Haskell函数的应用

const 是

const x = \_ -> x

(const id) 是一个接受一个参数并始终返回的函数 id 和

const id 1 2 = (\_ -> id) 1 2
             = id 2
             = 2

定义 foldr1 是

foldr1 f [x] = x
foldr1 f (x:xs) = f x (foldr1 f xs)

myLast' = foldr1 (const id)

myLast' [x] = foldr1 (const id) [x]
              {- definition of foldr1 -}
            = x

和

myLast' (x:xs) = foldr1 (const id) (x:xs)
                 {- definition of foldr1 -}
               = (const id) x (foldr1 (const id) xs)
                 {- definition of const -}  
               = (\_ -> id) x (foldr1 (const id) xs)
                 {- function application -}  
               = id (foldr1 (const id) xs)
                 {- definition of id -}  
               = foldr1 (const id) xs
                 {- definition of myLast' -}  
               = myLast' xs

这与 last' .

我严重依赖于 :t 在试图理解哈斯克尔的时候。在这种情况下:

Prelude> :t const id
const id :: b -> a -> a

也许能帮你看看发生了什么。