Haskell is an interesting playground for this since it tries so hard to make "pure" practical. Interestingly, they achieve this to a large degree using monads.

Haskell is an interesting playground for this since it tries so hard to make "pure" practical. Interestingly, they achieve this to a large degree using monads. I think that idea is fairly generalizable, though, that pure formalisms can embed impure ones as models and then have those be executed by an RTS.