Lambda the Ultimate

Sorry if this is OT, but I can't for the life of me figure out which paper I saw this in. I'm trying to gather some papers on the expression problem, and I recall one paper that was not specifically about the expression problem, but that mentioned a solution, hence my difficulty finding it.

The paper dealt with type classes, and one of the subsections specifically mentioned that the structure described in the paper incidentally implied a solution to the expression problem using type classes. So the mention of the expression problem was only an aside; not a footnote, it was discussed in a subsection of the paper itself. I don't recall if the solution described was actually novel, but IIRC the discussion in the paper implied that it was.

I remember the paper was in a two-column format if that helps, and I believe it was published after 2005, but I can't be certain. I'm almost certain I have it somewhere on my hard drive, but I haven't been able to find it in all my manual inspections, greping and file system searching. I'm pretty sure I came across it in the past 6 months, a year at the most, so it may have been posted to LtU. It doesn't seem to be on any list of links describing solutions to the expression problem, like this one.

If anyone has any clue, I'd very much appreciate a heads up!

The site for DSL'09, which took place two months ago in Oxford, is a treasure trove for DSL fans. While the blog format may be a bit confusing for a conference website, you should be able to find your way around to links to slides and paper versions of the presentations. Even better, the posts include various comments people made about the papers (each talk was followed by a comment from a discussant). Apparently one can even join in and post comments!

So tell us: What item caught your attention? Which paper should everyone here rush to read? Which DSL is downloadable and worth downloading?