Hi,
indeed, this is called "ap" in Control.Monad. So if we have an instance of
Monad, all that needs to be done to support the other instances is:
instance (SameContextAsTheMonadInstance) => Functor MyType where fmap =
liftM
instance (SameContextAsTheMonadInstance) => Applicative MyType where pure =
return; (<*>) = ap
Furthermore, this is only in the cases where we are defining the type from
scratch, and not using a library like monadLib or MTL, otherwise a simple
"deriving" is sufficient.
-Iavor
On Wed, Jan 5, 2011 at 12:29 PM, Tony Morris
On 06/01/11 04:58, Isaac Dupree wrote:
Tony, you're missing the point... Alexey isn't making a complete patch to GHC/base libraries, just a hacky-looking demonstration. Alexey is saying that in a class hierarchy (such as if Functor => Monad were a hierarchy, or for that matter "XFunctor"=>"XMonad" or Eq => Ord), it is still possible to define the superclass functions (fmap) in terms of the subclass functions (return and >>=) (such as writing a functor instance in which "fmap f m = m >>= (return . f)"). This has always been true in Haskell, it just might not have been obvious.
_______________________________________________ Haskell-prime mailing list Haskell-prime@haskell.org http://www.haskell.org/mailman/listinfo/haskell-prime Oh right sorry. I thought a stronger point was being made.
Then perhaps it's also worth pointing out that (<*>) can be written using (>>=) and return: f <*> a = f >>= \ff -> a >>= \aa -> return (ff aa)
-- Tony Morris http://tmorris.net/
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