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tphyahoo has asked for the
wisdom of the Perl Monks concerning the following question:
In the following code, I can call a module function using &, or not using &. Is there ever a scenario when it's preferrable to use the &? Where can I learn more about this?
#!perl
#test.pl
use strict;
use warnings;
#custom modules
use lib 'E:/data/entwicklungsArena/perl/development/modules';
#Works
use Misc::Logger;
print 'without &: ';
Misc::Logger::logMessage("test");
#Also works:
print 'with &: ';
&Misc::Logger::logMessage("test");
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Re: What is the point of the & sigil for function refs?
by merlyn (Cardinal) on Feb 04, 2005 at 12:14 UTC |
The & prefix unambiguously means "this is a user-defined subroutine call coming up next". You can omit it when you are using parens after your function name and your function name doesn't conflict with a built-in, or if you've declared/defined your function before you use it and the name doesn't conflict with a built-in.
The & prefix also disables prototype checking, but you shouldn't be using prototypes in the first place, so I consider that a very minor point, but if I didn't mention it, someone else would surely raise that flag. {grin}
In our Learning Perl book, we suggest that you always use the ampersand when you're first learning Perl, because otherwise you'll
be a bit befuddled when the following code doesn't work:
sub log {
print STDERR "logging: ", @_, "\n";
}
...
log("starting");
...
log("finishing");
...
Yes, you're not calling your log subroutine... you're calling the built-in "logarithm" function. Oops.
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[d/l] |
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> The & prefix also disables prototype checking, but you shouldn't be using prototypes in the first place,
> so I consider that a very minor point, but if I didn't mention it, someone else would surely raise that flag. {grin}
I was going to ask for clarification here as I thought prototypes were good ju-perl. How mistaken I was, a supersearch lead me to this node Gratuitous use of Perl Prototypes. Another step on the path to enlightenment.
Thanks,
R.
Pereant, qui ante nos nostra dixerunt!
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Re: What is the point of the & sigil for function refs?
by bpphillips (Friar) on Feb 04, 2005 at 13:21 UTC |
As merlyn eluded to above, you can omit parens when you use a & in front of the function call. This has an interesting "side-effect" in that the local @_ is implicitly passed to that sub even though no arguments are explicitly passed. For instance:
foo("\@_ has elements!"); # prints "Surprise! @_ has elements!"
bar("\@_ has no elements!"); # prints "Surprise!"
sub foo {
# absent parens say pass my @_
&baz;
}
sub bar {
# parens say only pass these arguments (nothing)
&baz();
}
sub baz {
print "Surprise! ",join("\t",@_),"\n";
}
This can be handy when golfing but might cause anyone who looks at your code (including you!) some headaches...
--Brian | [reply]
[d/l]
[select] |
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the local @_ is implicitly passed
It's even the same @_. This means that &foo(@_) and &foo; won't behave the same; see perlfaq7, "What's the difference between calling a function as &foo and foo()?". Demonstration:
use strict;
sub foo { pop }
sub bar {
print "bar before: @_";
print '&foo : ' . &foo . " <--- I am a theif";
print "bar after : @_ <--- no c";
}
sub baz {
print "baz before: @_";
print '&foo(@_) : ' . &foo(@_);
print "baz after : @_";
}
bar(qw/ a b c /); print '';
baz(qw/ a b c /);
__END__
bar before: a b c
&foo : c <--- I am a theif
bar after : a b <--- no c
baz before: a b c
&foo(@_) : c
baz after : a b c
ihb
See perltoc if you don't know which perldoc to read!
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[d/l]
[select] |
Re: What is the point of the & sigil for function refs?
by grinder (Bishop) on Feb 04, 2005 at 13:28 UTC |
Is there ever a scenario when it's preferrable to use the &
There is one dramatic difference that occurs when you call the function without a parameter list:
#! /usr/local/bin/perl -w
use strict;
sub inner {
print " inner: $_\n" for @_;
}
sub outer {
print "outer: $_\n" for @_;
inner;
print "\n";
}
sub outer_amp {
print "outer_amp: $_\n" for @_;
&inner;
print "\n";
}
outer( qw/ vroon gukguk faba / );
outer_amp( qw/ foomp tweedle zachitty / );
__RESULTS__
outer: vroon
outer: gukguk
outer: faba
outer_amp: foomp
outer_amp: tweedle
outer_amp: zachitty
inner: foomp
inner: tweedle
inner: zachitty
When using the &inner variant, whatever remains of @_ in the calling routine is passed to the called routine. The only time it's really useful is when doing out-of-the-ordinary stuff like autoloading. In general you should avoid it. It leads to bad surprises
PS: metasyntactic function call arguments courtesy of BooK's Acme::MetaSyntactic module, the donmartin theme.
- another intruder with the mooring in the heart of the Perl
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[d/l] |
Re: What is the point of the & sigil for function refs? (oldie)
by tye (Archbishop) on Feb 04, 2005 at 15:24 UTC |
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Re: What is the point of the & / ampersand sigil for function refs?
by dimar (Curate) on Feb 07, 2005 at 17:17 UTC |
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