[Haskell-cafe] Permutations

[Jake Mcarthur]

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On Nov 30, 2008, at 10:17 AM, Luke Palmer wrote:

> On Sun, Nov 30, 2008 at 9:06 AM, Jake Mcarthur <jake at pikewerks.com>  
> wrote:
>> Seems a bit easy, I think.
>>
>>   Data.List.permutations . nub
>
> That is not what he meant.  Given:
>
>  [1,1,2,2]
>
> The results should be:
>
>  [1,1,2,2]
>  [1,2,2,1]
>  [2,2,1,1]
>  [1,2,1,2]
>  [2,1,2,1]
>  [2,1,1,2]

Heh, after a couple more seconds of thought, reversing the two  
composed functions fixes it:

     nub . permutations

Of course, neither my previous nonsolution nor this solution are  
efficient for long lists, but I think it serves as a decent reference  
implementation at least.

- - Jake
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