StateT package:monadology

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.

Dangerous, because the MVar won't be released on exception.

Evaluate a state computation with the given initial state and return the final value, discarding the final state.

• evalStateT m s = liftM fst
  (runStateT m s)

Evaluate a state computation with the given initial state and return the final state, discarding the final value.

• execStateT m s = liftM snd
  (runStateT m s)

Run the StateT on an MVar, taking the initial state and putting the final state.

Map both the return value and final state of a computation using the given function.

• runStateT (mapStateT f m) = f .
  runStateT m

withStateT f m executes action m on a state modified by applying f.

• withStateT f m = modify f >> m