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GHC 9.10.3 · lts/ghc-9.10.x · 248f8f0 · 2026-10-05

Modulererebase-1.21.2Haskell2010

Control.Monad.State

  • 2 types
  • 3 classes
  • 12 values
typetype 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.

valuegets :: MonadState s m => (s -> a) -> m a
#

Gets specific component of the state, using a projection function supplied.

valuemodify :: 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.

valuemodify' :: MonadState s m => (s -> s) -> m ()
#

A variant of modify in which the computation is strict in the new state.

valueevalState
  1. :: State s a

    state-passing computation to execute

  2. -> s

    initial value

  3. -> a

    return value of the state computation

#

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

valueexecState
  1. :: State s a

    state-passing computation to execute

  2. -> s

    initial value

  3. -> s

    final state

#

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

valuerunState
  1. :: State s a

    state-passing computation to execute

  2. -> s

    initial state

  3. -> (a, s)

    return value and final state

#

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

classclass Monad m => MonadIO (m :: Type -> Type) where
#

Monads in which IO computations may be embedded. Any monad built by applying a sequence of monad transformers to the IO monad will be an instance of this class.

Instances should satisfy the following laws, which state that liftIO is a transformer of monads:

Methods

  • liftIO :: IO a -> m a

    Lift a computation from the IO monad. This allows us to run IO computations in any monadic stack, so long as it supports these kinds of operations (i.e. IO is the base monad for the stack).

    Example
    import Control.Monad.Trans.State -- from the "transformers" library

printState :: Show s => StateT s IO ()
printState = do
  state <- get
  liftIO $ print state

    Had we omitted liftIO, we would have ended up with this error:

    • Couldn't match type ‘IO’ with ‘StateT s IO’
 Expected type: StateT s IO ()
   Actual type: IO ()

    The important part here is the mismatch between StateT s IO () and IO ().

    Luckily, we know of a function that takes an IO a and returns an (m a): liftIO, enabling us to run the program and see the expected results:

    > evalStateT printState "hello"
"hello"

> evalStateT printState 3
3
Instances26MonadIO, …
classclass Monad m => MonadState s (m :: Type -> Type) | m -> s where
#

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

Methods

  • get :: m s

    Return the state from the internals of the monad.

  • put :: s -> m ()

    Replace the state inside the monad.

  • state :: (s -> (a, s)) -> m a

    Embed a simple state action into the monad.

Instances26MonadState, …
classclass (forall (m :: Type -> Type). Monad m => Monad (t m)) => MonadTrans (t :: (Type -> Type) -> Type -> Type) where
#

The class of monad transformers. For any monad m, the result t m should also be a monad, and lift should be a monad transformation from m to t m, i.e. it should satisfy the following laws:

Since 0.6.0.0 and for GHC 8.6 and later, the requirement that t m be a Monad is enforced by the implication constraint forall m. Monad m => Monad (t m) enabled by the QuantifiedConstraints extension.

Ambiguity error with GHC 9.0 to 9.2.2

These versions of GHC have a bug (https://gitlab.haskell.org/ghc/ghc/-/issues/20582) which causes constraints like

(MonadTrans t, forall m. Monad m => Monad (t m)) => ...

to be reported as ambiguous. For transformers 0.6 and later, this can be fixed by removing the second constraint, which is implied by the first.

Methods

  • lift :: Monad m => m a -> t m a

    Lift a computation from the argument monad to the constructed monad.

Instances32MonadTrans, …
newtypenewtype StateT s (m :: Type -> Type) 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.

Constructors

Instances39MonadAccum, MonadError, MonadReader, MonadState, MonadWriter, MonadSelect, …