State package:mtl

State monads. This module is inspired by the paper Functional Programming with Overloading and Higher-Order Polymorphism, Mark P Jones (http://web.cecs.pdx.edu/~mpj/) Advanced School of Functional Programming, 1995.

type State s = StateT s Identity

mtl Control.Monad.State.Lazy Control.Monad.State.Strict

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.

state :: MonadState s m => (s -> (a, s)) -> m a

mtl Control.Monad.State.Class Control.Monad.State.Lazy Control.Monad.State.Strict

Embed a simple state action into the monad.

newtype StateT s (m :: Type -> Type) a

mtl Control.Monad.State.Lazy Control.Monad.State.Strict

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.

StateT :: (s -> m (a, s)) -> StateT s (m :: Type -> Type) a

mtl Control.Monad.State.Lazy Control.Monad.State.Strict

class Monad m => MonadState s m | m -> s

mtl Control.Monad.State.Class Control.Monad.State.Lazy Control.Monad.State.Strict

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

evalState :: State s a -> s -> a

mtl Control.Monad.State.Lazy Control.Monad.State.Strict

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)
```

evalStateT :: Monad m => StateT s m a -> s -> m a

mtl Control.Monad.State.Lazy Control.Monad.State.Strict

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)

execState :: State s a -> s -> s

mtl Control.Monad.State.Lazy Control.Monad.State.Strict

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)
```

execStateT :: Monad m => StateT s m a -> s -> m s

mtl Control.Monad.State.Lazy Control.Monad.State.Strict

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)

mapState :: ((a, s) -> (b, s)) -> State s a -> State s b

mtl Control.Monad.State.Lazy Control.Monad.State.Strict

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

runState (mapState f m) = f . runState
m

mapStateT :: (m (a, s) -> n (b, s)) -> StateT s m a -> StateT s n b

mtl Control.Monad.State.Lazy Control.Monad.State.Strict

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

runStateT (mapStateT f m) = f .
runStateT m

runState :: State s a -> s -> (a, s)

mtl Control.Monad.State.Lazy Control.Monad.State.Strict

Unwrap a state monad computation as a function. (The inverse of state.)

runStateT :: StateT s m a -> s -> m (a, s)

mtl Control.Monad.State.Lazy Control.Monad.State.Strict

withState :: (s -> s) -> State s a -> State s a

mtl Control.Monad.State.Lazy Control.Monad.State.Strict

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

```
withState f m = modify f >> m
```

withStateT :: forall s (m :: Type -> Type) a . (s -> s) -> StateT s m a -> StateT s m a

mtl Control.Monad.State.Lazy Control.Monad.State.Strict

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

```
withStateT f m = modify f >> m
```