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 <tonymor...@gmail.com> wrote: > 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/ > > > > _______________________________________________ > Haskell-prime mailing list > Haskell-prime@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-prime >
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