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 <david.fe...@gmail.com> wrote: > 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 >
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