[Haskell-beginners] Re: permuting a list

Daniel Fischer daniel.is.fischer at web.de
Fri Feb 13 05:30:34 EST 2009

Am Freitag, 13. Februar 2009 10:23 schrieb Jan Snajder:
> Brent Yorgey wrote:
> > It seems everyone has just been reading the first few words of Jan's
> > email and not the actual content.  Jan is clearly trying to write a
> > *random list shuffling* function, not a function to generate
> > permutations.  Let's try to be helpful, people...
> > </rant>
> Thanks Brant, I forgot to mention explicitly that I need a random
> permutation.
> > Jan, this is tricky.  The type of permute is indeed (MArray a1 a IO)
> > => [a] -> IO [a], but this is fine, it just means that there has to be
> > some sort of mutable array which can store the things you are trying
> > to shuffle.
> >   This is not the problem.  The problem seems to be that
> > Haskell has no way to know what sort of array you want to use.  I was
> >
> > able to get the code to work, but it's sort of sneaky:
> > > {-# LANGUAGE FlexibleContexts, ScopedTypeVariables #-}
> I guess by 'sneaky' you mean this solution is GHC-specific?

It can be made a little less sneaky, you don't need FlexibleContexts, because 
the type signature

permute :: forall a. [a] -> IO [a]

works, too (code otherwise unchanged). 
But that still doesn't work with hugs :(

However, if you bring the array-creation to top level, it doesn't need any 
module-specific language extensions:

module Perms where

import Data.Array.MArray
import Data.Array.IO
import Control.Monad
import System.Random

-- call with toArray (length xs - 1) xs
toArray :: Int -> [a] -> IO (IOArray Int a)
toArray n xs = newListArray (0,n) xs

permute :: [a] -> IO [a]
permute xs = do
    let n = length xs - 1
    arr0 <- toArray n xs
    arr <- foldM swap arr0 [n, n-1 .. 1]
    getElems arr
      swap arr n = do
        x <- readArray arr n
        r <- randomRIO (0, n)
        y <- readArray arr r
        writeArray arr n y
        writeArray arr r x
        return arr

and it works in hugs, too (needs the -98 flag, because one of the imports 
needs ST.hs, which has foralled type signatures).


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