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From: Kasper Dupont <kasperd@cs.au.dk>
Newsgroups: comp.lang.beta
Subject: Re: Virtual attribute gives Seg Fault when accessed from pattern ref
Date: Mon, 03 May 1999 10:01:33 +0200
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Andrew Klaassen wrote:
> 
> Greetings:
> 
> I don't have a great handle on all of BETA's 'big words' (it
> took me half an hour just to find the words I needed for my
> subject line <G>), so I'll have to explain my question with
> code. The first program below compiles and runs. The second
> program compiles, but crashed with a 'Segmentation Fault' at
> the indicated point.  Why?
> 
> The first program:
> 
> ORIGIN '~beta/basiclib/v1.6/betaenv';
> ---program:descriptor---
> (* This program compiles and runs. *)
> (#
> 
> Function: (* a simple pattern *)
>   (# a: @integer;
>      Printname: (# do 'Function' -> putline; #);
>     enter a
>     do
>       a -> putint; newline;
>   #);
> 
> PatRef: ##Function;
> 
> do
> 
>   Function## -> PatRef##;
>   3 -> PatRef;
>   PatRef.Printname;
> 
> #)
> 
> The second program, which is very similar to the first:
> 
> ORIGIN '~beta/basiclib/v1.6/betaenv';
> ---program:descriptor---
> (* compiles but crashes *)
> (#
> 
> Function:
>   (# a: @integer;
>      (* Printname is now a virtual pattern *)
>      Printname:< (# do inner #);
>     enter a
>     do
>       a -> putint; newline;
>   #);
> 
> (* And we have a subpattern of Function, F1, which does
>    a final binding on Printname. *)
> F1: Function (# Printname:: (# do 'Function' -> putline; #); #);
> 
> PatRef: ##Function;
> 
> do
> 
>   F1## -> PatRef##;
>   3 -> PatRef; (* works *)
>   PatRef.Printname; (* crashed the program with a
>                        'Segmentation Fault' *)
> 
> #)
> 
> I'm not very familiar with the use of ## - there aren't many
> references to it in the BETA Language Intro, Tutorial, FAQ, etc -
> so I might be using it incorrectly.  If anyone could explain
> what's happening when the second program tries to execute
> PatRef.Printname, it would be much appreciated.
> 
> Thanks!
> 
> -----------== Posted via Deja News, The Discussion Network ==----------
> http://www.dejanews.com/       Search, Read, Discuss, or Start Your Own

You can not use an attribute on a
pattern directly, you must first
create an object from the pattern
before you can use the attribute.

Both your examples should get an
"An object is expected here" error
at compilation. To me it looks like
a bug in the compiler.


(**** should not compile ****)
PatRef.Printname;


(**** this should work ****)
(# TempObject: ^Function;
do
   &PatRef[]->TempObject[];
   TempObject.Printname;
#);


/KD