DeriveFoldable treatment of tuples is surprising

David Feuer david.feuer at
Tue Mar 21 20:34:20 UTC 2017

This seems much too weird:

*> :set -XDeriveFoldable
*> data Foo a = Foo ((a,a),a) deriving Foldable
*> length ((1,1),1)
*> length $ Foo ((1,1),1)

I've opened Trac #13465 [*] for this. As I write there, I think the
right thing is to refuse to derive Foldable for a type whose Foldable
instance would currently fold over components of a tuple other than
the last one.

I could go either way on Traversable instances. One could argue that
since all relevant components *must* be traversed, we should just go
ahead and do that. Or one could argue that we should be consistent
with Foldable and refuse to derive it.

What do you all think?


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