[Haskell-cafe] Permutations

[Miguel Mitrofanov]

First line is necessary, because the second line is written in  
assumption that the first element of a permutation does really exist.

On 30 Nov 2008, at 19:49, Andrew Coppin wrote:

> Miguel Mitrofanov wrote:
>> eqPerms [] = [[]]
>> eqPerms xs = [x:xt | x <- nub xs, xt <- eqPerms $ delete x xs]
>
> Well, that's one way... ;-)
>
> I'm still not precisely sure why the first line must exist, but it  
> seems no matter which way round you do it, that line is needed.  
> Probably due to some subtlety of the list monad...
>
> I'm not sure how efficient using the delete function is, but this  
> version definitely has the virtues of brevity and readbility.
>
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