Rust HKT

gistfile1.rs

// Option A.
trait Functor {
  fn fmap(self: &Self<A>, Fn(A) -> B) -> Self<B>
}

// Option B.
trait Functor<A,B> {
  fn fmap(&self, Fn(&A) -> &B) -> ???
}

// Option C.
trait Functor {
  type A
  type B
  fn fmap(&self, Fn(&Self::A) -> &Self::B) -> ???
}

impl Functor for Option<?> {
  fn fmap(self: &Option<A>, Fn(A) -> B) -> Option<B> { ... }
}

impl Functor for Iterator<?> {
  fn fmap(self: &Iterator<A>, Fn(A) -> B) -> Iterator<B> { ... } // Iterator has no generics!
}

impl Functor for Result<?> {
  fn fmap(self: &Result<A>, Fn(A) -> B) -> Result<B> { ... }
}