Re: [Haskell-cafe] Is generic information dumpable?

Thanks José! Will this make it into ghc-7.4? Bas 2011/11/4 José Pedro Magalhães :

Hi, Now, for the following datatype: data X a = X { myX :: a } deriving Generic You get the following -ddump-deriv output: ==================== Derived instances ==================== Derived instances:   instance GHC.Generics.Generic (Temp.X a_adY) where     GHC.Generics.from (Temp.X g1_aeG)       = GHC.Generics.M1           (GHC.Generics.M1 (GHC.Generics.M1 (GHC.Generics.K1 g1_aeG)))     GHC.Generics.to       (GHC.Generics.M1 (GHC.Generics.M1 (GHC.Generics.M1 (GHC.Generics.K1 g1_aeH))))       = Temp.X g1_aeH

  instance GHC.Generics.Datatype Temp.D1X where     GHC.Generics.datatypeName _ = "X"     GHC.Generics.moduleName _ = "Temp"

  instance GHC.Generics.Constructor Temp.C1_0X where     GHC.Generics.conName _ = "X"     GHC.Generics.conIsRecord _ = GHC.Types.True

  instance GHC.Generics.Selector Temp.S1_0_0X where     GHC.Generics.selName _ = "myX"

Generic representation:

  Generated datatypes for meta-information:     Temp.D1X     Temp.C1_0X     Temp.S1_0_0X

  Representation types:     Temp.Rep_X = GHC.Generics.D1                    Temp.D1X                    (GHC.Generics.C1                       Temp.C1_0X                       (GHC.Generics.S1 Temp.S1_0_0X (GHC.Generics.Par0 a_adY))) Still not perfect, in that the representation type should really appear as a type instance inside the Generic instance, but at least all the important information is printed.

Cheers, Pedro

2011/11/3 Bas van Dijk

...

2011/11/3 José Pedro Magalhães :

...

"-ddump-deriv" will print (most of) it.

But it doesn't print the most useful piece of information: the definition of Rep.

It would be great if this could be added.

Currently when I have a type that I want to know the Rep of, say:

data Foo = Bar Int         | Boo {hello :: String}           deriving Generic

I just convert it to a Rep and show it:

err = show $ from $ Boo "World"

However Reps don't have Show instances so GHC complains:

No instance for  (Show (D1 D1Foo (   C1 C1_0Foo (S1 NoSelector (Rec0 Int))                  :+: C1 C1_1Foo (S1 S1_1_0Foo (Rec0 String))                  )         x0        )  )  arising from a use of `show'

And there you go. This is the only time when I'm happy to see an error message :-)

Bas