I think it's a question of what one considers consistent. Is it more
consistent to treat tuples as transparent and consider every component with
type `a`, or is it more consistent to treat tuples as opaque and reuse the
existing Foldable instance for tuples even if it might cause a compile time
error?
On Tue, Mar 21, 2017, 4:34 PM David Feuer
This seems much too weird:
*> :set -XDeriveFoldable *> data Foo a = Foo ((a,a),a) deriving Foldable *> length ((1,1),1) 1 *> length $ Foo ((1,1),1) 3
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?
[*] https://ghc.haskell.org/trac/ghc/ticket/13465 _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users