NAME

Kavorka::Manual::MultiSubs - multi subs and multi methods

DESCRIPTION

Kavorka supports multi methods and multi subs:

multi method process (ArrayRef $x) { say "here" }
multi method process (HashRef $x) { say "there" }

__PACKAGE__->process( [] );    # here
__PACKAGE__->process( {} );    # there

Multi subs and multi methods must be named (cannot be anonymous coderefs).

This feature is shared with Perl 6 signatures, though Kavorka does not support some of Perl 6's more advanced features such as multi method prototypes. (Though method modifiers should more or less work with multi methods!) Kavorka includes both type constraints and value constraints in the dispatch decision, while Perl 6 only uses type constraints.

Multi methods versus multi subs

The word after multi (i.e. method in the above example) can be any Kavorka keyword that has been set up in the current lexical scope, provided the implementation class provides a non-undef invocation_style method (see Kavorka::Sub).

If the invocation_style is "fun" (like Kavorka::Sub::Fun), then the signature of each candidate function in package is checked in the order in which they were defined, and the first matching candidate is dispatched to.

If the invocation_style is "method" (like Kavorka::Sub::Method), then if no successful candidate is found in the current class, candidates in superclasses are also considered.

Long names

It is possible to define alternative "long names" for the candidates of a multi method or multi sub using the :long attribute:

multi fun process (ArrayRef $x) :long(process_array) {
   say "here";
}

multi fun process (HashRef $x) :long(process_hash) {
   say "there";
}

process($a);          # multi dispatch
process_array($b);    # single dispatch
process_hash($c);     # single dispatch

(Actually, :long isn't a real attribute; we just borrow the syntax. If you try to use attributes' introspection stuff, you won't find it.)

Prototypes, subroutine attributes, etc. declared on the multi subs will appear on the "long name" subs, but not the multi sub.

Definition at run time

Multi subs and multi methods are added to the symbol at run time (like methods, see "The Method Name" in Kavorka::Manual::Methods).

This means that it's possible to partly define a multi sub, call it, then further define it, before calling it again.

use Kavorka qw( multi fun );
use Try::Tiny;

multi fun plus_one (Int $x) { $x + 1 }

try {
   plus_one(41);      # 42
   plus_one(1.1);     # throws
};

multi fun plus_one (Num $x) { $x + 1 }

plus_one(41);      # 42
plus_one(1.1);     # 2.1

BUGS

Please report any bugs to http://rt.cpan.org/Dist/Display.html?Queue=Kavorka.

SEE ALSO

Kavorka::Manual, Kavorka::Manual::Signatures, Kavorka::Manual::PrototypeAndAttributes, Kavorka::Manual::Functions, Kavorka::Manual::Methods.

AUTHOR

Toby Inkster <tobyink@cpan.org>.

COPYRIGHT AND LICENCE

This software is copyright (c) 2013-2014, 2017 by Toby Inkster.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.

DISCLAIMER OF WARRANTIES

THIS PACKAGE IS PROVIDED "AS IS" AND WITHOUT ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE.

cpanm Kavorka

perl -MCPAN -e shell
install Kavorka