Krivine.hs

data Lam
  = Abs Lam
  | App Lam Lam
  | Var Int

data Stack = S [(Lam, Stack)]

krivine (App a b, S e, S s)     = krivine (a, S e, S ((b, S e):s))
krivine (Abs t,   S e, S (c:s)) = krivine (t, S (c:e), S s)
krivine (Var n,   S e, S s)     = krivine (fst (e !! n), snd (e !! n), S s)
krivine (t,       S e, S [])    = t

eval t = krivine (t, S [], S [])