State package:relude

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.

Embed a simple state action into the monad.

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.

Minimal definition is either both of get and put or just state

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)

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

Alias for flip evalState. It's not shorter but sometimes more readable. Done by analogy with using* functions family.

Alias for flip evalStateT. It's not shorter but sometimes more readable. Done by analogy with using* functions family.

Alias for flip execState. It's not shorter but sometimes more readable. Done by analogy with using* functions family.

Alias for flip execStateT. It's not shorter but sometimes more readable. Done by analogy with using* functions family.

Shorter and more readable alias for flip runState.

Shorter and more readable alias for flip runStateT.

>>> usingStateT 0 $ put 42 >> pure False
(False,42)