Deriving Functor

[Twan van Laarhoven]

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

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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)'

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