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What is the point of the & / ampersand sigil for function refs?

by tphyahoo (Vicar)
on Feb 04, 2005 at 12:02 UTC ( #428024=perlquestion: print w/ replies, xml ) Need Help??
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 (Sage) 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.

    -- Randal L. Schwartz, Perl hacker
    Be sure to read my standard disclaimer if this is a reply.

      > 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!
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

      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!

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

Re: What is the point of the & sigil for function refs? (oldie)
by tye (Cardinal) on Feb 04, 2005 at 15:24 UTC
Re: What is the point of the & / ampersand sigil for function refs?
by dimar (Curate) on Feb 07, 2005 at 17:17 UTC
    Where can I learn more about this?

    A somewhat related inquiry follows at: Subroutine Bewilderment with some helpful comments.

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