[Haskell-cafe] State Machine Composition

[Daniel Waterworth]

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 <nicolas at incubaid.com> 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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