Haskell Foldable Wats

Manuel Gómez targen at gmail.com
Wed Feb 24 18:27:21 UTC 2016

> El 24 feb 2016, a las 12:27, Chris Allen <cma at bitemyapp.com> escribió:
> You can't not-include the instances because we'll just end up with orphans
> so that's not cricket I think.

On Wed, Feb 24, 2016 at 1:41 PM,  <amindfv at gmail.com> wrote:
> I don't know what this means -- can you elaborate?
> (What I'm proposing is, since there is a sizeable number of people on both
> sides of the issue who don't seem to be coming closer to an agreement, we
> bring a vote *to the users* on whether to provide Foldable/Traversable
> instances for tuples of size 2 and greater. If users say they're useful, we
> keep/add 'em. If they find them confusing/not useful, we remove/don't add
> 'em)

If these instances were not included in base, then these instances
would nonetheless be made available to a large amount of code because
somebody will make a base-orphans library that will define these
instances, and many libraries written by authors who believe these
instances to be useful will depend on this package (or, worse yet,
define their own instances in their own packages which will clash with
each other and break things).  If any of these libraries end up in the
transitive closure of your packages’ dependencies, then you will have
these instances defined, regardless of your opinion of them, and
regardless of their exclusion from base.

To avoid this unhelpful outcome, if the community decided to forbid
these instances, some language extension would have to be designed to
forbid instance definitions.  This has been discussed previously.  If
the community did this, it would break a lot of code that does use
these instances, and there would be no workaround, as forbidding
instances would have to be as global as defining instances.  Changing
the fundamental property of type class instances that makes them not
opt-in/opt-out, but automatically imported from transitive
dependencies, would remove one of the properties of the language that
(as Edward often argues) is a significant part of what makes Haskell
more useful and healthy than some of its kin.

This is not a matter that can be resolved but by consensus, at least
not with the solutions that have been thus far proposed (simply
removing the instances or accepting the instances as they are).  We do
not appear to be approaching consensus on these particular solutions.
Accepting the instances as they are does have the benefit of already
being implemented and being used by a lot of code (but «we’ve always
done things this way» is not a good design criterion).  Adding more
instances for tuples would make the current situation more consistent,
although perhaps proponents of removing these instances would prefer
the current status quo: inconsistency, but less instances whose
existence they reject.

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