Is there a way to take a given monad's bind and wrap it to make it more
lazy? It doesn't seem like there should be, but I'm being hopeful.
On Mon, Sep 13, 2010 at 3:21 PM, Daniel Fischer
On Monday 13 September 2010 20:42:49, Alex Rozenshteyn wrote:
I'm trying to build a list where each entry depends on the previous one. Unfoldr seemed like a good idea at the time. Unfortunately, my values are monadic values (specifically RVars, from the random-fu package). Okay, shouldn't be a problem; I just monadic bind and...
-- example code updateCell :: Bool -> RVar Bool updateCell False = return False updateCell True = bernoulli (0.9 :: Double)
sim = sequence $ take 20 $ unfoldr (\x -> Just (x, x >>= updateCell))
(return True)
So you get
sequence [return True , return True >>= updateCell , (return True >>= updateCell) >>= updateCell , ((return True >>= updateCell) >>= updateCell) >>= updateCell , ... ]
runRVar sim DevURandom
[True,True,True,True,True,False,True,False,True,False,False,False,False, False,False,True,True,False,False,False]
That output shouldn't be possible if I'm doing things right... It appears that each cell has an independent history. I'm stumped. Advice on threading monad input in general and random-fu in specific would be appreciated.
What you want would be something like
iterateM :: (Monad m) => (a -> m a) -> a -> m [a] iterateM act start = do next <- act start rest <- iterateM act next return (start : rest)
but that would only work if the bind is sufficiently lazy, otherwise you'd have to iterate a given number of times.
-- Alex R