Hello Ghc people, I have been working on adding support for "derive Functor" to ghc. I have a patch that mostly works, but there are some problems: 1. Ghc reports the correct instance when I use -ddump-deriv:, but that instance is not subsequenctly used. Instead I get a warning like: Warning: No explicit method nor default method for `fmap' In the instance declaration for `Functor (Cont r)' What could be going on here? The full output is attached. 2. As far as I understand the code, there would need to be three traversals of a data type T to derive an instance Functor T: - First to determine whether the deriving should be allowed at all (checkSideConditions) - Then to determine the constraints needed (mk_data_eqn) - Finally to make the body of the instance (gen_Functor_binds) This seems a bit redundant, and currently the code only does the last pass. It would make more sense to report any errors during this pass as well. I don't understand how the constraints are determined in TcDeriv, for instance for the type data T1 a b c = T1 a deriving (Eq) it seems as if Eq constraints are added for all of {a,b,c}, but somehow only a constraint Eq a ends up in the final instance. How does that work? The (experimental) patch was too large to attach, it can be found at http://twan.home.fmf.nl/files/deriv-functor-experimental1.patch.gz By the way: is there any advantage (besides readability) to generating the code "fmap f" instead of "\x -> fmap f x"? Twan E:\Dev\ghc\ghc-head3\testsuite\tests\ghc-regress\deriving\should_run>e:\dev\ghc\ghc-head3\ghc\stage1-inplace\ghc -c drvrun-functor2.hs -fforce-recomp -ddump-deriv ==================== Derived instances ==================== InstInfo: forall r_aff. GHC.Base.Functor (Main.Cont r_aff) { GHC.Base.fmap f_afU (Main.Cont a1_afW) = Main.Cont (\ b1_afY -> a1_afW (\ b2_ag0 -> b1_afY (f_afU b2_ag0))) } InstInfo: forall r_afj w_afk. GHC.Base.Functor (Main.ReaderWriter r_afj w_afk) { GHC.Base.fmap f_ag4 (Main.RW a1_ag6) = Main.RW (\ b1_ag8 -> case a1_ag6 b1_ag8 of { ((a1_aga, a2_agc)) -> (f_ag4 a1_aga, a2_agc) }) } drvrun-functor2.hs:8:14: Warning: No explicit method nor default method for `fmap' In the instance declaration for `Functor (ReaderWriter r w)' drvrun-functor2.hs:11:14: Warning: No explicit method nor default method for `fmap' In the instance declaration for `Functor (Cont r)'