Haskell - 用警衛代替案件

Question

我想知道在這部分代碼中是否可以用守衛替換case語句：

firstFunction  :: String -> Maybe MyType
secondFunction :: MyType -> Integer
myFunction     :: String -> Maybe Integer
myFunction xs = case firstFunction xs of
    Nothing -> Nothing
    Just x  -> Just( secondFunction x )

先感謝您！

Answer 1

您可以使用模式防護 [Haskell-wiki] ，如：

myFunction :: String -> Maybe Integer
myFunction xs | Just x <- firstFunction xs = Just (secondFunction x)
              | otherwise = Nothing

但是，你在這里做什么基本上是“ fmap ”的結果firstFunction ，如：

myFunction :: String -> Maybe Integer
myFunction xs = fmap secondFunction (firstFunction xs)

fmap :: Functor f => (a -> b) -> fa -> fb用於在fmap :: Functor f => (a -> b) -> fa -> fb上“映射”。 現在Maybe是一個算子， 定義為 ：

 instance Functor Maybe where fmap _ Nothing = Nothing fmap f (Just a) = Just (fa) 

這基本上就是你在這里寫的邏輯。