Using one random variable multiple times in Haskell

It'd like to make a random sequence of opening braces and a matching sequence of closing braces. Here's my (partial) code:

shuffle :: [a] -> IO [a]
ps xs ys 开发者_JAVA技巧= map snd . sort $ zip xs ys

opens  = "[{("
closes = "]})"

parens = do
ord <- shuffle [0..length opens]
  let os  = ps ord opens
      cs  = reverse $ ps ord closes

It doesn't work since it generates a new ord for each shuffle. How do I make it remember the ord and use it for both shuffles?

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Don't shuffle them separately.

Shuffle the zipped sequence, and unzip when you're done.

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Have you seen Oleg's Provably perfect shuffle algorithms or the System.Random.Shuffle implementation?

Upon rereading your (very incomplete) code, it appears that it could be correct, making some assumptions about the surrounding code. Please show more so we can determine what might be wrong.

As an aside, using map fst . sortBy (comparing `on` fst) . zip xs or map fst . sortWith fst . zip xs would more clearly express that your intent is to order one list by another.