

 No GOTO

 No PRED and SUCC standard functions
Use DEC or INC on integer values instead.

 No built-in input/output facilities
No file types. I/O is provided by library routines.


Changed Features

 Standard procedure ENTIER instead of ROUND

 Syntax for REAL constants
3.0E+4 but not 3.0e+4

 Syntax for pointer type declarations
P = POINTER TO R
instead of
P = ^R

 Syntax for case statement
"|" instead of ";" as case separator.
ELSE clause.

Example
	CASE i * 3 - 1 OF
	  0: StdLog.String("zero")
	| 1..9: StdLog.String("one to nine")
	| 10, 20: StdLog.String("ten or twenty")
	ELSE StdLog.String("something else")
	END

 Procedure name must be repeated

Example
	PROCEDURE DrawDot (x, y: INTEGER);
	BEGIN
	END DrawDot;

 Case is significant
Small letters are distinguished from capital letters.

Example	"proc" is not the same as "Proc".

 String syntax
String literals are either enclosed between " or between '. There cannot be both single and double quotes in one string. String literals of length one are assignment-compatible to character variables.

Examples
	"That's great"	'Write "hello world" to the screen'
	ch := "x"
	ch := 'x'

 Comments
Comments are enclosed between (* and *) and may be nested.

 Set brackets
Set constants are given between { and } instead of [ and ].

Example	{0..2, 4, j..2 * k}

 Function syntax
Use keyword PROCEDURE for functions also, instead of FUNCTION.
Procedures with a return value always have a (possibly empty) parameter list in their declarations and in calls to them.
The function result is returned explicitly by a RETURN statement, instead of an assignment to the function name.

Example

	PROCEDURE Fun (): INTEGER;
	BEGIN
		RETURN 5
	END Fun;

	 instead of

	FUNCTION Fun: INTEGER;
	BEGIN
		Fun := 5
	END;

	n := Fun()   instead of   n := Fun

 Declarations
The sequence of declarations is
{ ConstDecl | TypeDecl | VarDecl} {ProcDecl | ForwardDecl}
instead of
[ConstDecl] [TypeDecl] [VarDecl] {ProcDecl}.

Forward declarations are necessary if a procedure is used before it is defined.

Example
	PROCEDURE ^ Proc;
	instead of
	PROCEDURE Proc; FORWARD;

 Procedure types
Procedures may not only be passed to parameters, but also to procedure-typed variables.

Example
	TYPE P = PROCEDURE (x, y: INTEGER);
	VAR v: P;
	v := DrawDot;	(* assign *)
	v(3, 5);	(* call DrawDot(3, 5) *)

 Explicit END instead of compound statement
BEGIN can only occur before a statement sequence, but not in it. IF, WHILE, and LOOP are always terminated by END.

 WITH statement
A WITH statement is a regional type guard, it does not imply a hidden variable and does not open a new scope.
See language reference for more details.

 ELSIF
IF statements can have several branches.

Example
	IF name = "top" THEN
		StdLog.Int(0)
	ELSIF name = "bottom" THEN
		StdLog.Int(1)
	ELSIF name = " charm" THEN
		StdLog.Int(2)
	ELSIF name = "beauty" THEN
		StdLog.Int(3)
	ELSE
		StdLog.String("strange")
	END

 BY instead of only DOWNTO in FOR
FOR loops may use any constant value as increment or decrement.

Example
	FOR i := 15 TO 0 BY -1 DO StdLog.Int(i, 0) END

 Boolean expressions use short-circuit evaluation
A Boolean expression terminates as soon as its result can be determined.

Example
	The following expression does not cause a run-time error when p = NIL:
	IF (p # NIL) & (p.name = "quark") THEN

 Constant expressions
In constant declarations, not only literals, but also constant expressions are allowed.

	Example
		CONST
			zero = ORD("0");
			one = zero + 1;

 Different operators
# is used instead of <> for inequality test.
& is used instead of AND for logical conjunctions.
~ is used instead of NOT for logical negation.

 Explicit conversion to included type with SHORT
Type inclusion for numeric types allows to assign values of an included type to an including type. To assign in the other direction, the standard procedure SHORT must be used.

Example
	int := shortint;
	shortint := SHORT(int)


New Features

 Hexadecimal numbers and characters

Example
	100H	(* decimal 256 *)
	0DX	(* carriage return *)

 Additional numeric types
LONGINT, SHORTINT, BYTE, SHORTREAL have been added.

 Symmetric set difference
Sets can be subtracted.

 New standard procedures
The new standard procedures INC, DEC, INCL, EXCL, SIZE, ASH, HALT, ASSERT, LEN, LSH, MAX, MIN, BITS, CAP, ENTIER, LONG and SHORT have been added.

 LOOP with EXIT
There is a new loop statement with an explicit exit statement. See language report for more details.

 ARRAY OF CHAR can be compared
Character arrays can be compared with the =, #, <, >, <=, and >= operators.

 Open arrays, multidimensional arrays
Arrays without predefined sizes can be defined, possibly with several dimensions.

Examples
	VAR a: POINTER TO ARRAY OF CHAR;
	NEW(a, 16)

	PROCEDURE ScalarProduct (a, b: ARRAY OF REAL; VAR c: ARRAY OF REAL);

	TYPE Matrix = ARRAY OF ARRAY OF REAL;
	PROCEDURE VectorProduct (a, b: ARRAY OF REAL; VAR c: Matrix);

 Pointer dereferencing is optional
The dereferencing operator ^ can be omitted.

Example
	root.next.value := 5
	instead of
	root^.next^.value := 5

 Modules
Modules are the units of compilation, of information hiding, and of loading. Information hiding is one of the main features of object-oriented programming. Various levels of information hiding are possible: complete hiding, read-only / implement-only export, full export.
See language report for more details.

 Type extension
Record types (pointer types) can be extended, thus providing for polymorphism. Polymorphism is one of the main features of object-oriented programming.

 Methods
Procedures can be bound to record types (pointer types), thus providing late binding. Late binding is one of the main features of object-oriented programming. Such procedures are also called methods.

 String operator
The string (sequence of characters) that is contained in an array of character can be selected by using the $-selector.

 Record attributes
Records are non-extensible by default, but may be marked as EXTENSIBLE, ABSTRACT, or LIMITED.

 Method attributes
Methods are non-extensible by default, but may be marked as EXTENSIBLE, ABSTRACT, or EMTPY. Newly introduced methods must be marked as NEW.