Maybe primitive

maybe.pony

type MaybeT[A] is (A|None)

primitive Maybe
  fun just[A: Any val](a: A) : MaybeT[A] => a
  fun nothing[A: Any val]() : MaybeT[A] => None
  fun fmap[A: Any val, B: Any val](v: MaybeT[A], f: {(A): B}): MaybeT[B] =>
    match v
    | None => None
    | let v' : A => f(v')
    end

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 : MaybeT[U32] = Maybe.just[U32](1)
    // let maybeInt1 : U32 = None // b.
    let maybeInt11 = Maybe.fmap[U32, U32](maybeInt1, inc)
    let maybeInt21 = Maybe.fmap[U32, U32](maybeInt11, inc)
    let maybeString = Maybe.fmap[U32, String](maybeInt21, {(a) => a.string()})
    let maybePrint = Maybe.fmap[String, None](maybeString, {(a) => env.out.print(a)})