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From: reino@cs.eur.nl (Reino de Boer)
Newsgroups: comp.lang.pascal
Subject: Re: TP bug???
Message-ID: <1989Dec15.080929.16028@cs.eur.nl>
Date: 15 Dec 89 08:09:29 GMT
References: <RON.89Dec13163843@clarity.Princeton.EDU> <1033@dutrun.UUCP>
Distribution: comp
Organization: Erasmus Universiteit Rotterdam, dept. CS (Informatica)
Lines: 92

winfave@dutrun.UUCP (Alexander Verbraeck) writes:

>In article <RON.89Dec13163843@clarity.Princeton.EDU> ron@clarity.Princeton.EDU (Ronald Beekelaar) writes:
>>
>>I was trying to compile the following TP code, but everytime the compiler hang
>>and even ctrl-break didn't work anymore. I had to reboot the computer.
>>
>>Unit mycolor;
>>Uses Crt;

My copy of Turbo Pascal gives (as expected) the following compiler
error: INTERFACE expected.

>>Interface
>>Function color(fore, back: byte): byte;
>>Implementation
>>Const stringcolor: byte = color(blue, red);
>>Function color(fore, back: byte): byte;
>>   begin
>>      color:= (fore and $8F) + (back and $70)
>>   end;
>>End.

The following (though syntactically correct) still generates a compiler
error (as expected by anyone who read the manual):

unit mycolor;
interface
uses  crt;
function color( fore, back : byte ) : byte;
implementation
const stringcolor : byte = color( blue, red );
{                          ^                      }
{ Compiler error: cannot evaluate this expression }
function color( fore, back : byte ) : byte;
  begin
    color := ( fore and $8F ) + ( back and $70 )
  end;
end.

>It looks like a real bug. 

It isn't.

>I tried to compile the code using TP 5.5 and
>PANG, the compiler hang. 

What strange version of Turbo Pascal are you using ?

>The strange thing is, that the line counter
>shows 0 lines compiled. When you introduce a bug, the bug is correctly
>reported and the compiler does not hang. The bug is independent of the
>name 'color'. To try it in a quick way, I used a's, b's and c's and the
>same error occurred..... strange..... 
>The following program DOES compile, but stops your computer when it is
>runned.

>program test;
>uses Crt;

>function abc:byte;
>begin
>  abc:=1;
>end;

>const sc=abc;

>begin
>  writeln;
>end.

It doesn't (and shouldn't) compile at all. I refer to pages 15--16 of
the Turbo Pascal (R) Reference Guide (Version 5.0 and 5.5):
	Since the compiler has to be able to completely evaluate a constant
	expression at compile time, the following constructs are {\it not\/}
	allowed in constant expressions:
	.
	.
	o  function calls (except those noted in the following text)
	.
	.
	The following standard functions are allowed in constant
	expressions:
	abs, ...., trunc
and as expected, no user defined functions here.

Reino

-- 
Reino R. A. de Boer
Erasmus University Rotterdam ( Informatica )
e-mail: reino@cs.eur.nl