[Haskell-cafe] State Machine Composition

[Daniel Waterworth]

I must admit, I haven't looked into arrows in a great deal of detail,
perhaps I should.

Daniel

2011/12/22 Ertugrul Söylemez <es at ertes.de>:
> 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/
>
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