[Haskell-cafe] A practical Haskell puzzle

[Yitzchak Gale]

Thanks to everyone for the nice solutions to this puzzle,
here and on reddit:

http://www.reddit.com/r/haskell/comments/fu6el/a_practical_haskell_puzzle/

There were two basic approaches. One was to use GADTs and
higher-rank types to reduce the amount of type trickery needed.
One nice example is apfelmus' solution here in this thread,
and several people on reddit suggested using use thrists package:

http://hackage.haskell.org/package/thrist

The other approach is to use some kind of generics.

In any case, there does not appear to be any reasonable way
to handle this simple and common situation in Haskell without
extensions. I challenge the Haskell community to add these
extensions to the Haskell standard in Haskell 2012!

Lennart proposed using type-level numbers and reification, but
I'm not sure about the full details of that solution. Does it use
Haskell extensions, and if so, which ones?

Thanks,
Yitz