At Standard Chartered they are using relations as data structures in their Haskell-like language.
It works very well. (The biggest hurdle for a really nice integration is IMHO stuff like typing the join operator. But even the dynamically-typed only version was useful.)
Yeah, I've heard the praise. I tried to rally Gergo into porting the idea to GHC. (Using the new pluggable constraint solvers in GHC seems like the best bet.)
I still remember when I hadn't had any clue about functional programming, and I was moving from stuff like QBasic and C to Python. I thought dicts were awesome. And they are---by comparison to only having arrays.
Relations do everything dicts do, but you don't have to decide on the structure beforehand. Exactly the same argument that Codd had.
It works very well. (The biggest hurdle for a really nice integration is IMHO stuff like typing the join operator. But even the dynamically-typed only version was useful.)