Haskell要列出的元组列表？ [英] Haskell List of tuples to list?

问题描述

是否可以将元组列表 [（Int，Int）] 转换为对任何输入大小有效的通用方法？ ..我在各种问题中看到这是不可能的一般性的例子：

pre $ type X = [（Int，Int）]


func :: X - > [Int]


解决方案

元组应该被转换成列表。我假设你想让它们平展 - 例如， [（1,2），（3,4）] 应该变成 [1， 2,3,4] 。

如果元组的两个元素的类型相同，可以做这样的事情：

  tupleToList :: [（a，a）]  - > [a] 
 tupleToList（（a，b）：xs）= a：b：tupleToList xs 
 tupleToList _ = [] 
  

在一般情况下，这种翻译是不可能的。我可以想象的一件事是使用或者来包装这两种不同的类型：

  tupleToList :: [（a，b）]  - > [a b] 
 tupleToList（（a，b）：xs）=左a：右b：tupleToList xs 
  

Is it possible to convert a list of tuples [(Int,Int)] as a generic way which valid to any input size ? .. i saw in various questions thats its not possible generically

example :

type X = [(Int,Int)]


func :: X -> [Int]

解决方案

Your question is not very certain about how the tuples should be converted into a list. I assume that you want to have them flattend - for instance, [(1,2),(3,4)] should become [1,2,3,4].

This translation is only possible, if the two elements of your tuple are of the same type, than you can do something like this:

tupleToList :: [(a,a)] -> [a]
tupleToList ((a,b):xs) = a : b : tupleToList xs
tupleToList _          = []

In the general case, such a translation is impossible. One thing I could imagine to make the impossible possible is to use Either to wrap up the two different types:

tupleToList :: [(a,b)] -> [Either a b]
tupleToList ((a,b):xs) = Left a : Right b : tupleToList xs

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