Control.Monad.Trans.State.Lazy

Lazy state monads, passing an updatable state through a computation. See below for examples.

Some computations may not require the full power of state transformers:

• For a read-only state, see Control.Monad.Trans.Reader.
• To accumulate a value without using it on the way, see Control.Monad.Trans.Writer.

In this version, sequencing of computations is lazy, so that for example the following produces a usable result:

 evalState (sequence $ repeat $ do { n <- get; put (n*2); return n }) 1

For a strict version with the same interface, see Control.Monad.Trans.State.Strict.

The State monad

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.

state

Arguments

:: Monad m

=> (s -> (a, s))

pure state transformer

-> StateT s m a

equivalent state-passing computation

Construct a state monad computation from a function. (The inverse of runState.)

runState

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.)

evalState

Arguments

:: State s a

state-passing computation to execute

-> s

initial value

-> 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.

• evalState m s = fst (runState m s)

execState

Arguments

:: State s a

state-passing computation to execute

-> s

initial value

-> s

final state

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)

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

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

• runState (mapState f m) = f . runState m

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

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

• withState f m = modify f >> m

The StateT monad transformer

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.

Constructors

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

Instances

MonadTrans (StateT s)

Monad m => Monad (StateT s m)

Functor m => Functor (StateT s m)

MonadFix m => MonadFix (StateT s m)

MonadPlus m => MonadPlus (StateT s m)

(Functor m, Monad m) => Applicative (StateT s m)

(Functor m, MonadPlus m) => Alternative (StateT s m)

MonadIO m => MonadIO (StateT s m)

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

Evaluate a state computation with the given initial state and return the final value and state. (inverse of StateT)

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.

• evalStateT m s = liftM fst (runStateT m 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.

• execStateT m s = liftM snd (runStateT m s)

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

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

• runStateT (mapStateT f m) = f . runStateT m

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

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

• withStateT f m = modify f >> m

State operations

get :: Monad m => StateT s m s

Fetch the current value of the state within the monad.

put :: Monad m => s -> StateT s m ()

put s sets the state within the monad to s.

modify :: Monad m => (s -> s) -> StateT s m ()

modify f is an action that updates the state to the result of applying f to the current state.

• modify f = get >>= (put . f)

modify' :: Monad m => (s -> s) -> StateT s m ()

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

• modify' f = get >>= (($!) put . f)

gets :: Monad m => (s -> a) -> StateT s m a

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

• gets f = liftM f get

Lifting other operations

liftCallCC :: CallCC m (a, s) (b, s) -> CallCC (StateT s m) a b

Uniform lifting of a callCC operation to the new monad. This version rolls back to the original state on entering the continuation.

liftCallCC' :: CallCC m (a, s) (b, s) -> CallCC (StateT s m) a b

In-situ lifting of a callCC operation to the new monad. This version uses the current state on entering the continuation. It does not satisfy the laws of a monad transformer.

liftCatch :: Catch e m (a, s) -> Catch e (StateT s m) a

Lift a catchE operation to the new monad.

liftListen :: Monad m => Listen w m (a, s) -> Listen w (StateT s m) a

Lift a listen operation to the new monad.

liftPass :: Monad m => Pass w m (a, s) -> Pass w (StateT s m) a

Lift a pass operation to the new monad.

Examples

State monads

Parser from ParseLib with Hugs:

 type Parser a = StateT String [] a
    ==> StateT (String -> [(a,String)])

For example, item can be written as:

 item = do (x:xs) <- get
        put xs
        return x

 type BoringState s a = StateT s Identity a
      ==> StateT (s -> Identity (a,s))

 type StateWithIO s a = StateT s IO a
      ==> StateT (s -> IO (a,s))

 type StateWithErr s a = StateT s Maybe a
      ==> StateT (s -> Maybe (a,s))

Counting

A function to increment a counter. Taken from the paper "Generalising Monads to Arrows", John Hughes (http://www.cse.chalmers.se/~rjmh/), November 1998:

 tick :: State Int Int
 tick = do n <- get
           put (n+1)
           return n

Add one to the given number using the state monad:

 plusOne :: Int -> Int
 plusOne n = execState tick n

A contrived addition example. Works only with positive numbers:

 plus :: Int -> Int -> Int
 plus n x = execState (sequence $ replicate n tick) x

Labelling trees

An example from The Craft of Functional Programming, Simon Thompson (http://www.cs.kent.ac.uk/people/staff/sjt/), Addison-Wesley 1999: "Given an arbitrary tree, transform it to a tree of integers in which the original elements are replaced by natural numbers, starting from 0. The same element has to be replaced by the same number at every occurrence, and when we meet an as-yet-unvisited element we have to find a 'new' number to match it with:"

 data Tree a = Nil | Node a (Tree a) (Tree a) deriving (Show, Eq)
 type Table a = [a]

 numberTree :: Eq a => Tree a -> State (Table a) (Tree Int)
 numberTree Nil = return Nil
 numberTree (Node x t1 t2) = do
     num <- numberNode x
     nt1 <- numberTree t1
     nt2 <- numberTree t2
     return (Node num nt1 nt2)
   where
     numberNode :: Eq a => a -> State (Table a) Int
     numberNode x = do
         table <- get
         case elemIndex x table of
             Nothing -> do
                 put (table ++ [x])
                 return (length table)
             Just i -> return i

numTree applies numberTree with an initial state:

 numTree :: (Eq a) => Tree a -> Tree Int
 numTree t = evalState (numberTree t) []

 testTree = Node "Zero" (Node "One" (Node "Two" Nil Nil) (Node "One" (Node "Zero" Nil Nil) Nil)) Nil
 numTree testTree => Node 0 (Node 1 (Node 2 Nil Nil) (Node 1 (Node 0 Nil Nil) Nil)) Nil