DeriveFoldable treatment of tuples is surprising

Sven Panne svenpanne at
Wed Mar 22 08:12:27 UTC 2017

2017-03-21 22:29 GMT+01:00 Edward Kmett <ekmett at>:

> [... 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.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <>

More information about the Glasgow-haskell-users mailing list