Control-Monad-ST2

A variation on the ST monad with two type parameters.

The ST2 monad is like the ST monad, but with finer-grained control over access to mutable state. The phantom type parameters r and w are used to track the read and write dependencies of the computation. If a computation of type ST2 r w a is polymorphic in w then it does not write any external state. If it is also polymorphic in r then it does not read any external state.