如何比较Haskell中的两个列表？

Question

In Haskell, how can you compare two lists to check if they are equal? Also the order shouldn't matter.

Example:

[1,2] = [2,1]

I tried all (flip elem [1,2,3]) [2,1] , but this returns true ...

Thanks.

Answer 1

Something like this?

import Data.List (sort)
areEqual a b = sort a == sort b

OUTPUT:
*Main> areEqual [1,2] [2,1]
True

Answer 2

As Eq a => Eq [a] ( http://www.haskell.org/ghc/docs/7.4.1/html/libraries/base-4.5.0.0/Data-Eq.html )

you DON'T need extra code to compare lists for equality.

[1,2] == [2,1]

If you want to compare lists as bags, then a bag is a MultiSet , so look for a Multiset package

import "multiset" Data.MultiSet as M

-- or

import "multiset" Data.IntMultiSet as M   -- if you deal with Ints 

M.fromList [1,2] == M.fromList [2,1] 

Answer 3

A good data structure for "collection without order or repetitions" is from the module Data.Set :

import qualified Data.Set as S

sameElems xs ys = S.fromList xs == S.fromList ys

This does, however, consider [1,1] to be equal to [1] , which may not be what you want.