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Purescript效果:防止深嵌套

purescript

1

M. Walker · 技术社区 · 6 年前

马上 main

main :: Eff (dom :: DOM) (Maybe (Eff (dom :: DOM) (Maybe Element)))

我想要以下内容:

main :: Eff (dom :: DOM) (Maybe Element)

findItem :: forall e. IsElement e => e -> Eff (dom :: DOM) (Maybe Element)
findItem e = getElementsByClassName "thing" (toElement e) >>= (item 0)

-- main :: Eff (dom :: DOM) (Maybe Element)
main = (map findItem) <$> (window >>= document >>= body)

最好的方法是什么?

当然,我可以做到以下几点:

findItem :: forall e. IsElement e => Maybe e -> Eff (dom :: DOM) (Maybe Element)
findItem (Just e) = getElementsByClassName "findItemEdit" (toElement e) >>= (item 0)
findItem Nothing = pure Nothing

main :: Eff (dom :: DOM) (Maybe Element)
main = findItem =<< body =<< document =<< window

但我不想 Maybe 在中处理 findItem 作用

3 回复 | 直到 6 年前

1

3

ephrion 6 年前

使用 traverse :: forall a b eff. (a -> Eff eff b) -> Maybe a -> Eff eff (Maybe b) . 该函数更通用,但这就是您想要使用它的方式。每当你发现自己想“交换”两种类型时,比如 Maybe a -> Eff eff (Maybe a) 或 Maybe (List a) -> List (Maybe a) ,您可以使用 traverse 或 sequence .

具体来说 main 看起来像:

main :: Eff (dom :: DOM) (Maybe Element)
main = do
  mdocBody <- window >>= document >>= body
  mmitem <- traverse findItem mdocBody
  -- mmitem has type `Maybe (Maybe Element)`
  -- we can use `join` to collapse
  pure (join mmitem)

或者,作为无点,

main :: Eff (dom :: DOM) (Maybe Element)
main =
  window >>= document >>= body 
    >>= map join <<< traverse findItem

2

3

Sam Thomson 6 年前

您可以使用 MaybeT monad变压器:

main = runMaybeT do
  b <- MaybeT (window >>= document >>= body)
  MaybeT (findItem b)

MaybeT 是的新类型 Maybe 包裹在单子里 m (在我们的情况下 m级 是 Eff (dom :: DOM) ):

newtype MaybeT m a = MaybeT (m (Maybe a))

及其 bind 以您希望的方式处理嵌套和取消嵌套:

bind (MaybeT x) f = MaybeT do
  x >>= case _ of
    Nothing -> pure Nothing
    Just y -> case f y of MaybeT m -> m

3

0

Fyodor Soikin 6 年前

我能想到的最好办法就是使用 Maybe 本质上需要 findItem 从您的第一个示例中 芬迪特姆 从您的第二个示例中:

main :: Eff (dom :: DOM) (Maybe Element)
main = maybe (pure Nothing) findItem =<< body =<< document =<< window