Module

Control.Monad.State.Trans

This module defines the state monad transformer, StateT.

#StateT

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

The state monad transformer.

This monad transformer extends the base monad with the operations get and put which can be used to model a single piece of mutable state.

The MonadState type class describes the operations supported by this monad.

Constructors

Instances

#runStateT

runStateT :: forall s m a. StateT s m a -> s -> m (Tuple a s)

Run a computation in the StateT monad.

#evalStateT

evalStateT :: forall s m a. Functor m => StateT s m a -> s -> m a

Run a computation in the StateT monad, discarding the final state.

#execStateT

execStateT :: forall s m a. Functor m => StateT s m a -> s -> m s

Run a computation in the StateT monad discarding the result.

#mapStateT

mapStateT :: forall s m1 m2 a b. (m1 (Tuple a s) -> m2 (Tuple b s)) -> StateT s m1 a -> StateT s m2 b

Change the result type in a StateT monad action.

#withStateT

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

Modify the final state in a StateT monad action.

Re-exports from Control.Monad.State.Class

#MonadState

class MonadState :: Type -> (Type -> Type) -> Constraintclass (Monad m) <= MonadState s m | m -> s where

The MonadState s type class represents those monads which support a single piece of mutable state of type s.

  • state f updates the state using the function f.

An implementation is provided for StateT, and for other monad transformers defined in this library.

Laws:

  • do { get ; get } = get
  • do { put x ; put y } = put y
  • do { put x ; get } = put x $> x
  • do { s <- get ; put s } = pure unit

Members

#put

put :: forall m s. MonadState s m => s -> m Unit

Set the state.

#modify_

modify_ :: forall s m. MonadState s m => (s -> s) -> m Unit

#modify

modify :: forall s m. MonadState s m => (s -> s) -> m s

Modify the state by applying a function to the current state. The returned value is the new state value.

#gets

gets :: forall s m a. MonadState s m => (s -> a) -> m a

Get a value which depends on the current state.

#get

get :: forall m s. MonadState s m => m s

Get the current state.

Re-exports from Control.Monad.Trans.Class

#MonadTrans

class MonadTrans :: ((Type -> Type) -> Type -> Type) -> Constraintclass MonadTrans t  where

The MonadTrans type class represents monad transformers.

A monad transformer is a type constructor of kind (* -> *) -> * -> *, which takes a Monad as its first argument, and returns another Monad.

This allows us to add additional effects to an existing monad. By iterating this process, we create monad transformer stacks, which contain all of the effects required for a particular computation.

The laws state that lift is a Monad morphism.

Laws:

  • lift (pure a) = pure a
  • lift (do { x <- m ; y }) = do { x <- lift m ; lift y }

Members

Modules