Proposal: add liftA4 and liftA5 to match liftM4 and liftM5

[David Feuer]

I don't feel strongly about it, but that seems like a reasonable plan.

On Thu, Nov 6, 2014 at 1:31 AM, Michael Snoyman <michael at snoyman.com> wrote:

> 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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