| Safe Haskell | Safe |
|---|---|
| Language | Haskell2010 |
State
Contents
Synopsis
- type State s = StateT s Identity
- runState :: State s a -> s -> (a, s)
- evalState :: State s a -> s -> a
- execState :: State s a -> s -> s
- mapState :: ((a, s) -> (b, s)) -> State s a -> State s b
- withState :: (s -> s) -> State s a -> State s a
- newtype StateT s (m :: * -> *) a = StateT {
- runStateT :: s -> m (a, s)
- evalStateT :: Monad m => StateT s m a -> s -> m a
- execStateT :: Monad m => StateT s m a -> s -> m s
- mapStateT :: (m (a, s) -> n (b, s)) -> StateT s m a -> StateT s n b
- withStateT :: (s -> s) -> StateT s m a -> StateT s m a
- class Monad m => MonadState s (m :: * -> *) | m -> s where
- gets :: MonadState s m => (s -> a) -> m a
- modify :: MonadState s m => (s -> s) -> m ()
- modify' :: MonadState s m => (s -> s) -> m ()
- class (MonadState s m, MonadState t n) => Zoom (m :: * -> *) (n :: * -> *) s t | m -> s, n -> t, m t -> n, n s -> m
- zoom :: Zoom m n s t => LensLike' (Zoomed m c) t s -> m c -> n c
- type family Zoomed (m :: * -> *) :: * -> * -> *
State
type State s = StateT s Identity #
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.
Arguments
| :: State s a | state-passing computation to execute |
| -> s | initial state |
| -> (a, s) | return value and final state |
Unwrap a state monad computation as a function.
(The inverse of state.)
Arguments
| :: State s a | state-passing computation to execute |
| -> s | initial value |
| -> a | return value of the state computation |
Arguments
| :: State s a | state-passing computation to execute |
| -> s | initial value |
| -> s | final state |
StateT
newtype StateT s (m :: * -> *) a #
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.
Instances
evalStateT :: Monad m => StateT s m a -> s -> m a #
Evaluate a state computation with the given initial state and return the final value, discarding the final state.
evalStateTm s =liftMfst(runStateTm s)
execStateT :: Monad m => StateT s m a -> s -> m s #
Evaluate a state computation with the given initial state and return the final state, discarding the final value.
execStateTm s =liftMsnd(runStateTm s)
withStateT :: (s -> s) -> StateT s m a -> StateT s m a #
withStateT f mm on a state modified by
applying f.
withStateTf m =modifyf >> m
MonadState
class Monad m => MonadState s (m :: * -> *) | m -> s where #
Minimal definition is either both of get and put or just state
Methods
Return the state from the internals of the monad.
Replace the state inside the monad.
state :: (s -> (a, s)) -> m a #
Embed a simple state action into the monad.
Instances
| MonadState s m => MonadState s (MaybeT m) | |
| MonadState s m => MonadState s (ListT m) | |
| MonadState s m => MonadState s (LogicT m) | |
| MonadState s m => MonadState s (ListT m) | |
| (Functor m, MonadState s m) => MonadState s (Free m) | |
| (Monoid w, MonadState s m) => MonadState s (WriterT w m) | |
| (Monoid w, MonadState s m) => MonadState s (WriterT w m) | |
| Monad m => MonadState s (StateT s m) | |
| Monad m => MonadState s (StateT s m) | |
| MonadState s m => MonadState s (IdentityT m) | |
| MonadState s m => MonadState s (ExceptT e m) | Since: mtl-2.2 |
| (Error e, MonadState s m) => MonadState s (ErrorT e m) | |
| (Functor f, MonadState s m) => MonadState s (FreeT f m) | |
| (Stream s, MonadState st m) => MonadState st (ParsecT e s m) | |
| MonadState s m => MonadState s (ReaderT r m) | |
| MonadState s m => MonadState s (ContT r m) | |
| (Monad m, Monoid w) => MonadState s (RWST r w s m) | |
| (Monad m, Monoid w) => MonadState s (RWST r w s m) | |
gets :: MonadState s m => (s -> a) -> m a #
Gets specific component of the state, using a projection function supplied.
modify :: MonadState s m => (s -> s) -> m () #
Monadic state transformer.
Maps an old state to a new state inside a state monad. The old state is thrown away.
Main> :t modify ((+1) :: Int -> Int)
modify (...) :: (MonadState Int a) => a ()This says that modify (+1) acts over any
Monad that is a member of the MonadState class,
with an Int state.
modify' :: MonadState s m => (s -> s) -> m () #
A variant of modify in which the computation is strict in the
new state.
