The point is that there are two reasonable ways to do it, and the deriving mechanism, as a rule, does not make choices between reasonable alternatives.
On Tue, Mar 21, 2017 at 5:05 PM, Jake McArthur <jake.mcart...@gmail.com> wrote: > 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-us...@haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
