[Haskell] String permutation

[Robert Dockins]

On Jul 26, 2006, at 12:44 AM, Sukit Tretriluxana wrote:

> Dear expert Haskellers,
>
> I am a newbie to Haskell and try to write several algorithms with  
> it. One of them is the string permutation which generates all  
> possible permutations using the characters in the string. For  
> example, if I say,
>
> permute "abc"
>
> It will return the the result as
>
> ["abc","acb","bca","bac","cab","cba"]
>

[snip]

 > I am more than certain that this simple program can be rewritten  
to be more succinct and possibly more efficient using Haskell's  
features. So I would like to ask if any of you would kindly show me >  
an example.


About the shortest way I can think of, using the good ol' list  
monad.  This isn't exactly efficient, but there you are...


import Data.List

permute :: String -> [String]
permute [] = [[]]
permute str = do
    x  <- str
    xs <- permute (delete x str)
    return (x:xs)



Rob Dockins

Speak softly and drive a Sherman tank.
Laugh hard; it's a long way to the bank.
           -- TMBG