state package:transformers

Construct a state monad computation from a state transformer function.

Construct a state monad computation from a state transformer function.

Construct a state monad computation from a function. (The inverse of runState.)

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.

Convert an accumulation (append-only) computation into a fully stateful computation.

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

• evalState m s = fst (runState m
  s)

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.

• execState m s = snd (runState 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)

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

• runState (mapState f m) = f . runState
  m

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

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

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.

• withState f m = modify f >> m

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

• withStateT f m = modify f >> m