haskell或者String（NestedList a） - 为什么它不工作 [英] haskell Either String (NestedList a)- why doesn't it work

问题描述

我试图添加像普通列表一样在嵌套列表上工作的函数。我想要使​​用Either字符串（Nested a），以便它返回错误或附加列表。但它一直在失败。我没有在任何地方做NestedList [NestedList a]。为什么它表示它预期[NestedList（NestedList a）]

I am trying to append function that works on Nested Lists like regular lists. I want to use Either String (Nested a) so that it returns error or the appended list. But it keeps failing. I am not doing NestedList[NestedList a] anywhere. Why does it say it expected [NestedList (NestedList a)]

module Main where

data NestedList a=Elem a | List[NestedList a] deriving Show

flatten ::NestedList a->[a]
flatten (Elem x)=[x]
flatten (List(x:xs))=flatten(x)++flatten(List xs)
--flatten NestedList (x:xs)=flatten(x)++flatten(xs)
flatten(List [])=[]

------------------

count::[a]->Int
count a=length (a)

-----------------
append::NestedList a->NestedList a->Either String (NestedList a)
append (_) (Elem a)=Left "Elements are not allowed"
append (Elem a) (_)=Left "Elements are not allowed"
append (List a) (List b)=Right (List (flatten(List a)++flatten(List b)))
-------------------

main=do
        print(flatten (List[Elem 1, Elem 2,List[Elem 1, Elem 2 ]]))
        print(count(flatten (List[Elem 1, Elem 2,List[Elem 1, Elem 2 ]])))
        print(append (List[List[Elem 1, Elem 2 ]]) (List[Elem 1, Elem 2,List[Elem 1, Elem 2 ]]    ))  

给我这个错误

gives me this error

   flatten_list.hs:18:52:
   Couldn't match type `a' with `NestedList a'
      `a' is a rigid type variable bound by
      the type signature for
        append :: NestedList a
                  -> NestedList a -> Either String (NestedList a)
      at flatten_list.hs:15:9
       Expected type: [NestedList (NestedList a)]
  Actual type: [NestedList a]
In the first argument of `List', namely `a'
In the first argument of `flatten', namely `(List a)'
In the first argument of `(++)', namely `flatten (List a)'

flatten_list.hs:18:69:
    Couldn't match type `a' with `NestedList a'
  `a' is a rigid type variable bound by
      the type signature for
        append :: NestedList a
                  -> NestedList a -> Either String (NestedList a)
      at flatten_list.hs:15:9
    Expected type: [NestedList (NestedList a)]
  Actual type: [NestedList a]
In the first argument of `List', namely `b'
In the first argument of `flatten', namely `(List b)'
In the second argument of `(++)', namely `flatten (List b)'

推荐答案

import Data.Traversable

append :: NestedList a -> NestedList a -> Either String (NestedList a)
append (Elem x) (Elem y) = Right $ List [Elem x, Elem y] 
append (Elem _) _ = Left ""
append _ (Elem _) = Left "" 
append (List xs) (List ys) = fmap List $ sequenceA $ zipWith append xs ys 

你在你的问题中写的追加似乎只是想扁平化和连接两个清单，这是毫无意义的，因为你扔掉了所有的结构。如果这是所需的行为，那么只需使用一个常规列表：

The append you wrote in your questions seems to want to just flatten and concatenate the two lists, which is rather pointless, since you are throwing away all the structure. If this is desired behavior, then just use a regular list:

append' :: NestedList a -> NestedList a -> Either String [a]
append' (Elem x) (Elem y) = Right $ [x,y]
append' (Elem _) _ = Left ""
append' _ (Elem _) = Left "" 
append' a b        = Right $ flatten a ++ flatten b

<你也可以根据 free monad ：

import Control.Monad.Free
type NestedList = Free []

flatten :: NestedList a -> [a]
flatten = retract 

append :: NestedList a -> NestedList a -> Either String (NestedList a)
append (Pure x) (Pure y) = Right $ Free [Pure x, Pure y] 
append (Pure _) _ = Left ""
append _ (Pure _) = Left "" 
append (Free xs) (Free ys) = fmap Free $ sequenceA $ zipWith append xs ys 

这个定义与你给出的定义是同构的。

This definition is isomorphic to the one you gave.

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