State monads, passing an updatable state through a computation.

Some computations may not require the full power of state transformers:

• For a read-only state, see Control.Monad.Trans.Reader.
• To accumulate a value without using it on the way, see Control.Monad.Trans.Writer.

This version is lazy; for a strict version, see Control.Monad.Trans.State.Strict, which has the same interface.