Lambda the Ultimate

When I heard of Amazon FPS (an overview of which can be found here), I knew a DSL must be hidden somewhere.

And indeed, the GateKeeper language is a special-purpose language for specifying (declarative) payment instructions.

Once again we see the importance of language design skills in today's marketplace...

So what do LtU readers think of the design of the GateKeeper language?

I've been reading and trying to understand First-class Polymorphism with Type Inference (FCPWI); the "monads as first-class values" example in FCPWI was quite interesting. I was wondering if there were any languages that currently implement first-class polymorphism (FCP), and whether they support type inference.

From my searches, it would seem that MLF supports FCP, though MLF is still in its infancy; higher-rank types seem to have the same expressiveness as FCP, but I'm not certain as I'm still a type newbie. If that equivalency is valid, then the Morrow interpreter qualifies. Anything else?

The system described in FCPWI seems interesting, but I haven't seen it implemented anywhere, so I was just wondering what its limitations are.