编译我的自定义 ZipList 应用程序实例需要什么？

Question

这是我的自定义列表 class，用于创建自定义 ZipList class。我想创建一个 ZipList 的应用实例。

import Control.Applicative

data List a =
  Nil
  | Cons a (List a)
  deriving (Eq, Show)

instance Functor List where
  fmap f (Cons x xs) = Cons (f x) (fmap f (Cons (head' xs) (tail' xs)))
  fmap _ Nil = Nil

instance Applicative List where
  pure x = Cons x Nil
  (<*>) fs xs = Cons (head' fs $ head' xs) ((<*>) (tail' fs) (tail' xs))

head' :: List a -> a
head' (Cons a _) = a

tail' :: List a -> List a
tail' (Cons _ l) = l

take' :: Int -> List a -> List a
take' 0 _ = Nil
take' _ Nil = Nil
take' i xs = Cons (head' xs) (take' (i-1) (tail' xs))

newtype ZipList' a =
  ZipList' (List a)
  deriving (Eq, Show)

instance Functor ZipList' where
  fmap f (ZipList' xs) = ZipList' $ fmap f xs

instance Applicative ZipList' where
  pure x = ZipList' (Cons x Nil)
  (<*>) fs xs = ZipList' (go fs xs)
    where go gs ys = Cons (head' gs $ head' ys) (go (tail' gs) (tail' ys))
          go Nil _ = Nil
          go _ Nil = Nil

我得到的错误是：

chap17/ZipList_applicative.hs:38:30: Couldn't match expected type ‘List (r0 -> b)’ …
                with actual type ‘ZipList' (a -> b)’
    Relevant bindings include
      xs :: ZipList' a
        (bound at /Users/ebs/code/haskell/book/chap17/ZipList_applicative.hs:38:12)
      fs :: ZipList' (a -> b)
        (bound at /Users/ebs/code/haskell/book/chap17/ZipList_applicative.hs:38:9)
      (<*>) :: ZipList' (a -> b) -> ZipList' a -> ZipList' b
        (bound at /Users/ebs/code/haskell/book/chap17/ZipList_applicative.hs:38:3)
    In the first argument of ‘go’, namely ‘fs’
    In the first argument of ‘ZipList'’, namely ‘(go fs xs)’
chap17/ZipList_applicative.hs:38:33: Couldn't match expected type ‘List r0’ …
                with actual type ‘ZipList' a’
    Relevant bindings include
      xs :: ZipList' a
        (bound at /Users/ebs/code/haskell/book/chap17/ZipList_applicative.hs:38:12)
      fs :: ZipList' (a -> b)
        (bound at /Users/ebs/code/haskell/book/chap17/ZipList_applicative.hs:38:9)
      (<*>) :: ZipList' (a -> b) -> ZipList' a -> ZipList' b
        (bound at /Users/ebs/code/haskell/book/chap17/ZipList_applicative.hs:38:3)
    In the second argument of ‘go’, namely ‘xs’
    In the first argument of ‘ZipList'’, namely ‘(go fs xs)’
Compilation failed.

它说它需要来自 go 的 List (r0 -> b) 类型，而不是 ZipList'。我不明白为什么，去 returns 列表，而不是 ZipList'...

Answer 1

您的 go 将 ZipList 作为输入，但在 (<*>) fs xs 中 xs 和 fs 是 ZipList'。你应该打开新类型：

ZipList' fs <*> ZipList' xs = ZipList' (go fs xs)

还有，你的go是错误的。每当程序评估任一参数中的空列表时，它都会失败并出现运行时错误，因为 go gs ys 案例匹配所有内容，并且无法访问它之后的案例。你应该改为这样做：

go (Cons f fs) (Cons x xs) = Cons (f x) (go fs xs)
go _ _ = Nil

作为一般规则，您应该避免使用 head、tail 和其他部分函数（即可能抛出异常的函数）。 Cons x xs 上的模式匹配同时绑定了头部和尾部，因此此后的 head 和 tail 没有太大用处。很少有人真正证明使用偏函数是合理的。

编译我的自定义 ZipList 应用程序实例需要什么？

What is needed to get my custom ZipList applicative instance to compile?

haskell

applicative