New-style sub strangeness

ChrisDennis has asked for the wisdom of the Perl Monks concerning the following question:

Hello Perl Monks

This is my first question here, so please don't laugh.

Can anyone explain to me what's going on in this code?

#!/usr/bin/perl
use strict;
use warnings;
sub foo ($) {
    my $x = shift;
    print "foo gets '$x'\n";
    return ($x+2, 33);
}
my @a = foo(42);   
print "a=@a\n";   
@a = sort(foo(42)); 
print "a=@a\n";  
@a = (sort foo(42));  
print "a=@a\n";    
@a = sort foo(42);  
print "a=@a\n";
[download]

which produces the following output on my Linux system with Perl 5.10.1:

foo gets '42'
a=44 33
foo gets '42'
a=33 44
a=42
a=42
[download]

If I use old-style subs (i.e. &foo instead of foo), then it produces 'a=33 44' three times, which is what I'd expect.

Comment on New-style sub strangeness Select or Download Code

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Re: New-style sub strangeness by Corion (Patriarch) on Jun 07, 2011 at 15:42 UTC
When you look at the output you get, you'll notice that `sort foo(42)` never calls `foo`. This is because Perl sees it as: `> perl -MO=Deparse -e "sub foo($); sort foo(42);" sort foo 42` [download] ... which means "sort the list with a single element '42' by using `foo` as the comparison operator". If you use the "perl4" method of calling the subroutine (via `&foo(...)`), Perl knows what you really mean, but you could also tell this to Perl by adding a comma after the `sort` invocation, to tell `sort` that it gets a parameter list instead of a sort block: `> perl -MO=Deparse -e "sub foo($){44,33}; print for sort( foo(42), );" sub foo ($) { 44, 33; } ; print $_ foreach (sort foo(42)); > perl -wle "sub foo($){44,33}; print for sort( foo(42), );" 33 44` [download]	[reply] [d/l] [select]
Re^2: New-style sub strangeness by ChrisDennis (Sexton) on Jun 07, 2011 at 16:43 UTC
Aha! The penny drops with regard to following 'sort' by the name of a subroutine. Thank you for all three answers. I didn't know about -MO=Deparse -- that could be very useful. cheers, Chris	[reply]
Re: New-style sub strangeness by choroba (Cardinal) on Jun 07, 2011 at 15:38 UTC
I'd expect it four times when using &foo, and I also get it. B::Deparse to help you understand: perl -MO=Deparse -e 'use strict; use warnings; sub foo ($) { my $x = shift; print "foo gets '$x'\n"; return ($x+2, 33); } my @a = foo(42); print "a=@a\n"; @a = sort(foo(42)); print "a=@a\n"; @a = (sort foo(42)); print "a=@a\n"; @a = sort foo(42); print "a=@a\n"; ' sub foo ($) { use warnings; use strict 'refs'; my $x = shift @_; print "foo gets \n"; return $x + 2, 33; } use warnings; use strict 'refs'; my(@a) = foo(42); print "a=@a\n"; @a = sort(foo(42)); print "a=@a\n"; @a = (sort foo 42); print "a=@a\n"; @a = (sort foo 42); print "a=@a\n"; -e syntax OK [download] In other words, `sort foo(42)` is understood as "sort the list containing 42 by sorting function foo".	[reply] [d/l] [select]
Re: New-style sub strangeness by wind (Priest) on Jun 07, 2011 at 15:40 UTC
Just read sort. It allows you to use a subroutine to set an alternative sorting method. `sub numerical { $a <=> $b } @a = sort numerical (4,33); print "a = @a\n";` [download]	[reply] [d/l]