destructive update extension, the horror!

gistfile1.hs

evalThen :: Env a -> Lambda a -> (Maybe (Lambda a) -> b)
evalThen env (Var a) then = then $ M.lookup a env
evalThen env abs@(Abs {}) then = then $ Just abs
evalThen env (App fun arg) then = then $ case fun of
  Abs id body -> evalThen env arg >>= \a -> evalThen (M.insert id a env) body
  _           -> Nothing
evalThen env p@(Prim {}) = then $ Just p

eval env term = eval env term id

exec env term = eval env term (\x -> case x of
  (Var a) -> alter (\y -> x) a env
  _ -> env)