I suspect it's possible to do something like that with multi parameter type classes, but I tend to find that trying to do pseudo dependently typed things in Haskell tends to bite me sooner or later. Daniel On 21 December 2011 22:24, Nicolas Trangez wrote:

On Wed, 2011-12-21 at 18:34 +0000, Daniel Waterworth 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

Any way to do something along the lines of

type StateChange a = SC a a

handleChange :: Monad m => StateChange ConnectionState -> m () handleChange (SC Closed Opening) = return () handleChange (SC Opening Closed) = return () handleChange (SC Closed Open) = return ()

where the last line would yield a type checking error at compile time, and the compiler can warn about handleChange not being exhaustive (for the valid state change steps)?

Nicolas

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I must admit, I haven't looked into arrows in a great deal of detail, perhaps I should. Daniel 2011/12/22 Ertugrul Söylemez :

Daniel Waterworth 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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