Proposal: Add bifunctor related classes to base

[João Cristóvão]

> The previous discussion about methods on Either had some mention of
adding bifunctors to base, but no one wrote up the details

I was implicitly leaving that to someone more qualified than me ;)

This proposal looks great to me, and I agree that it is the right
abstraction for functions like mapLeft and such, instead of Arrows.
Still, some of the underlying assumptions made are not entirely clear to me:

> If someone goes into the documentation for Data.Either looking for a way
to map both parameters,
> they will not, of course, be directed to the bifunctors package, even
though it provides a good means of doing what they want.

So, what you are proposing is that mapLeft and such are _not_ standalone
functions in Data.Either, but instead we just add documentation refering
them to bifunctors as the correct abstraction, a merge of my "Proposal 1"
with "Proposal 8". Although I'm much more confortable with this than
forwarding user to arrows, I still see a point in adding this basic
functions in Data.Either.

It is my understanding that Data.List has a very complete API mainly for
historical reasons, and that nowadays it is recomended to use Foldable and
Traversable for many of those operations. The same applies to Data.Maybe.
But a novice user will then look at Data.Either and Data.Tuple, and think:
are these second class citizens?
I guess this is an entirely different discussion, that happend many times
in the past with no relevant consensus...

An additional question: would the arrow operators then be defined in terms
of bifunctors?

Cheers



2014-04-26 8:31 GMT+01:00 Andreas Abel <andreas.abel at ifi.lmu.de>:

> I retract my bikeshedding attempt.
>
> On 25.04.14 8:18 PM, Dan Doel wrote:
>
>> ​In my code, I use (<&>) from lens when I need a flipped fmap:
>>
>>      let ys = xs <&> \x -> ...
>>
>> and do use Applicative for.
>>
>
> Maybe & and <&> become standard at some point, and then I am maybe fine to
> use them.  Only that "<&>", unlike "for", does not evoke any association to
> iteration in my brain.
>
>
> --
> Andreas Abel  <><      Du bist der geliebte Mensch.
>
> andreas.abel at ifi.lmu.de
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