[Haskell-cafe] State Machine Composition

[Ertugrul Söylemez]

Daniel Waterworth <da.waterworth at gmail.com> wrote:

> I made this simple state machine combinator library today. I think it
> works as a simple example of a good use for GADTs.
>
> https://gist.github.com/1507107

Aren't your examples all special cases of the generic automaton arrow?
There are two ways to represent it, both with their advantages and
disadvantages:

    newtype Auto a b = Auto (a -> (b, Auto a b))

    countFrom :: Int -> Auto a Int
    countFrom n = Auto (\_ -> (n, countFrom (succ n)))

or:

    data Auto a b = forall s. Auto s ((a, s) -> (b, s))

    countFrom :: Int -> Auto a Int
    countFrom n0 = Auto n0 (\(_, s) -> (s, succ s))

These state machines have local state and can be composed using
applicative and arrow interfaces:

    liftA2 (+) (countFrom 3) (countFrom 5)

    proc x -> do
        n1 <- countFrom 10 -< ()
        n2 <- someOtherMachine -< x
        anotherMachine -< n1 + n2


Greets,
Ertugrul


-- 
nightmare = unsafePerformIO (getWrongWife >>= sex)
http://ertes.de/
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 836 bytes
Desc: not available
URL: <http://www.haskell.org/pipermail/haskell-cafe/attachments/20111222/18e2fbfb/attachment.pgp>