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

Moduletransformers-0.6.1.1Haskell2010

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:

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.

  • 2 types
  • 21 values

The State monad

7 declarations
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.

valuestate
  1. :: Monad m
  2. => (s -> (a, s))

    pure state transformer

  3. -> StateT s m a

    equivalent state-passing computation

#

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

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.)

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.

The StateT monad transformer

5 declarations
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

Instances12MonadTrans, Monad, Functor, MonadFix, MonadFail, Applicative, …

State operations

6 declarations
valueget :: Monad m => StateT s m s
#

Fetch the current value of the state within the monad.

valueput :: Monad m => s -> StateT s m ()
#

put s sets the state within the monad to s.

valuemodifyM :: Monad m => (s -> m s) -> StateT s m ()
#

A variant of modify in which the new state is generated by a monadic action.

valuegets :: Monad m => (s -> a) -> StateT s m a
#

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

Lifting other operations

5 declarations
valueliftCallCC :: 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.

valueliftCallCC' :: 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 uniformity property (see Control.Monad.Signatures).

Examples

0 declarations

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