haskell错误函数 [英] haskell error function

问题描述

我有一个函数 first ，类型为： Int - > [a] - > （错误（[a]，[a]））和第二个函数 second 类型： [a] - > [a] - > [a]

我正在尝试使用上述函数创建第三个函数。
这个函数的类型是： [Int] - > [a] - >错误[a]

我已经得到这些类型的解决方法，所以不能更改它们。

这是我试过的：

  last :: [Int]  - > [a]  - >错误[a] 
 last（x：xs）list = second（first x list）
  

您可以将使用错误函数的函数的输出传递给其他函数吗？

解决方案假设错误是错误monad ，您可以使用monadic绑定操作符（）和 uncurry 函数：

  z（x：xs）list = F x list>> = return。 uncurry Q 
  

uncurry 有两个参数（又名咖喱函数）成对的函数。这意味着 uncurry Q ::（[a]，[a]） - > [a]

绑定运算符从monad中获取一个值并将其传递给monadic函数。这里我们提取由 F 返回的Error monad的值，并将它传递给 Q ，现在变成了一元函数可以在一对列表上运行，这要归功于 return 和 uncurry 。

I have one function first with type: Int -> [a] -> (Error ([a],[a])) and a second function second with type: [a] -> [a] -> [a]

I am trying to make a third function now that uses the above functions. the type I have for this function is: [Int] -> [a] -> Error [a]

I have been given these types to work around so cant change them.

This is what I tried:

last :: [Int] -> [a] -> Error [a]
last (x:xs) list = second (first x list)

Can you pass outputs from functions that use the error function in to others?

解决方案

Assuming Error is an error monad, you can use the monadic bind operator (>>=) and the uncurry function:

z (x:xs) list = F x list >>= return . uncurry Q

uncurry transforms Q from a function with two arguments (aka a curried function) into a function on pairs. This means that uncurry Q :: ([a], [a]) -> [a]

The bind operator takes a value out of a monad and passes it into a monadic function. Here we're extracting the value of the Error monad returned by F and passing it to Q, now turned into a monadic function that works on a pair of lists, thanks to return and uncurry.

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