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GHC 9.10.3 · lts/ghc-9.10.x · c74966e · 2026-09-27

Modulemtl-2.3.1Haskell2010

Control.Monad.State.Lazy

Lazy state monads.

This module is inspired by the paper Functional Programming with Overloading and Higher-Order Polymorphism, Mark P Jones (http://web.cecs.pdx.edu/~mpj/) Advanced School of Functional Programming, 1995.

  • 2 types
  • 1 class
  • 13 values
  • Packagemtl-2.3.1
  • Exports16
  • LanguageHaskell2010
  • LicenceBSD-3-Clause
  • SourceLazy.hs

MonadState class

4 declarations
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.

Instances15MonadState, …
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.

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

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

The State monad

6 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.

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

6 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

Instances19MonadAccum, MonadError, MonadReader, MonadState, MonadWriter, MonadSelect, …

module Control.Monad.Trans

Examples

0 declarations

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

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
            (newTable, newPos) <- return (nNode x table)
            put newTable
            return newPos
    nNode::  (Eq a) => a -> Table a -> (Table a, Int)
    nNode x table
       = case (findIndexInList (== x) table) of
         Nothing -> (table ++ [x], length table)
         Just i  -> (table, i)
    findIndexInList :: (a -> Bool) -> [a] -> Maybe Int
    findIndexInList = findIndexInListHelp 0
    findIndexInListHelp _ _ [] = Nothing
    findIndexInListHelp count f (h:t)
       = if (f h)
         then Just count
         else findIndexInListHelp (count+1) f t

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

sumTree is a little helper function that does not use the State monad:

sumTree :: (Num a) => Tree a -> a
sumTree Nil = 0
sumTree (Node e t1 t2) = e + (sumTree t1) + (sumTree t2)