Since: mtl-2.2
Zoom
class (MonadState s m, MonadState t n) => Zoom (m :: * -> *) (n :: * -> *) s t | m -> s, n -> t, m t -> n, n s -> m #
This class allows us to use zoom in, changing the State supplied by
many different Monad transformers, potentially quite
deep in a Monad transformer stack.
Minimal complete definition
Instances
| Zoom m n s t => Zoom (MaybeT m) (MaybeT n) s t | |
| Zoom m n s t => Zoom (ListT m) (ListT n) s t | |
| Zoom m n s t => Zoom (IdentityT m) (IdentityT n) s t | |
| Zoom m n s t => Zoom (ExceptT e m) (ExceptT e n) s t | |
| (Functor f, Zoom m n s t) => Zoom (FreeT f m) (FreeT f n) s t | |
| (Error e, Zoom m n s t) => Zoom (ErrorT e m) (ErrorT e n) s t | |
| (Monoid w, Zoom m n s t) => Zoom (WriterT w m) (WriterT w n) s t | |
| Monad z => Zoom (StateT s z) (StateT t z) s t | |
| Monad z => Zoom (StateT s z) (StateT t z) s t | |
| (Monoid w, Zoom m n s t) => Zoom (WriterT w m) (WriterT w n) s t | |
| Zoom m n s t => Zoom (ReaderT e m) (ReaderT e n) s t | |
| (Monoid w, Monad z) => Zoom (RWST r w s z) (RWST r w t z) s t | |
| (Monoid w, Monad z) => Zoom (RWST r w s z) (RWST r w t z) s t | |
zoom :: Zoom m n s t => LensLike' (Zoomed m c) t s -> m c -> n c infixr 2 #
Run a monadic action in a larger State than it was defined in,
using a Lens' or Traversal'.
This is commonly used to lift actions in a simpler State
Monad into a State Monad with a larger State type.
When applied to a Traversal' over
multiple values, the actions for each target are executed sequentially
and the results are aggregated.
This can be used to edit pretty much any Monad transformer stack with a State in it!
>>>flip State.evalState (a,b) $ zoom _1 $ use ida
>>>flip State.execState (a,b) $ zoom _1 $ id .= c(c,b)
>>>flip State.execState [(a,b),(c,d)] $ zoom traverse $ _2 %= f[(a,f b),(c,f d)]
>>>flip State.runState [(a,b),(c,d)] $ zoom traverse $ _2 <%= f(f b <> f d <> mempty,[(a,f b),(c,f d)])
>>>flip State.evalState (a,b) $ zoom both (use id)a <> b
zoom::Monadm =>Lens's t ->StateTt m a ->StateTs m azoom:: (Monadm,Monoidc) =>Traversal's t ->StateTt m c ->StateTs m czoom:: (Monadm,Monoidw) =>Lens's t ->RWSTr w t m c ->RWSTr w s m czoom:: (Monadm,Monoidw,Monoidc) =>Traversal's t ->RWSTr w t m c ->RWSTr w s m czoom:: (Monadm,Monoidw,Errore) =>Lens's t ->ErrorTe (RWSTr w t m) c ->ErrorTe (RWSTr w s m) czoom:: (Monadm,Monoidw,Monoidc,Errore) =>Traversal's t ->ErrorTe (RWSTr w t m) c ->ErrorTe (RWSTr w s m) c ...
type family Zoomed (m :: * -> *) :: * -> * -> * #
This type family is used by Zoom to describe the common effect type.
Instances
| type Zoomed (MaybeT m) | |
Defined in Control.Lens.Zoom | |
| type Zoomed (ListT m) | |
Defined in Control.Lens.Zoom | |
| type Zoomed (IdentityT m) | |
Defined in Control.Lens.Zoom | |
| type Zoomed (ExceptT e m) | |
Defined in Control.Lens.Zoom | |
| type Zoomed (FreeT f m) | |
Defined in Control.Lens.Zoom | |
| type Zoomed (ErrorT e m) | |
Defined in Control.Lens.Zoom | |
| type Zoomed (WriterT w m) | |
Defined in Control.Lens.Zoom | |
| type Zoomed (StateT s z) | |
Defined in Control.Lens.Zoom | |
| type Zoomed (StateT s z) | |
Defined in Control.Lens.Zoom | |
| type Zoomed (WriterT w m) | |
Defined in Control.Lens.Zoom | |
| type Zoomed (ReaderT e m) | |
Defined in Control.Lens.Zoom | |
| type Zoomed (RWST r w s z) | |
Defined in Control.Lens.Zoom | |
| type Zoomed (RWST r w s z) | |
Defined in Control.Lens.Zoom | |