[RFC] Implement type inheritance

[faultyserver]

While talking about how to re-implement IO as a type rather than a module, it seemed to make the most sense to have IO be a supertype that other types can inherit from. For example, a Socket is an IO and a FileDescriptor is an IO. With the original idea of using module composition, the semantics would be more that Socket is a type that acts like an IO.

Assuming that inheritance is added to the language, the question then becomes how it should be represented in the syntax. I see two decent options for how this could work:

• Following influence from Ruby and Crystal, use the < operator as a keyword:

deftype Foo < Bar
  # ...
end

• Borrow the syntax from C++, using ::

deftype Foo : Bar
  # ...
end

I like the second option more, because it fits nicely with the existing syntax for type restrictions on method parameters.

The other consideration is how this will work in the interpreter itself. The simplest solution seems to be adding another property, supertype, to TType. Then, TType#ancestors will call #ancestors on the supertype and append the result to the list it already created.

Since Myst doesn't support singleton classes or other metaprogramming concepts like Ruby/Crystal does, I think this implementation will be sufficient for most cases.