On Fri, 2010-04-02 at 14:51 -0400, Christopher Lane Hinson wrote:
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,
What I meant was:
instance ArrowLoop a => ArrowLoop (SF a) where loop (SF f) = SF $! proc (dyn, b, r, s) -> do rec ((c, d), s, f') <- f -< (dyn, (b, d), r, s) returnA -< (c, s, loop f')
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
Hmm. I guess to avoid (some) problems:
class (Monad m, Arrow a) => ArrowKleisli m a where liftMonad ∷ (b -> m c) -> a c liftMonad = liftKleisli . Kleisli liftKleisli ∷ Kleisli m b c -> a b c liftKleisli = liftMonad . runKleisli
Given:
instance ArrowKleisli CHP (Kleisli CHP) instance ArrowKleisli IO (Kleisli CHP)
And:
someFunc = liftIO . print :: (Show b, MonadIO m) => b -> m ()
Which liftIO is run in:
liftMonad someFunc :: Kleisli CHP String ()
ghci> :t liftMonad (someFunc) :: Kleisli CHP String () <interactive>:1:0: No instance for (ArrowKleisli m (Kleisli CHP)) arising from a use of `liftMonad' at <interactive>:1:0-19 Possible fix: add an instance declaration for (ArrowKleisli m (Kleisli CHP)) In the expression: liftMonad (someFunc) :: Kleisli CHP String () As IO is popular it is particularly likely to run into this problem. Adding:
class Arrow a ⇒ ArrowIO a where liftAIO ∷ (b → IO c) → a b c
instance ArrowKleisli IO a ⇒ ArrowIO a where liftAIO = liftMonad
Solves problem: ghci> :t liftAIO someFunc :: Kleisli CHP String () liftAIO someFunc :: Kleisli CHP String () :: Kleisli CHP String () Regards