Deriving Data does not work for attached code

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

Trac metadata

Trac field	Value
Version	7.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	mmitar@gmail.com
Operating system
Architecture