GADTs in the wild

[Yitzchak Gale]

Simon Peyton-Jones wrote:
> This message is to invite you to send me your favourite example of using a
> GADT to get the job done.  Ideally I’d like to use examples that are (a)
> realistic, drawn from practice (b) compelling and (c) easy to present
> without a lot of background.

Last year I presented the following simple puzzle to the
community:

http://www.haskell.org/pipermail/haskell-cafe/2011-February/089719.html

That puzzle actually came up in a real-life software project
in Haskell that I was working on.

Several solutions were submitted that used various
Haskell extensions such as rank N types and generics,
as well as a rather cumbersome Haskell 98 solution.

But in my opinion, by far the best solution, using only
GADTs, was submitted by Eric Mertens:

http://hpaste.org/44469/software_stack_puzzle

Eric's solution could now be simplified even further
using data kinds.

Yitz