[Haskell-beginners] Randomness, lists, and unfoldr

[Sriram Durbha]

related question: is this also an example of  para morphism?

here is my current understanding
cata morphism -> fold/reduce [ e.g. list of numbers -> sum of numbers]
ana morphism ->  unfold/reproduce depending on input [ e.g. one number ->
list of reapeated instances of same number]
        some functions like map can be written both as a cata and as an ana.
hylo morphism -> ana followed by cata [ e.g. recursion trees of function
calls are ana, and the return path to the final evaluation is cata]
para morphism -> kind of ana but depends on input and output generated so
far fib, fact etc?

Im not sure i understood the last one well.

Thanks
Cheers
Ram


On Mon, Sep 13, 2010 at 11:42 AM, Alex Rozenshteyn <rpglover64 at gmail.com>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)
>
> > 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.
>
> --
>           Alex R
>
>
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