transformers-0.2.1.0: Concrete functor and monad transformersContentsIndex
Control.Monad.Trans.State.Strict
Portabilityportable
Stabilityexperimental
Maintainerlibraries@haskell.org
Contents
The State monad
The StateT monad transformer
State operations
Lifting other operations
Examples
State monads
Counting
Labelling trees
Description

Strict state monads, passing an updateable state through a computation. See below for examples.

In this version, sequencing of computations is strict in the state. For a lazy version, see Control.Monad.Trans.Writer.Lazy, which has the same interface.

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

Synopsis
type State s = StateT s Identity
state :: (s -> (a, s)) -> State s a
runState :: State s a -> s -> (a, s)
evalState :: State s a -> s -> a
execState :: State s a -> s -> s
mapState :: ((a, s) -> (b, s)) -> State s a -> State s b
withState :: (s -> s) -> State s a -> State s a
newtype StateT s m a = StateT {
runStateT :: s -> m (a, s)
}
evalStateT :: Monad m => StateT s m a -> s -> m a
execStateT :: Monad m => StateT s m a -> s -> m s
mapStateT :: (m (a, s) -> n (b, s)) -> StateT s m a -> StateT s n b
withStateT :: (s -> s) -> StateT s m a -> StateT s m a
get :: Monad m => StateT s m s
put :: Monad m => s -> StateT s m ()
modify :: Monad m => (s -> s) -> StateT s m ()
gets :: Monad m => (s -> a) -> StateT s m a
liftCallCC :: ((((a, s) -> m (b, s)) -> m (a, s)) -> m (a, s)) -> ((a -> StateT s m b) -> StateT s m a) -> StateT s m a
liftCallCC' :: ((((a, s) -> m (b, s)) -> m (a, s)) -> m (a, s)) -> ((a -> StateT s m b) -> StateT s m a) -> StateT s m a
liftCatch :: (m (a, s) -> (e -> m (a, s)) -> m (a, s)) -> StateT s m a -> (e -> StateT s m a) -> StateT s m a
liftListen :: Monad m => (m (a, s) -> m ((a, s), w)) -> StateT s m a -> StateT s m (a, w)
liftPass :: Monad m => (m ((a, s), b) -> m (a, s)) -> StateT s m (a, b) -> StateT s m a
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
:: (s -> (a, s))pure state transformer
-> State s aequivalent state-passing computation
Construct a state monad computation from a function. (The inverse of runState.)
runState
:: State s astate-passing computation to execute
-> sinitial state
-> (a, s)return value and final state
Unwrap a state monad computation as a function. (The inverse of state.)
evalState
:: State s astate-passing computation to execute
-> sinitial value
-> areturn value of the state computation

Evaluate a state computation with the given initial state and return the final value, discarding the final state.

execState
:: State s astate-passing computation to execute
-> sinitial value
-> sfinal state

Evaluate a state computation with the given initial state and return the final state, discarding the final value.

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.

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

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

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
runStateT :: s -> m (a, s)
show/hide Instances
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.

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.

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.

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

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

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

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

Lifting other operations
liftCallCC :: ((((a, s) -> m (b, s)) -> m (a, s)) -> m (a, s)) -> ((a -> StateT s m b) -> StateT s m a) -> StateT s m a
Uniform lifting of a callCC operation to the new monad. This version rolls back to the original state on entering the continuation.
liftCallCC' :: ((((a, s) -> m (b, s)) -> m (a, s)) -> m (a, s)) -> ((a -> StateT s m b) -> StateT s m a) -> StateT s m a
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 :: (m (a, s) -> (e -> m (a, s)) -> m (a, s)) -> StateT s m a -> (e -> StateT s m a) -> StateT s m a
Lift a catchError operation to the new monad.
liftListen :: Monad m => (m (a, s) -> m ((a, s), w)) -> StateT s m a -> StateT s m (a, w)
Lift a listen operation to the new monad.
liftPass :: Monad m => (m ((a, s), b) -> m (a, s)) -> StateT s m (a, b) -> 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.math.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
             (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)
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