Re: Data.Monoid

26 Oct 2005

      oh. pure haskell98... i forgot that old problem. :/

well, i still miss the arrow-(&&&)-version of that monoid.
but, to newtype it, may be the better solution.

[code]
newtype ParArrow f a b = ParArrow (f a b) -- monoid iff b is monoid
newtype SerArrow f a b = SerArrow (f a b) -- monoid on endomorphisms
instance (Arrow f) => Arrow (ParArrow f) where
  ... -- derived
instance (Arrow f) => Arrow (SerArrow f) where
  ... -- derived
instance (Arrow f, Monoid b) => Monoid (ParArrow f a b) where
  mempty = pure (const mempty)
  mappend a b = (a &&& b) >>> pure (uncurry mappend)
instance (Arrow f) => Monoid (SerArrow f a a) where
  mempty = pure id
  mappend = (>>>)
[/code]

Am Dienstag, 25. Oktober 2005 19:05 schrieb Iavor Diatchki:
...
Hello,
These instances overlap, which is not allowed in Haskell.
-Iavor
On 10/24/05, Marc A. Ziegert  wrote:
...
Excerpt (revised):
[code]
class Sequence m where
  sequence :: [m a] -> m [a]
instance Monad m => Sequence m where
  sequence = Control.Monad.sequence
instance (Arrow f) => Sequence (f a) where
  sequence []     = pure (const [])
  sequence [f]    = f >>> pure (:[])
  sequence (f:fr) = (f &&& sequence fr) >>> pure (uncurry (:))