[Haskell-beginners] permuting a list

[Thomas Davie]

On 12 Feb 2009, at 10:20, Jan Snajder wrote:

> Hi!
>
> I'm trying to write a list permutation function, and there is in  
> fact a
> nice explanation of how to do it here:
> http://sneakymustard.com/2008/12/23/shuffling-in-haskell
>
> But for the start I wanted to keep things simple and avoid monad
> transformers (since I'm not into this yet). Instead, I'd like to  
> write a
> function of type:
>
>> permute :: [a] -> IO [a]

o.O

Why not keep things simple and just write a pure function?

permute :: [a] -> [[a]]
permute xs = [s:ps | (s,ss) <- select xs, ps <- permute ss]

select :: [a] -> [(a,[a])]
select []     = []
select (x:xs) = (x,xs) : [(s,x:ss) | (s,ss) <- select xs]

Bob