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
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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