Deprecate Foldable for Either

[Henrik Nilsson]

Hi,

Francesco Ariis wrote:

 > I am not trying to be polemic, just to see where the community
 > stands:

Well, personally, I would not miss neither Foldable nor Traversable
instances of (Either b) or ((,) b) for even a fraction of a second.

And I know many people who fully agrees.

Henning summed it up very well, I think:

https://mail.haskell.org/pipermail/libraries/2016-February/026678.html

I am completely and utterly unconvinced by arguments along the lines
that just because one of the standard types *can* be seen as a container
type it must be seen that way, by everyone, no exceptions. That was
never the intent of these types, as evidenced by their names and as
evidenced by the inherent asymmetry of their instances. Pretending
otherwise just creates confusion, of which there are plenty of 
instances, with more coming as per Andreas' initial mail. Further
this generality buys very little if anything in practice as anyone
who really needs the functionality can define their own instances or,
better, introduce a custom type with a well-chosen name that clearly
reflects the application in question.

So, indeed, "all consistent but consistently useless", as Henning
eloquently put it. To be concrete: after all, why would I write,
for an x of type Either Int, say,

foldl (+) 0 x

and not simply

either (const 0) id x

?

I know what I find more readable, at least, and I'd be willing
to bet on what is more maintainable as well as time passes and
code is passed from one group of maintainers to another.

Yes, I did see:

Kris Nuttycombe wrote:

 > As with all of these discussions, the point of having Either be
 > Foldable is not that you're going to call foldMap on an Either value 
 > directly.

But I must say that strikes me as supremely odd: why on Earth should
one *not* make direct use of instance methods if they make so much sense
as is claimed?

Well, I don't really expect any changes. This is just to say that there
still are lots of people who find the push to instantiate Foldable and
Traversable as widely as possible very unfortunate.

Best,

/Henrik




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