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.
A state monad parameterized by the type s of the state to carry. The return function leaves the state unchanged, while >>= uses the final state of the first computation as the initial state of the second.
A state transformer monad parameterized by: * s - The state. * m - The inner monad. The return function leaves the state unchanged, while >>= uses the final state of the first computation as the initial state of the second.
Construct a state monad computation from a state transformer function.
Construct a state monad computation from a function. (The inverse of runState.)
Evaluate a state computation with the given initial state and return the final value, discarding the final state. *  m s = fst (runState m >
Evaluate a state computation with the given initial state and return the final value, discarding the final state. *  m s = liftM fst > (runStateT m
Evaluate a state computation with the given initial state and return the final state, discarding the final value. *  m s = snd (runState m >
Evaluate a state computation with the given initial state and return the final state, discarding the final value. *  m s = liftM snd > (runStateT m
Map both the return value and final state of a computation using the given function. *  (mapState f m) = f . runState
Map both the return value and final state of a computation using the given function. *  (mapStateT f m) = f . > runStateT
Unwrap a state monad computation as a function. (The inverse of state.)
withState f m executes action m on a state modified by applying f. *  f m = modify f >>
withStateT f m executes action m on a state modified by applying f. *  f m = modify f >>