1Syntax::Keyword::MultiSUusbe(r3)Contributed Perl DocumenStyanttiaoxn::Keyword::MultiSub(3)
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6 "Syntax::Keyword::MultiSub" - multiple dispatch on subroutines
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9 use v5.26;
10 use Syntax::Keyword::MultiSub;
11 use experimental 'signatures';
12 multi sub max() { return undef; }
14 multi sub max($x) { return $x; }
15 multi sub max($x, @more) { my $y = max(@more);
16 return $x > $y ? $x : $y; }
17 say max(1, 2, 15, 3, 4); # prints 15
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21 This module provides a new keyword, "multi", to put before subroutine
22 declarations, which permits multiple distinct function bodies to be
23 provided, which take different parameters. A call to a "multi sub" will
24 invoke whichever function body best fits the arguments passed.
25 Currently this module can only make dispatching decisions based on the
27 number of arguments as compared to the number of signature parameters
28 each body was expecting. It requires perl version 5.26 or above, in
29 order to get enough support from signatures. Note also enabling this
30 module does not enable the "signatures" feature; you must do that
31 independently.
32
34 multi
35 multi sub NAME (SIGNATURE) { BODY... }
36 Declares an alternative for the "multi sub" of the given name. Each
38 alternative will be distinguished by the number of parameters its
39 signature declares. If the signature includes optional parameters, this
40 alternative is considered to cover the entire range from none to all of
41 the optional ones being present. The ranges of parameter count covered
42 by every alternative to a given function name must be non-overlapping;
43 it is a compiletime error for two function bodies to claim the same
44 number of parameters.
45 Each of the non-final alternatives for any given name must use only
47 scalar parameters (though some may be optional); but as a special-case,
48 the final alternative may end in a slurpy parameter (either an array or
49 a hash). If this is the case then it will be considered for dispatch if
50 none of the previous alternatives match, as long as it has at least the
51 minimum number of required parameters present.
52
54 Future::AsyncAwait
55 As of Future::AsyncAwait version 0.55 a cross-module integration test
56 asserts that the "multi" modifier can be applied to "async sub".
57 use Future::AsyncAwait;
59 use Syntax::Keyword::MultiSub;
60 async multi sub f () { return "nothing"; }
62 async multi sub f ($key) { return await get_thing($key); }
63
65 • Much better error checking and diagnostics for function bodies that
66 don't use signatures.
67 • Cross-module testing with Object::Pad (for "multi method"). This
69 may require a better combined implementation, to be aware of method
70 resolution order, inheritence, etc...
71 • An eventual consideration of type assertions or value testing, as
73 well as simple argument count.
74 This particular task is likely to be a large undertaking as it
76 spans several other areas of language. As well as types on
77 parameters, it would be nice to put them on lexical variables,
78 object slots, "match/case" comparisons, and so on. It would be a
79 shame to invent a special mechanism for one of these areas that
80 could not be reüsed by the others.
81
83 Paul Evans <leonerd@leonerd.org.uk>
84perl v5.36.1 2023-07-13 Syntax::Keyword::MultiSub(3)