[Haskell-cafe] Textbook example of instance Foldable ((,)a)

[Barak A. Pearlmutter]

True. And their use in Arrow is a great example of pairs that do *not*
work well as instances of Foldable as currently defined. Arrow is
using pairs as abstract Cartesian products, and the natural
isomorphism A×B≅B×A is used implicitly all the time. The Foldable
definition for pairs breaks that symmetry inappropriately.

Cheers,

--Barak.