Perl 6
SMOP Multi Sub Dispatch: Revision 4
Multi subs in Perl 6 support two very nasty features:
* lexical variants of outer-or-global multi subs
* variant disambiguation
What do they mean?
^^ Lexical Variants
.pre
my multi foo (Foo $a) {...};
for 1..4 {
my multi foo (Bar $a) {...};
# foo(Bar) is visible here
}
# but not here
.pre
^^ Variant Disambiguation
.pre
class A {};
class B is A {};
multi foo (A $a) { say 1 };
multi foo (B $a) { say 2 };
foo(A.new());
foo(B.new());
.pre
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
* lexical variants of outer-or-global multi subs
* variant disambiguation
What do they mean?
^^ Lexical Variants
.pre
my multi foo (Foo $a) {...};
for 1..4 {
my multi foo (Bar $a) {...};
# foo(Bar) is visible here
}
# but not here
.pre
^^ Variant Disambiguation
.pre
class A {};
class B is A {};
multi foo (A $a) { say 1 };
multi foo (B $a) { say 2 };
foo(A.new());
foo(B.new());
.pre
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
