Monday, September 5, 2011

Long weekend - Part 2: Dynamic Dispatch

Alas, Sunday was a wasted day due to a bad (legit) windows 7 ISO burn during my computer upgrades. I got to reinstall the OS just so I could re-download and burn the iso (this time checking md5's).

But now I have a beefy new I7 sitting under my desk. Woot.

Today I went through and did some more compiler and code-gen bugfixes so that I can tell you things without being a damned dirty liar. Tangent now properly supports dynamic dispatch for simple enums. Adding to our last example:

Type foo => enum{ values{ a, b+c } };
stuff (x: foo) => void { debug "in stuff."; }
stuff (x: foo.a.Type) => void { debug "in a stuff."; } 
entrypoint => void {
  debug " + "\"entrypoint\"" + @";
  stuff foo.b+c;
  stuff foo.a;

Here we overload stuff so that it can take any foo, but if it gets foo.a then that more specific implementation should be used. So you get the nice output:

in stuff.
in a stuff.

Yes yes, this isn't actually dynamic dispatch; just overloading methods on parameters. Once I can actually store variables, it'll be dynamic dispatch (another post for another time). C# supports it if the method takes a dynamic, but that isn't statically checked, few people know about it, and it's not super important in C# (plus dynamic methods are limited in where they can be used). In Tangent though using type context for terms that are ambiguous in other languages (but clear to humans) is the core of what it's trying to experiment with. This sort of method refinement on types/capabilities is a big feature that is likely to be a large part of the language idioms as it grows.

The other thing shown here is the Type member. This is autogenerated onto every type at the moment. For constant/sealed things like enums, it is typed as simply another reference to the constant (anonymous) type that enum values have. For classes, it is typed to return a kind of T. This makes sense since we're guaranteed that any subtypes will fit into that kind. It allows us to get static typing when working with types of variables for cloning, serialization, and factory sort of things.

2 days left in the long weekend. I hope to get another little bit of work and a post done, but I suspect work and (moreso) Disgaea 4 will derail that plan.

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