Question

如果我们在monadPlusSDif，Maybe选择MonadPlus作为tdif :: Int -> Int -> Maybe Int tdif x y | y == 0 = Nothing | otherwise = Just (div x y) monadPlusSDif :: MonadPlus m => Int -> Int -> m Int monadPlusSDif x y = guard (y /= 0) >> return (div x y)的数据类型，它们是否等效？

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Answer 1

好吧，对于Maybe，MonadPlus实例是

instance MonadPlus Maybe where
  mempty = Nothing

和guard实现为

guard b = if b then return () else mempty
--      = if b then Just   () else Nothing

有了这些知识，您可以使用等式推理来推断，当m为Maybe时，您可以替换原始代码

monadPlusSDif x y = guard (y /= 0) >> return (div x y)

与

monadPlusSDif x y = (if y /= 0
  then Just ()
  else Nothing) >> Just (div x y)

或

monadPlusSDif x y
  | y /= 0    = Just () >>= \_ -> Just (div x y)
  | otherwise = Nothing >>= \_ -> Just (div x y)

或

monadPlusSDif x y
  | y /= 0    = Just (div x y)
  | otherwise = Nothing

或

monadPlusSDif x y
  | y == 0    = Nothing
  | otherwise = Just (div x y)

所以你看到功能是相同的。

Answer 2

如果m ~ Maybe，这些函数将具有等效行为，但它们的编译字节代码表示可能会有所不同。你也可以用普通MonadPlus monad的实际守卫来实现它

monadPlusSDif :: MonadPlus m => Int -> Int -> m Int
monadPlusSDif x y
    | y == 0 = mzero
    | otherwise = return $ x `div` y

然后你可以用它作为

bigEquation :: Int -> Int -> Maybe Int
bigEquation x y = do
    z1 <- monadPlusSDif x y
    z2 <- monadPlusSDif (x + y) (x - y)
    z3 <- monadPlusSDif y x
    return $ z1 + z2 * z3

并且编译器能够在该上下文中找出它应该Maybe使用m。

Haskell Monad相当于

2 个答案: