[norswap]

It looks like Oz [1] declarative variables could help implement a lot of this stuff. Essentially, its variables are pointers that can be assigned to only once. As such you can pass an unbound variable to a function and have it bound to produce a result.

You can also do "unification" between two variables, meaning that it will try to make the two variables equal to each other. Typically, one or both of the two variables hold a partial data structure - a data structure that references unbound variables - and it will binds some of these variables to make the unified variables compatible, if possible.

[1] http://en.wikipedia.org/wiki/Oz_%28programming_language%29

[aplummer]

I was also lost at functor...

[norswap]

Which is not mentioned in my comment or in the wikipedia page :)

But I get your point. Oz is a nice language which suffers from poor marketing and lack of support. It's a really lovely language though. It's the closest take I've seen so far of enabling expressibility by using a small number of orthogonal primitives. That's basically all the "multi-paradigm" talk is about.

If someone is interested to seriously hack on Oz, I think Peter Van Roy [1] would be interested, esp. if you can pitch it as a research project somehow (easier to get funding). I can make introductions if necessary.

[1] http://www.info.ucl.ac.be/~pvr/cvvanroy.html