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

[Serguey Zefirov]

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.

This is somewhat strange situation.

Was it a design decision?