Interestingly, in the case of dir-traverse,

foldMapA = (fmap fold .) . traverse

ends up being faster than

foldMapA f = getAp . foldMap (Ap . f)

...which I did not expect. I suppose we should benchmark this before adding it to base.

Cheers,
Vanessa McHale

On 5/9/19 12:49 PM, chessai . wrote:

I've also defined this in multiple of my own projects/codebases, and I provided it as a motivation for introducing Data.Monoid.Ap in the first place.

I'm +1 on the inclusion of foldMapA.

On Thu, May 9, 2019, 11:10 AM Matt <parsonsmatt@gmail.com> wrote:

I've personally defined `foldMapA` in at least three private projects, and I've one-off written it probably over a dozen times. Each time I've used something like `fmap k . traverse f` where `k` is one of `mconcat`, `fold`, `join`, etc. I appreciate the subtle discussion on the implementation for performance and I think it'd be awesome to have this defined in `base`.

Matt Parsons

On Tue, May 7, 2019 at 10:36 PM David Feuer <david.feuer@gmail.com> wrote:

On Wed, May 8, 2019, 12:12 AM Bryan Richter <b@chreekat.net> wrote:

Hi David,

At the risk of invoking the gods of Language Blorp, I will note that as a working programmer I know exactly what Applicative, Traversable, and Monoid are (from Vanessa's original proposal), but the unfortunately-named getAp is something I will only learn about begrudgingly.

That seems unfortunate. Learning to use such types is pretty useful. I'd recommend that every Haskell programmer get to know all the types in Data.Monoid and come to an understanding of what they're good for.

What you consider "so simple we don't need to define it" took a rather lengthy email to describe. Are you sure it's not worth actually defining?

So ... that long post was about trying to prove what I intuitively thought *must* be true. In the end, I wasn't quite able to finish the proof, but I did at least manage to convince myself that my intuition was correct. It's true that this sort of intuition takes a certain amount of time to develop. In the case of a really important operation, yeah, we should package it up. But is this operation important enough? I'm not really convinced yet.

If nothing else, the next time someone searches Hoogle for a function matching its type signature, perhaps it will be an opportunity for someone like me to peer beneath the hood and learn something new.

That's valid. But ... there are lots of opportunities for that sort of thing already. Is it worth the API clutter to add another one?

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