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
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.
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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