Perl 6
SMOP Multi Sub Dispatch: Revision 6
Multi subs in Perl 6 support two features:

* lexical variants of outer-or-global multi subs
* variant disambiguation

What do they mean?

^^ Lexical Variants

my multi foo (Foo $a) {...};
for 1..4 {
my multi foo (Bar $a) {...};
# foo(Bar) is visible here
# but not here

^^ Variant Disambiguation

class A {};
class B is A {};
multi foo (A $a) { say 1 };
multi foo (B $a) { say 2 };

Most languages that support multi dispatch doesn't support this feature, this is usually seen as a compile-time error, since the two variant are ambiguous. But this is a very important feature to Perl 6, since that's how objects can overload most operators. Which means you need a disambiguation code to sort all the matching candidates.

^^ How to make it work

The dispatching would implement the following steps:

* manual traversal of the lexical scope searching for variants: This is not a simple lookup because we need the variant declared in an inner scope not to hide the declaration in an outer scope.
* check if package CANDO this sub: sub dispatch is 'no strict', meaning that if the symbol is not found in the current lexical scope, it might be in the package.
* grep all the variants for ACCEPTS
* sort them according to a "pluggable disambiguation code"
* invoke the topmost variant on the sorted list

^^ API

module P6SubDispatch {
only sub dispatch ($name, $lexpad, $package) {...}
role P6VariantDisambiguator {
only sub sort (*@variants) {...}

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