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

Barak A. Pearlmutter barak at pearlmutter.net
Wed Nov 25 19:56:49 UTC 2020


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.


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