理解Haskell中的let-expression

Question

我是Haskell的新手,目前正在研究它进行考试.我一直在从learnyouahaskell学习它.我不理解组函数中的以下let-expression.

splitWhen :: (a -> Bool) -> [a] -> ([a],[a])    
splitWhen p xs = (takeWhile (not . p) xs, dropWhile (not . p) xs)

group :: (Eq a) => [a] -> [[a]]
group [] = []
group (x:xs) = let (group1, rest) = splitWhen (/=x) xs
                   in (x:group1) : group rest

我知道什么是splitWhen: splitWhen even [1,2,3] => ([1],[2,3])

我知道功能组产生如下:

group [1,1,2,1,3,3,3] => [[1,1],[2],[1],[3,3,3]]

我不明白它是如何工作的.有人可以向我解释一下吗？谢谢.

Answer 1

let表达式是用于立即应用匿名函数的语法糖.那是,

let (group1, rest) = splitWhen (/=x) xs
    in (x:group1) : group rest

相当于

(\(group1, rest) -> (x:group1) : group rest) (splitWhen (/=x) xs)

换句话说let,将函数体的位置和函数的参数反转.相比

let name =  value in body
(  \name -> body  )  value