Re: Proposal: add liftA4 and liftA5 to match liftM4 and liftM5

I am a bit alert about this discussion because it seems that we have quite different ideas about how the AMP implementation should affect the base libraries. 1. Where can we see and discuss the proposed changes? Not on https://www.haskell.org/haskellwiki/Functor-Applicative-Monad_Proposal Not on https://ghc.haskell.org/trac/ghc/ticket/9586 2. Imho, the reasonable thing is to rewrite all of F/A/M base functions such that they use the minimal F/A/M constraints. This of course includes liftM :: (Functor m) => (a -> b) -> m a -> m b liftM2 :: (Applicative m) => (a -> b -> c) -> m a -> m b -> m c and sequence and friends even if the M postfix can then "only explained historically" (HT). One can say "M" stands for "effectful", but the minimal type class to realize the effect is chosen from F/A/M. If we burn bridges, we should do it properly. Cheers, Andreas On 07.11.2014 20:35, Edward Kmett wrote:

I don't want them for rewriting liftM4 and liftM5, I want them in their own right.

It doesn't do anyone any good to just have random asymmetries in the API like that.

It just means a user goes to reach for a tool, doesn't find it and flails around.

-Edward

On Fri, Nov 7, 2014 at 1:37 PM, John Lato mailto:jwlato@gmail.com> wrote:

I still don't think it's worth adding liftA4 and liftA5 just so that liftM4+ can be rewritten.

Very weakly -0.1

On Fri Nov 07 2014 at 10:24:27 AM David Feuer mailto:david.feuer@gmail.com> wrote:

Another point: using `liftA` or `liftM`, specialized to the relevant type, may reduce code size in some cases. With f <$> a <*> b <*> c and such, you have to hope that you either get some benefit from the inlining or that CSE is able to save you from the duplication.

On Fri, Nov 7, 2014 at 7:47 AM, Jacques Carette mailto:carette@mcmaster.ca> wrote:

On 2014-11-07 5:30 AM, Henning Thielemann wrote:

On Fri, 7 Nov 2014, Andreas Abel wrote:

I hope the same happens for sequence, mapM and the like!

sequence :: (Applicative m) => [m a] -> m [a] sequence = foldr (\ x xs -> (:) <$> x <*> xs) (pure [])

Actually, this is an example, where liftA2 shows its advantage:

sequence = foldr (liftA2 (:)) (pure [])

This looks much clearer to me than decoding the mixture of infix and uninfixed infix operators. It simply says, that 'sequence' is like 'id = foldr (:) []' but with everything lifted to an applicative functor.

I agree. I have lots of code which looks really clean because I can use liftA2 (and even liftA3) in exactly the way above. Having to eta expand everything obscures the real meat of what is going on.

Jacques

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