[Haskell-cafe] Is generic information dumpable?

[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 <v.dijk.bas at gmail.com>

> 2011/11/3 José Pedro Magalhães <jpm at cs.uu.nl>:
> > "-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
>
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