haskell - 如何将 Maybe 值注入(inject) MaybeT

Question

说我有一些foo :: Maybe Int例如，我想将它与 bar :: Int -> MaybeT (Writer String) Int 绑定(bind)，这样做的惯用方法是什么？

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是一种耻辱，但是做 a hoogle search ，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 更可取.我只是把它放在某个方便的模块中。