我试图理解函数组合是如何工作的。
常见示例是:
(.) :: (b -> c) -> (a -> b) -> a -> c
f . g = x -> f (g x)
很容易理解,什么是函数组合。
现在考虑:
(fmap . fmap) (+5) (Just (Just 4))
让我们考虑第一个fmap
实现:
fmap . fmap = x -> fmap ( fmap x )
示例将如何执行?
fmap (+5)
要先执行吗?
与示例相关的x
值是多少?
(fmap . fmap) (+5) (Just (Just 4))
= { def. (.) }
fmap (fmap (+5)) (Just (Just 4))
= { fmap g (Just x) = Just (g x), where g = fmap (+5) ; x = Just 4 }
Just (fmap (+5) (Just 4))
= { fmap g (Just x) = Just (g x), where g = (+5) ; x = 4 }
Just (Just ((+5) 4))
= { def. (+) }
Just (Just 9)