Re: Fun with junctions (was Sets vs Junctions)

[Damian Conway]

Thomas Sandlaß wrote:
    class Source[Language ::To] is Str {
        multi sub *coerce:as (Any $data, To ::Lang) {
            return Lang.serialize($data)
        }
    }

What is the return type of &*coerce:as?
Sorry, I was too lazy (well, I'd claim I was thinking at a much higher level, 
but it amounts to the same ;-) and I made a mistake in the signature of the 
coercion.

Here's a fully typed, and correctly parameterized, version:
      class Source[Language ::To] is Str {
          multi sub *coerce:as (Any $data, Source[To] ::Lang)
              returns Source[To]
          {
              return Lang.serialize($data)
          }
      }
      class YAML is Language {
          method serialize($data) returns Source[YAML] {
              ...
              return Source[YAML].new($source_code);
          }
      }
Or we could cheat in the coercion, and just do it all with strings:
      class Source[Language ::To] {
          multi sub *coerce:as (Any $data, Source[To] ::Lang)
              returns Str
          {
              return Lang.serialize($data)
          }
      }
      class YAML is Language {
          method serialize($data) returns Str {
              ...
              return $source_code;
          }
      }
As for the MMD table, that's an implementation issue, but presumably it'd have 
to contain separate entries for each instantiation of the generic Source class:

      2-ary &*coerce:as<Any,Source[Perl]>
      2-ary &*coerce:as<Any,Source[Python]>
      2-ary &*coerce:as<Any,Source[Lisp]>
      2-ary &*coerce:as<Any,Source[C::Sharp]>
      # etc.
or (more likely, given the dynamic nature of Perl) a single entry that's a 
"trampoline" that instantiates particular parameterizations of the generic 
class as they're needed:

      2-ary &*coerce:as<Any,Source[*]>
Damian