I've briefly gone through the Ur demo at http://impredicative.com/ur/demo/

Ur looks very impressive, so the natural question I'm asking myself is:  How does it stack up against haskell frameworks, and why can't Ur be implemented in Haskell?  

I'm thinking mainly of the safety guarantees, not necessarily the performance guarantees, GC-less execution, or even non-lazy evaluation.

And I can't answer those.. any takers?

Alexander