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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 resed by
the others.
AUTHOR
Paul Evans <leonerd@leonerd.org.uk>
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