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Hash (not) returned by subroutine

by Anonymous Monk
on Jan 27, 2020 at 09:49 UTC ( #11111922=perlquestion: print w/replies, xml ) Need Help??

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

Hello,
I did not expect that $result in the code below is set, since it is not returned by the called subroutine. Why it happens?
Thanks.

$ cat test.pl use strict; use lib ('.'); use Test qw (get_data); use Data::Dumper; my $result = get_data(); print Dumper $result; $cat Test.pm package Test; use strict; require Exporter; our @ISA = qw(Exporter); our @EXPORT = qw(); our @EXPORT_OK = qw( get_data ); #------------- sub get_data { my $data = {a=>'b'}; } 1; $ perl test.pl $VAR1 = { 'a' => 'b' };

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Re: Hash (not) returned by subroutine
by soonix (Canon) on Jan 27, 2020 at 09:58 UTC
    perldoc return says:
    (In the absence of an explicit return, a subroutine, eval, or do FILE automatically returns the value of the last expression evaluated.)
    … which here is the value of the assignment statement (i.e. the value assigned last)
Re: Hash (not) returned by subroutine
by tobyink (Canon) on Jan 27, 2020 at 12:06 UTC

    Further to soonix's answer, if you wish to not return anything from a function, use:

    sub get_data { ...; return; # undef in scalar context or the empty list in list conte +xt }

    or:

    sub get_data { ...; return undef; # undef in scalar context and a one-item list conta +ining undef in list context }

    Usually the former is preferred.

    For object-oriented modules, if you don't have anything useful to return for a method, return $self is a good idea because it allows method calls to be easily chained, like:

    sub enable_warnings { my $self = shift; $self->{warnings} = 1; return $self; } sub enable_errors { my $self = shift; $self->{errors} = 1; return $self; } sub disable_warnings { my $self = shift; $self->{warnings} = 0; return $self; } sub disable_errors { my $self = shift; $self->{errors} = 0; return $self; } sub run_process { my $self = shift; ...; return $result; } # Now instead of doing this: $widget->enable_warnings; $widget->enable_errors; my $result = $widget->run_process; # We can do this: my $result = $widget->enable_warnings->enable_errors->run_process;

    A final note. Doing this:

    sub bleh { return $result; }

    Is actually very slightly slower than:

    sub bleh { $result; }

    So if you have a small function that gets called a lot and you want to optimize, removing return and just allowing the last value evaluated to fall through as the return value may give you a slight boost.

      > may give you a slight boost

      Note that perl5200delta says:

      In certain situations, when return is the last statement in a subroutine's main scope, it will be optimized out.

      map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]
        "...certain situations...return...optimized out"

        Doesn’t such a method make the behavior of a programming language unpredictable? Regards, Karl

        «The Crux of the Biscuit is the Apostrophe»

        perl -MCrypt::CBC -E 'say Crypt::CBC->new(-key=>'kgb',-cipher=>"Blowfish")->decrypt_hex($ENV{KARL});'Help

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