[Haskell-cafe] Template Haskell sees into abstract data types

[Simon Peyton-Jones]

|  Data.Map.Map and Data.Set.Set are exported abstractly, without
|  exposing knowledge about their internal structure.
|  
|  I cannot directly create my own class instances for them because of
|  that. But I found that I can write Template Haskell code that could do
|  that - those data types could be reified just fine.

I've created a ticket for this http://hackage.haskell.org/trac/ghc/ticket/4222.  

In writing it down I identify several design questions that need to be addressed before we can "fix" this bug.  Help welcome.

Simon