[Haskell-cafe] Ur vs Haskell

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