Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!usc!elroy.jpl.nasa.gov!jpl-devvax!lwall
From: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall)
Newsgroups: comp.lang.perl
Subject: Re: perl bug with $1, affected by subroutine call
Message-ID: <7298@jpl-devvax.JPL.NASA.GOV>
Date: 6 Mar 90 22:43:13 GMT
References: <JBW.90Mar5215526@bucsf.bu.edu> <7291@jpl-devvax.JPL.NASA.GOV> <JBW.90Mar6151520@bucsf.bu.edu>
Reply-To: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall)
Organization: Jet Propulsion Laboratory, Pasadena, CA
Lines: 62

In article <JBW.90Mar6151520@bucsf.bu.edu> jbw@bucsf.bu.edu (Joe Wells) writes:
: Huh?  Why is "&sub1" in "&one(&sub1)" being evaluated in an array context?
: I intended it to be the single scalar argument to "&one".

You can intend all you want--the single item passed to a subroutine call is
a LIST, which can have any number of arguments.  If you want a version
of Perl that treats 1 argument differently from 0 arguments or 2 arguments,
you can go back to Perl version 1.0, which made the distinction.  It was a
headache.  Perl now believes that lists can have 1 element.

:    Since the "if" is the last statement of the sub block, it has to be
:    evaluated in the context that the subroutine was called.
: 
: Huh?  Does this mean that an "if" statement has a value, unlike in C?  If
: so, is there a precise specification of the value of an "if" statement?  I
: can't find that in the manual.

If you look up subroutines you find:

     do SUBROUTINE (LIST)
             Executes a SUBROUTINE declared by a sub declaration,
             and returns the value of the last expression
             evaluated in SUBROUTINE.

What was the last expression evaluated in your subroutine?

: If an "if (EXPR) BLOCK" statement is evaluated in a certain context, does
: that mean that EXPR is always evaluated in the same context?  It seems
: that EXPR should always be evaluated in a scalar context, and the value of
: the "if" statement should be the value of BLOCK (evaluated in the
: appropriate context).

That's precisely what I was suggesting as a possible remedy in my article.

:    So the /(x)/ has it's array context behavior.
: 
: Question about this.  Consider this statement:
: 
:     @a = /xxxxxxx/;
: 
: What is the value of @a if the match succeeds?  (), right?
: 
: What is the value of @a if the match fails?  (), right?

At this point the documentation lies slightly.  It implies that
you'd get a null array either way.  In fact, it only pays attention
to the array context if there are any parens or if it fails, so you'd get
an array with a value of (1) if it matches, and () if it doesn't.
Kind weird.

I will fix the man page.  Thanks.

chdir '/usr/spool/news/comp/lang/perl' || die "Not a \"normal\" news server.\n";
sub bynum {$a - $b;} $/ = '\0'; $* = 1; $| = 1;
opendir(DIR,'.'); @arts = sort bynum readdir(DIR);
srand(time); print "Not ";
while ($art = $arts[rand @arts]) {
    open art; $_ = <art>; next unless s/\n-- *\n.*Randal [^\0]*/\n/;
    s/[^\0]*\n\n//; next unless $_; eval $_; last unless $@;
}

Larry