Free Maybe MaybeT（作家 Nat）

特定

data Maybe a = Just a
             | Nothing

我們有

data Free Maybe a
     = Pure a
     | Free (Just (Free Maybe a))
     | Free Nothing

這相當於

data Hopes a
     = Confirmed a
     | Possible (Hopes a)
     | Failed

或等效（如果你承諾首先評估 fst 元素）(Nat, Maybe a)，aka MaybeT (Writer Nat) a with

data Nat = Z | S Nat
data Writer Nat a = Writer Nat a
data MaybeT (Writer Nat) a = MaybeT (Nat, Maybe a)