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

4 Nov 2011

      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