Jan Snajder wrote:
Brent Yorgey wrote:
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 #-}
import Data.Array.MArray import Data.Array.IO import Control.Monad import System.Random permute :: forall a. (MArray IOArray a IO) => [a] -> IO [a] permute xs = do let n = length xs - 1 arr0 <- (newListArray (0, n) xs :: IO (IOArray Int a)) arr <- foldM swap arr0 [n..1] getElems arr where 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
The type class constraint is not needed because IOArray can hold any element type anyway. (It's unboxed arrays that only work for certain element types). Thus, you can write FlexibleConstraints extension and simply write permute :: forall a. [a] -> IO [a] instead. Also, I think that specifying the type of arr0 as (arr0 :: IOArray Int a) <- newListArray (0, n) xs should work as well. -- http://apfelmus.nfshost.com