http://qs321.pair.com?node_id=11147265


in reply to Test if a string is a sub name

You can use exists and defined to find out: 
#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

sub My::Filters::known {}
sub My::Filters::forward;

for my $string (qw(My::Filters::known
                   My::Filters::unknown
                   My::Filters::forward)
) {
    say join "\t", $string, defined &$string, exists &$string;
}

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Output: 

My::Filters::known    1    1
My::Filters::unknown        
My::Filters::forward        1

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map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]

Replies are listed 'Best First'.
Re^2: Test if a string is a sub name
by BillKSmith (Monsignor) on Oct 06, 2022 at 15:15 UTC
    I do not understand how your expression &$string works. The documentation for exists tells us that the expression must return the name of a subroutine. It appears to me that this would only be true for your expression if $string is a symbolic reference (which is prohibited by use strict). I would expect you need string interpolation (perhaps "&$string") but this syntax throws an error that the argument of exists is not a subroutine.
    Bill
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      It seems the documentation uses the term "subroutine name" in quite a confusing way. Check the examples:
      Use of a subroutine call, rather than a subroutine name, as an argument to exists is an error. 
      exists ⊂    # OK
exists &sub();  # Error

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      So, in this context, "subroutine name" includes the ampersand.

      Considering strict, note the exceptions to strict 'refs': 

      $bar = \&{'foo'};
&$bar;

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      We are not calling the subroutine, we're just checking it exists/is defined.

      map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]
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Re^2: Test if a string is a sub name
by clscott (Friar) on Oct 05, 2022 at 22:42 UTC
    That's great. Thanks very much
    --
    Clayton
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