Re: [Haskell-cafe] generating random lists until some predicate holds

28 Dec 2008

      On Sun, Dec 28, 2008 at 2:39 PM, Nicholas O. Andrews wrote:
...
Hi all,
What's the best way to implement the following Python code in Haskell?
It is purposefully written in a functional style (and as a result will
kill your recursion stack every other run).
Here's my solution, using MonadRandom (from Hackage).  There may be more
infinite-listy ways of doing it, but I wasn't able to make it come out
clean.

import Control.Monad.Random

many n = sequence . replicate n

untilM p m = do
  x <- m
  if p x then return x else untilM p m

getList :: MonadRandom m => m [Int]
getList = many 10 $ getRandomR (0,9)

main = print =<< evalRandIO (untilM (all even) getList)

# begin Python
...
from random import *
def genList ():
   return [randint(0,9) for x in range(10)]
def randWhile (predicate):
   result = genList ()
   if predicate(result):
       return result
   else:
       return randWhile (predicate)
def allEven (list):
   return reduce(lambda x,y: x and y, [x%2 == 0 for x in list])
print randWhile (allEven)
# End Python
Thanks!
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Re: [Haskell-cafe] generating random lists until some predicate holds

Luke Palmer