On Mon, Jun 29, 2009 at 4:31 PM, Ross Paterson
On Mon, Jun 29, 2009 at 02:37:56PM -0400, David Menendez wrote:
How about liftA2?
2 is a scary number. Do you have an example in mind where a customized liftA2 would be a big win?
I don't know about a big win, but my preference for Applicative has always been to define <*> and liftA2 as co-primitives, like so: class Functor f => Applicative f where pure :: a -> f a liftA2 :: (a -> b -> c) -> f a -> f b -> f c (<*>) :: f (a -> b) -> f a -> f b (<*>) = liftA2 ($) liftA2 f a b = fmap f a <*> b This is how the old arrows package defined Sequence, one of the precursors to Applicative., and it's analogous to the argument that
= and join should both be members of Monad.
Back in 2006 I pointed out that a custom liftA2 would be sufficient to
fix frisby's problems with *> and <*.
http://www.haskell.org/pipermail/libraries/2006-October/005902.html
Specifically, if liftA2 is defined like so:
liftA2 f (P a) (P b) = P $ PMap (uncurry f) (Then a b)
and (*>) is defined using liftA2,
(*>) = liftA2 (const id)
then (*>) is naturally the same as Frisby's custom (->>), without the
need for a custom definition.
--
Dave Menendez