freeVars.hs

freeVars :: Exp -> [Id]
freeVars = nub . go [] where
  go env (Var v)        | v `elem` env = []
                        | otherwise = [v]
  go _   (Nat _)        = []
  go _   (Op _)         = []
  go env (Lam arg body) = go (arg:env) body
  go env (App f x)      = go env f ++ go env x