Deriving Data does not work for attached code

[mitar]

For attached code I get this error:

Test2.hs:19:48:
    Could not deduce (Real r, Random r)
      from the context (Data r, Typeable1 t)
      arising from a use of `f'
    Possible fix:
      add (Real r, Random r) to the context of
        the type signature for `dataCast1'
        or the instance declaration
    In the first argument of `gcast1', namely `f'
    In the expression: gcast1 f
    In an equation for `dataCast1': dataCast1 f = gcast1 f

[mitar]

Ups, I attached wrong test code. Please delete that one. I am attaching correct file now.

[mitar]

Correct file for this ticket

[mitar]

I also overwrote first attachment with correct version.

[simonpj]

You have

data (Real r, Show r, Random r, Data r)
  => RandomFoo r = RandomFoo (RandomBar r) 
  deriving (Typeable, Data)

As you'll see if you say -ddump-deriv (I'm improving the layout) you get a Data instance with (Real r, Random r) in the context? Why? Because the instance pattern matches on the constructor.

Solution: simply remove the context from the data declaration, giving

data RandomFoo r = RandomFoo (RandomBar r) 
  deriving (Typeable, Data)

The context (which is a Haskell mis-feature) does you no good, and as you have seen, it does you harm.

Simon