[Haskell-cafe] understanding generics

[Anatoly Yakovenko]

i am trying to understand how data.data works.  How would i traverse
the subterms of one type and build another type out of them.
something like this:

> gmapQ (\a -> Right a) (Just "hello")

<interactive>:1:0:
    Inferred type is less polymorphic than expected
      Quantified type variable `d' escapes
    In the first argument of `gmapQ', namely `(\ a -> Right a)'
    In the expression: gmapQ (\ a -> Right a) (Just "hello")
    In the definition of `it':
        it = gmapQ (\ a -> Right a) (Just "hello")


Thanks,
Anatoly