Proposal: add liftA4 and liftA5 to match liftM4 and liftM5

[Michael Snoyman]

David: I think the resistance you're seeing is coming from the fact that-
at least in my experience- liftMN is not considered good, idiomatic Haskell
code, since the idea is expressed *better* by <$> and <*>. There's been a
downside until now that <$> and <*> introduced a different constraint
(Applicative instead of Monad) but, as you already mention, AMP will solve
that.

So- at least for me- adding in liftA2... would be a step *backwards*, since
it encourages people to avoid the idiomatic solution for the non-idiomatic
solution. In fact, I wouldn't be surprised if you'd get less resistance to
the idea of deprecating liftM2... (though please *don't* propose that,
there's no need to break backwards compatibility).

But let me ask something else: why not just change the type signature of
liftM2... to have an Applicative constraint instead of a Monad constraint?
Besides the funny naming, that would seem to address your concern, without
increasing the number of non-idiomatic combinators.

On Thu, Nov 6, 2014 at 6:44 AM, David Feuer <david.feuer at gmail.com> wrote:

> Sure you could, but that would be kind of silly. liftMN should either be
> defined as liftAN or should be defined using the Monad ops as they have in
> the past. I was trying to make Base a little smaller by using the first
> approach, but it's not a big deal to repeat everything with specializations
> and unfoldings.
>
> On Wed, Nov 5, 2014 at 11:14 PM, Carter Schonwald <
> carter.schonwald at gmail.com> wrote:
>
>> umm.... you can use  <*> to define the liftAN operations right? Couldn't
>> you just directly use <*> and pure to define the liftMN ones?
>>
>> On Wed, Nov 5, 2014 at 9:32 PM, David Feuer <david.feuer at gmail.com>
>> wrote:
>>
>>> Well, I'm looking to define  liftM = liftA, liftM2 = liftA2, liftM3 =
>>> liftA3, and (with a modified definition of ap) I'm getting that to work,
>>> but that leaves liftM4 and liftM5 hanging.
>>>
>>> On Wed, Nov 5, 2014 at 9:30 PM, John Lato <jwlato at gmail.com> wrote:
>>>
>>>> Does anyone actually want these?  I would have thought we should go the
>>>> other way and deprecate `liftM3+` in favor of using `<*>`.
>>>>
>>>> On Thu Nov 06 2014 at 10:26:36 AM David Feuer <david.feuer at gmail.com>
>>>> wrote:
>>>>
>>>>> Since Applicative is supposed to be important now, I figure we should
>>>>> get these in.
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