I've got a pretty good implementation of multimethods on CPAN. I'm biased, but I think it's the best underlying implementation, though I've done almost zero work on making the syntax nice.

use strict;
use warnings;
use experimental qw( signatures builtin );
use feature qw( say );
use builtin qw( true false );
use Sub::MultiMethod -all, -lexical;
use Types::Common -types, -lexical;

package Foo { use Class::Tiny };
package Bar { use Class::Tiny };

multifunction frobnicate => (
  positional => [ InstanceOf['Foo'] ],
  code       => sub ( $o ) { return 'It was a Foo.' },
);

multifunction frobnicate => (
  positional => [ InstanceOf['Bar'] ],
  code       => sub ( $o ) { return 'It was a Bar.' },
);

multifunction frobnicate => (
  positional => [ HashRef, ArrayRef ],
  code       => sub ( $h, $a ) { return 'They were a hashref and array
+ref.' },
);

multifunction frobnicate => (
  named      => [ message => Str, truth => Bool ],
  code       => sub ( $arg ) { $arg->truth ? $arg->message : 'Truth wa
+s false' },
);

say frobnicate( Foo->new );
say frobnicate( Bar->new );
say frobnicate( {}, [] );

say frobnicate(   message => "This will be seen",     truth => true   
+ );
say frobnicate(   message => "This will NOT be seen", truth => false  
+ );

say frobnicate( { message => "This will be seen",     truth => true  }
+ );
say frobnicate( { message => "This will NOT be seen", truth => false }
+ );

say frobnicate( [], undef, 123 );  # dies; no implementation available
[download]

Thank you! I saw it already. But for unknown reasons, I have not managed it to pass [ InstanceOf['Foo'] ] to positional 🤪😎

«The Crux of the Biscuit is the Apostrophe»

As expected, frobnicate cannot be imported - "there can be only one".

I don't understand this sentence, but I also don't see anything in Methods.pm that would allow it to be exported. What happens if you use Exporter to make frobnicate exportable? A brief look at the implementation of multimethods suggests it should work fine.

When I call it with Exporter like this:

package Methods {
    use strict;
    use warnings;
    use feature qw(isa);
    use Logic::Easy;
    use Exporter qw(import)
    no warnings qw(redefine);
    our @EXPORT_OK = qw(frobnicate);

    sub frobnicate : Multi(frobnicate) {
        SIG [$o] where {$o isa 'Foo'};
    qq(foo);
    }

    sub frobnicate : Multi(frobnicate) {
        SIG [$o] where {$o isa 'Bar'};
    qq(bar);
    }
    1;
}

__END__
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#!/usr/bin/env perl

use strict;
use warnings;
use lib q(.);
use Bar;
use Foo;
use Methods qw(frobnicate);
use feature qw(say);

my $gizmo = Foo->new();
my $mojo  = Bar->new();

# dd $gizmo, $mojo;

say frobnicate($_) for
    ($gizmo, $mojo);

__END__
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I get:

karl@h3002993:~/src/perl/mmdemo$ ./run.pl
Logic::Easy control failed at Methods.pm line 16.
 at Methods.pm line 16. </p>
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The same result with say frobnicate($gizmo);. With say frobnicate($mojo); i get bar.

With the loop and Methods::frobnicate($_) i get the expected result.

«The Crux of the Biscuit is the Apostrophe»

Or are you asking how to implement SIG ?

edit

after looking into the source of Logic::Easy (which has no POD) it seems that SIG is implemented by source filter.

I can't see any mention of multi-methods there ¹ but since Perl doesn't do any static parameter checks when calling a sub², you'll have to construct one single sub frobnicate() which checks the arguments against patterns and delegates via dispatch table to specific sub implementations.

Maybe message the author?

HTH! :)

updates

1) well I didn't spend much more guessing effort into this undocumented 0.02 version from MAR 26, 2005

2) well, it does prototype checks when compiling, but that's another thing

Multimethods of Logic

«The Crux of the Biscuit is the Apostrophe»

1. Logic
2. No¹⁾

I just want to write frobnicate($object) and not Methods::frobnicate($object). With use Exporter qw(import); our @EXPORT_OK =qw(frobnicate); and use Methods qw(frobnicate); only one frobnicate is found²⁾ and with Methods->frobnicate() nothings happens.

¹⁾ I could read the sources for this (could take a while ;-).
²⁾ This yields "Logic::Easy multi dispatch failed" from Logic::Easy line 94.

«The Crux of the Biscuit is the Apostrophe»