A maybe type, with Functor interface, in Pony

functor.pony

class val Maybe[A: Any val]
  let _v : (A | None)
  new val just(a: A) => _v = a
  new val nothing() => _v = None
  fun val fmap[B: Any val](f: {(A): B}) : Maybe[B] =>
    match _v
    | None => Maybe[B].nothing()
    | let v' : A => Maybe[B].just(f(v'))
  end

interface Functor[A: Any val]
  fun fmap[B: Any val](f: {(A): B}) : Functor[B]

actor Main
  new create(env: Env) =>
    let inc : {(U32) : U32} = {(a:U32) => a + 10}
    // This is the only line you have to change (a. or b.)
    // Whichever it is, your .fmapped code stays the same.
    let maybeInt1 : Maybe[U32] = Maybe[U32].just(1) // a.
    // let maybeInt1 : Maybe[U32] = Maybe[U32].nothing() // b.

    maybeInt1.fmap[U32](inc)
    .fmap[U32](inc)
    .fmap[String]({(a: U32) => a.string()})
    .fmap[None]({(a: String) => env.out.print(a)})