On Friday 28 January 2011 15:13:44 Jan Behrens wrote:
On Sunday 02 January 2011 13:04:30 you wrote:
The patches attached to http://hackage.haskell.org/trac/ghc/ticket/4834 make Applicative a superclass of Monad. Default definitions are provided for backwards compatibility.
I looked at the Prelude of the referenced documentation in the ticket: http://bifunctor.homelinux.net/~bas/doc/ghc/html/libraries/base-4.4.0.0/Pre lude.html
Would it be useful to include liftM and ap in the Prelude, so that you can use it as a default implementation for fmap, when defining Functor instances?
I have to correct myself. It should be "liftA" and "ap", as liftA is more abstract than liftM and can thus be used as a default fmap implementation for BOTH monads and applicative functors.
Then you can write...
instance Functor Something where fmap = liftM
instance Applicative Something where pure x = ... (<*>) = ap
instance Monad Something where (>>=) = ...
This way you don't need to write explicit implementations of fmap and <*> for every monad you define.
The example would read then as follows: instance Functor Something where fmap = liftA instance Applicative Something where pure x = ... (<*>) = ap instance Monad Something where (>>=) = ...
An alternative name for liftM would be fmapMonad...
There is a similar approach in Data.Traversable:
fmapDefault :: Traversable t => (a -> b) -> t a -> t b
However as Functor, Applicative and Monad are part of the Prelude, it might be useful, to include a default implementation for fmap in the Prelude.
Regards Jan
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