[Haskell-beginners] Randomness, lists, and unfoldr

[Alex Rozenshteyn]

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 <daniel.is.fischer at web.de>wrote:

> 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
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