Proposal: add liftA4 and liftA5 to match liftM4 and liftM5

[John Lato]

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 <david.feuer at gmail.com> wrote:

> Another point: using `liftA` or `liftM`, specialized to the relevant type,
> may reduce code size in some cases. With f <$> a <*> b <*> c and such, you
> have to hope that you either get some benefit from the inlining or that CSE
> is able to save you from the duplication.
>
> On Fri, Nov 7, 2014 at 7:47 AM, Jacques Carette <carette at mcmaster.ca>
> wrote:
>
>> On 2014-11-07 5:30 AM, Henning Thielemann wrote:
>>
>>>
>>> On Fri, 7 Nov 2014, Andreas Abel wrote:
>>>
>>>  I hope the same happens for sequence, mapM and the like!
>>>>
>>>>  sequence :: (Applicative m) => [m a] -> m [a]
>>>>  sequence = foldr (\ x xs -> (:) <$> 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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