maybe.pony

class Maybe[A : Equatable[A] val] is Equatable[Maybe[A]]
  let v : (A val | None)

  new val create(thing: A) => v = thing
  new val empty() => v = None
  
  fun eq(that: box->Maybe[A]): Bool =>
    match (this.v, that.v)
    | (let v' : A, let v'' : A) => v' == v''
    | (let n' : None, let n'' : None) => true
    | (_, _) => false
    end

actor Main
  new create(env: Env) =>
    let full1  = Maybe[String]("hello")
    let full2  = Maybe[String]("hello")
    let full3  = Maybe[String]("world")
    let empty1 = Maybe[String].empty()
    let empty2 = Maybe[String].empty()
    env.out.print((full1 == empty1).string()) //f
    env.out.print((full1 == full2).string())  //t
    env.out.print((full1 == full3).string())  //f
    env.out.print((empty1 == empty2).string()) //t