Proposal: add liftA4 and liftA5 to match liftM4 and liftM5

[David Feuer]

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 <carette at 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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