[Haskell-beginners] Request for Another State Monad Example

[Mike Sullivan]

>
> data FibState = F {previous, current :: Integer}
> fibState0 = F {previous = 1, current = 0}
>
> currentFib :: State FibState Integer
> currentFib = gets current
>
> nextFib :: State FibState Integer
> nextFib = do
>        F p c <- get
>        let n = p+c
>        put (F c n)
>        return n
>
> does that help?
>

Thank you, Daniel, for responding so quickly. I've played around with your
Fibonacci generator, but I'm afraid I'm not yet confident enough with monads
to get it to give me a meaningful answer. How do you seed its state, and how
would you go about printing out (for example) the first 5 numbers in the
Fibonacci sequence?

Mike



On Wed, Sep 17, 2008 at 1:39 PM, Daniel Fischer <daniel.is.fischer at web.de>wrote:

> Am Mittwoch, 17. September 2008 20:05 schrieb Mike Sullivan:
> > Hi All,
> >
> > As I'm sure all Haskell beginners do, I'm having a bit of a struggle
> > wrapping my head around all of the uses for monads. My current
> frustration
> > is trying to figure out how to use the state monad to attach some
> > persistent state between different calls to a function. I have two
> > questions that I would appreciate it if somebody could help me with.
> >
> > The ubiquitous state monad example for Haskell tutorials seems to be a
> > random number generator, with a function like the following (from
> > http://www.haskell.org/all_about_monads/html/statemonad.html#example):
> >
> > getAny :: (Random a) => State StdGen a
> > getAny = do g      <- get
> >             (x,g') <- return $ random g
> >             put g'
> >             return x
> >
> > My first question is very basic, but here it goes: I see it everywhere,
> but
> > what does the "=>" signify? Specifically, in this example what does
> > "(Random a) =>" do in the type signature?
>
> It describes a required context, here it means "for any type 'a' which is
> an
> instance of the typeclass Random, getAny has the type State StdGen a".
>
> >
> > My second question is more of a request, I suppose. I think it would be
> > useful to get another example that does not have the added complications
> of
> > dealing with the Random package, and saves more than one piece of data as
> > state. How would one go about (for example) creating a Fibonacci sequence
> > generator that saves the last state, such that on each call it returns
> the
> > next number in the Fibonacci sequence?
>
> data FibState = F {previous, current :: Integer}
> fibState0 = F {previous = 1, current = 0}
>
> currentFib :: State FibState Integer
> currentFib = gets current
>
> nextFib :: State FibState Integer
> nextFib = do
>        F p c <- get
>        let n = p+c
>        put (F c n)
>        return n
>
> does that help?
>
> >
> > Thank you,
> > Mike
>
> Cheers,
> Daniel
>
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