2017-03-21 22:29 GMT+01:00 Edward Kmett
[... In general I think the current behavior is the least surprising as it "walks all the a's it can" and is the only definition compatible with further extension with Traversable. [...]
OTOH, the current behavior contradicts my intuition that wrapping a type into data/newtype plus using the deriving machinery is basically a no-op (modulo bottoms etc.). When I e.g. wrap a type t, I would be very surprised if the Eq/Ord instances of the wrapped type would behave differently than the one on t. I know that this is very handwavy argument, but I think the current behavior is *very* surprising. Somehow the current behavior seems to be incompatible with the FTP, where pairs are given a special treatment (if that't the right/intuitive choice is a completely different topic, though). Given the fact that "deriving Foldable" is quite old and therefore hard to change, I would at least suggest a big, fat warning in the documentation, including various examples where intuition and implementation do not necessarily meet.