Hi all,



I am trying to randomly reorder a list (e.g. shuffle a deck of cards) . My initial approach is to treat it as an array, generate a list of unique random numbers between 0 and n - 1, then use those numbers as new indexes. I am using a function to generate random numbers in the State monad as follows:



randInt∷  Int →  State StdGen Int
randInt x = do g ←  get
              (v,g') ←  return $ randomR (0, x) g
              put g'
              return v



This is pretty much straight from the documentation. My function for the new indexes is:



-- return a list of numbers 0 to x-1 in random order                                       randIndex∷ Int → StdGen → ([Int], StdGen)
randIndex x = runState $ do
   let randIndex' acc r
           | (length acc ≡ x) = acc
           | (r `elem` acc) ∨ (r ≡  (−1)) = do
               r' ← randInt (x − 1)
               randIndex' acc r'
           | otherwise = do
               r' ← randInt (x − 1)
               randIndex' r:acc r'
       in
       randIndex' [] (−1)



This fails to compile on




  Couldn't match expected type `[a]'
          against inferred type `State StdGen b'
   In a stmt of a 'do' expression: r' <- randInt (x - 1)
   In the expression:
       do { r' <- randInt (x - 1);
            randIndex' acc r' }




I can see what's happening here - it's treating randIndex' as the second argument to randInt instead of invisibly putting the State in there. Or am I going about this completely the wrong way?


Thanks,



G




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