This is getting ridiculous. The main problem with the `Foldable` instance for tuples seems to be that it's asymmetric (takes only the second element into account). Yet tuples are already asymmetric given their `Functor` instance! No one seems to complain about that although for a beginner it's a total WAT (judging from my own experience and people that I explained Haskell to). Why does `succ <$> (1, 2)` evaluate to `(1, 3)` and not `(2, 3)`? Current definitions are entirely consistent, and consistency is the most valuable property.

And given that `Traversable` gives a rise to a `Foldable` similarly to how a `Monad` gives a rise to an `Applicative`, it is the same stupid mistake not to reflect this relation in the type system.

The ship has sailed, but not when the `Foldable` instance defined. It has sailed when the `Functor` instance was defined, everything else is a follow-up. Just get over it already.

On Thu, Feb 25, 2016 at 11:28 AM, Jeremy <voldermort@hotmail.com> wrote:
Several suggestions have been made in this thread. The one with the highest
cost-to-benefit ratio would seem to be removing the Foldable constraint from
Traversable, as it is simply not required by the Traversable definition.
Lennart actually tried recompiling base with this "and it hardly needed any
changes".

I suggest that we consider this change first, as it brings the benefit of
allowing Traversable types without having to define possibly nonsensical
Foldable instances, and according to Lennart's experiment, has trivial
impact.

Lennart, what changes did you have to make? Anybody care to recompile their
pet project with this change and see if there's any significant impact?



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