Hi Maarten,

The problem here is that your instance of GShow MyFancyType defines gshow, but the

function that is defined generically is actually gshowsPrec, with the others being given

defaults. For this to work as you'd expect it to, you have to define gshowsPrec in the

instance GShow MyFancyType. This is a bit unfortunate, but because gshowsPrec has

a generic default, it cannot have the usual default (like showsPrec does).

Cheers,

Pedro

On Sat, Feb 1, 2014 at 12:11 PM, Maarten Faddegon <haskell-cafe@maartenfaddegon.nl> wrote:

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