[Haskell-cafe] generating random lists until some predicate holds
Luke Palmer
lrpalmer at gmail.com
Sun Dec 28 17:12:15 EST 2008
On Sun, Dec 28, 2008 at 2:39 PM, Nicholas O. Andrews <nandrews at vt.edu>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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