Functors

[Mercerenies]

Parameterized modules, a la OCaml. This is going to be our primary means of ad hoc polymorphism.

Syntax:

trait Eq { X : Type }
  val == ( X X -- Bool )
end

mod IntIsEq
  require Eq { Int }

  fun == ( Int Int -- Bool )
    Prelude.=
  end

end

Some notes:

• Functors take modules as arguments, constrained by traits. Currently, all types except function types are represented as modules. At some point, function types will be represented as well, making it possible to pass function types to functors, but there are workarounds in the meantime.
• Traits are structurally typed, which means you don't have to explicitly declare that a module satisfies a trait. If it has the right shape, it satisfies it. A require declaration is recommended, as it will verify at compile-time that the trait is satisfied, but it's not required.

[Mercerenies]

Updated note on syntax

trait Eq
  type t
  val == ( Self.t Self.t -- Bool )
end

mod IntIsEq
  type t = Int
  require Eq

  fun == ( Int Int -- Bool )
    Prelude.=
  end

end

[Mercerenies]

Functors are nearly completely implemented. Still unresolved are:

• Aliasing within functors is insane and broken (#9)
• Functors cannot themselves declare aliases (#17)
• Functors cannot include modules or declare resource synonyms (#19 and #20)
• Functors cannot require traits (#21)
• Record syntax inside functors (#18)

[Mercerenies]

The initial implementation is complete, and all remaining concerns have been split into other, smaller issues. So I'm closing this one.