当函数使用IO时,可能使用fmap

Question

我有一个函数,我想使用Maybeval.通常我会这样做func <$> val.但现在假设func使用IO monad.func <$> val会回来的Maybe (IO ()).所以我必须定义一个新的运算符:

(<$$>) :: Monad m => (a -> m b) -> Maybe a -> m ()
(<$$>) func (Just val) = func val >> return ()
(<$$>) func Nothing    = return ()

所以现在我可以写func <$$> val,但是有更好的方法吗？

Answer 1

mapM_fromData.Foldable可能是最好的匹配：

Prelude Data.Foldable> :set -XScopedTypeVariables
Prelude Data.Foldable> :t \f (a :: Maybe a) -> Data.Foldable.mapM_ f a
\f (a :: Maybe a) -> Data.Foldable.mapM_ f a
  :: Monad m => (a -> m b) -> Maybe a -> m ()

如果您想要更专业的类型，还有maybe：

Prelude> :t \f -> maybe (return ()) (f $)
\f -> maybe (return ()) (f $)
  :: Monad m => (a -> m ()) -> Maybe a -> m ()