Control.Monad.Trans.State.Strict (original) (raw)

type State s = StateT s Identity Source #

A state monad parameterized by the type s of the state to carry.

The [return](/package/base-4.14.1.0/docs/Control-Monad.html#v:return "Control.Monad") function leaves the state unchanged, while >>= uses the final state of the first computation as the initial state of the second.

state Source #

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](Control-Monad-Trans-State-Strict.html#v:runState "Control.Monad.Trans.State.Strict").)

runState Source #

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](Control-Monad-Trans-State-Strict.html#v:state "Control.Monad.Trans.State.Strict").)

evalState Source #

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.

execState Source #

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.

The StateT monad transformer

newtype StateT s m a Source #

A state transformer monad parameterized by:

The [return](/package/base-4.14.1.0/docs/Control-Monad.html#v:return "Control.Monad") function leaves the state unchanged, while >>= uses the final state of the first computation as the initial state of the second.

State operationsLifting other operations

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

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

ExamplesState 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