On Fri, 2 Apr 2010, Maciej Piechotka wrote:
1. How to interpret ArrowLoop? I have two possible implementations:
type RunSF a = a Dynamic ()
data SF a b c = SF (a (Dynamic, b, RunSF, Set Unique) (c, Set Unique, SF a b c))
(...)
instance ArrowLoop (SF a) where loop (SF f) = loop' f undefined where loop' g d = proc (dyn, b, r, s) -> do ((c, d'), s, g') <- g <- (dyn, (b, d), r, s) returnA -< (c, s, loop' g' d')
instance ArrowLoop a => ArrowLoop (SF a) where loop (SF f) = SF $! proc (d, b, r, s) -> do rec ((c, d), s, f') <- f -< (d, (b, d), r, s) returnA -< (c, s, loop f')
Neither of these compile through my eyeball, but I don't think it should be possible for SF to be an Arrow-anything unless 'a' is also.
2. Why there is no ArrowIO in arrows? I.e.
class Arrow a => ArrowIO a where liftAIO :: Kleisli IO b c -> a b c
(possibly
class Arrow a => ArrowST a where liftAST :: Kleisli ST b c -> a b c )
It would only be a convenience typeclass, and in that case why not just have a generic ArrowKleisli with: (i -> m o) -> a i o
3. Why switch is needed? How to interpret switch with current continuation?
I think switch is equivalent to ArrowChoice but do I miss something?
They are not equivalent. A switch, roughly, provides a way to persistently replace a running segment of a program with a different program. ArrowChoice is just a way of implementing if-then-else flow control in an Arrow, which might be useful, but is not the point of FRP. Imagine a light switch that remains on or off after you toggle it, compared to a pressure switch that requires constant supervision. Friendly, --Lane