[Haskell-cafe] Mixing own and derived instances with Generic Deriving Mechanism

[Maarten Faddegon]

Dear Pedro, Cafe,

Thanks again for helping me out last December. I have been
playing a bit more with deriving show and now ran into an
interesting problem mixing my own instances with derived
instances. Hope you can enlighten me there!

 > {-# LANGUAGE DeriveGeneric #-}
 > module Test where
 > import GHC.Generics
 > import Generics.Deriving.Show

The Generic Deriving Mechanism adds the keyword 'default' to
class definitions.  With this keyword we can define a
type-generic definition of that method when not given. For
example, if we define our own MyData type, we can derive the
GShow methods:

 > data MyData = MyData MyFancyType deriving Generic
 > instance GShow MyData

We can also still give our own definition, for example if we want
values of the MyFancyType to always be shown as the same string:

 > data MyFancyType = MyFancy1 | MyFancy2 deriving Generic
 > instance GShow MyFancyType where
 >       gshow _ = "Fancy!"

There is something strange here though: when we use gshow
directly on a MyFancyType value our own instance definition is
used, evaluating as expected to "Fancy!".

 > ex1 = gshow MyFancy1

But as soon as we are inside a derived method, we will continue
using derived instances even though we defined our own. The
example below evaluates to "MyData MyFancy1", rather than "MyData
Fancy!":

 > ex2 = gshow (MyData MyFancy1)

The default methods of GShow are defined in terms of methods from
GShow' which operate on the type-representation. From this
representation I do not see a way to recover the information
that a type has a GShow instance.  Am I correct (I hope not :) or
is there a way out?

Cheers,

Maarten Faddegon