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

17 Jul 2009


      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 (,)?

    class Functor f => Applicative f where

        pure :: a -> f a
        pair :: f a -> f b -> f (a,b)
        (<*>) :: f (a -> b) -> f a -> f b

        (<*>) = fmap (uncurry ($)) . pair
        pair fa fb = pure (,) <*> fa <*> fb

Best wishes,
Wolfgang

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

Wolfgang Jeltsch