Can you expand on that, please? Because it seems to me that state fulness propagates up through the system. I can't imagine a system that was pure in the large but side-effecting in the small. I'd be interested to hear how that works...
If only there was an effect system that could guarantee purity and at the same time not be in the way (i.e. allow full or Scala-like type inference). Then purity would be guaranteed by types, like it is in Haskell. (N.B. I'm not asking for Haskell's purity by default. I advocate impure as default, with a type guarantting purity.)