Syntax::Keyword::MultiSub(3pm)

NAME
"Syntax::Keyword::MultiSub" - multiple dispatch on subroutines

SYNOPSIS
   use v5.26;
   use Syntax::Keyword::MultiSub;
   use experimental 'signatures';

   multi sub max()          { return undef; }
   multi sub max($x)        { return $x; }
   multi sub max($x, @more) { my $y = max(@more);
                              return $x > $y ? $x : $y; }

   say max(1, 2, 15, 3, 4);  # prints 15

DESCRIPTION
This module provides a new keyword, "multi", to put before subroutine declarations, which permits multiple distinct function bodies to be provided, which take different parameters. A call to a "multi sub" will invoke whichever function body best fits the arguments passed.

Currently this module can only make dispatching decisions based on the number of arguments as compared to the number of signature parameters each body was expecting. It requires perl version 5.26 or above, in order to get enough support from signatures. Note also enabling this module does not enable the "signatures" feature; you must do that independently.

KEYWORDS

  multi
   multi sub NAME (SIGNATURE) { BODY... }

Declares an alternative for the "multi sub" of the given name. Each alternative will be distinguished by the number of parameters its signature declares. If the signature includes optional parameters, this alternative is considered to cover the entire range from none to all of the optional ones being present. The ranges of parameter count covered by every alternative to a given function name must be non-overlapping; it is a compiletime error for two function bodies to claim the same number of parameters.

Each of the non-final alternatives for any given name must use only scalar parameters (though some may be optional); but as a special-case, the final alternative may end in a slurpy parameter (either an array or a hash). If this is the case then it will be considered for dispatch if none of the previous alternatives match, as long as it has at least the minimum number of required parameters present.

WITH OTHER MODULES

  Future::AsyncAwait
As of Future::AsyncAwait version 0.55 a cross-module integration test asserts that the "multi" modifier can be applied to "async sub".

   use Future::AsyncAwait;
   use Syntax::Keyword::MultiSub;

   async multi sub f () { return "nothing"; }
   async multi sub f ($key) { return await get_thing($key); }

TODO

•   Much better error checking and diagnostics for function bodies that don't use signatures.

•   Cross-module testing with Object::Pad (for "multi method"). This may require a better combined implementation, to be aware of method resolution order, inheritence, etc...

•   An eventual consideration of type assertions or value testing, as well as simple argument count.

    This particular task is likely to be a large undertaking as it spans several other areas of language. As well as types on parameters, it would be nice to put them on lexical variables, object slots, "match/case" comparisons, and so on. It would be a shame to invent a special mechanism for one of these areas that could not be reüsed by the others.

AUTHOR
Paul Evans <leonerd@leonerd.org.uk>