Proposal: add liftA4 and liftA5 to match liftM4 and liftM5

[Jacques Carette]

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