Re: proposal #3335: make some Applicative functions into methods, and split off Data.Functor

On Fri, Jul 17, 2009 at 9:56 AM, Wolfgang Jeltsch wrote:

Am Dienstag, 30. Juni 2009 00:01 schrieb David Menendez:

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On Mon, Jun 29, 2009 at 4:31 PM, Ross Paterson wrote:

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On Mon, Jun 29, 2009 at 02:37:56PM -0400, David Menendez wrote:

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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

Wouldn’t it make more sense to define (<*>) and pair as co-primitives, where pair = liftA2 (,)?

Defining pair in terms of liftA2 is simpler than defining liftA2 in terms of pair. pair = liftA2 (,) -- just partial application liftA2 f a b =uncurry f <$> pair a b -- creates and destroys an intermediate representation -- Dave Menendez http://www.eyrie.org/~zednenem/