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 };
foo(A.new());
foo(B.new());

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) {...}
}