[Haskell-cafe] Why doesn't this work?

Michael Vanier mvanier at cs.caltech.edu
Mon Apr 25 02:16:29 EDT 2005

I've been trying to generate an infinite list of random coin flips in GHC
6.4, and I've come across some strange behavior:

import System.Random

data Coin = H | T deriving (Eq, Show)

-- Generate a random coin flip.
coinFlip :: IO Coin
coinFlip = do b <- getStdRandom random
              return (bool2coin b)
              bool2coin True  = H
              bool2coin False = T

-- Generate an infinite list of coin flips.
coinFlips :: IO [Coin]
coinFlips = sequence cfs
            where cfs = (coinFlip : cfs)

-- Print n of them.
test :: Int -> IO ()
test n = do f <- coinFlips
            print (take n f)

Now when I do "test 1" (for instance), it hangs forever.  It seems as if
there is some kind of strictness constraint going on that I don't
understand.  My understanding is that cfs is an infinite list of (IO Coin),
sequence lifts this to be IO [Coin] where [Coin] is an infinite list, and
then test should extract the infinite list of coin flips into f, take some
number of them, and print them.  But instead, the system appears to be
trying to compute all the coin flips before taking any of them.  Why is
this, and how do I fix it?



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