haskell - 如何将 Maybe 值注入 MaybeT

Question

假设我有一些foo :: Maybe Int并且我想将它与例如绑定bar :: Int -> MaybeT (Writer String) Int，那么惯用的方法是什么？

我可以定义自己的liftMaybe函数，然后使用它，例如：

let liftMaybe = maybe (fail "Nothing") return in liftMaybe foo >>= bar

但是有没有更惯用（或至少简洁）的方式来做到这一点？

Answer 1

MaybeT . return :: (Monad m) => Maybe a -> MaybeT m a

我认为它没有标准名称很遗憾，但是进行hoogle 搜索，我们看到relude包使用hoistMaybe：

hoistMaybe :: Applicative m => Maybe a -> MaybeT m a

---

更一般的形式是

liftMaybe :: (MonadPlus m) => Maybe a -> m a
liftMaybe = maybe mzero return

这比使用fail. 我只是把它放在某个方便的模块中。