まちがってた。

{-# OPTIONS_GHC -fglasgow-exts #-}
module Test where

import System.Random
import Data.Array.IArray
import Data.Array.ST
import Control.Monad.ST

hoge :: forall a i e. (IArray a e, Ix i, Random i) => a i e -> StdGen -> (a i e, StdGen)
hoge ia gi = runST act
  where
    act :: forall s. ST s (a i e, StdGen)
    act = do
      ma <- thaw ia
      let _ = ma :: STArray s i e
      g' <- shuffle ma gi (range bo)
      ra <- unsafeFreeze ma
      return (ra, g')
    bo@(_, last) = bounds ia
    shuffle a g [] = return g
    shuffle a g (i:is) = do
        let (j, g') = randomR (i, last) g
        x <- readArray a i
        y <- readArray a j
        writeArray a i y
        writeArray a j x
        shuffle a g' is