 | Haskell Core Libraries (base package) | Parent | Contents | Index |
| Control.Monad.State |
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State monads.
This module is inspired by the paper Functional Programming with Overloading and Higher-Order Polymorphism, Mark P Jones (http://www.cse.ogi.edu/~mpj/) Advanced School of Functional Programming, 1995.
See below for examples.
| class (Monad m) => MonadState s m | m -> s where | ||
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| modify :: (MonadState s m) => (s -> s) -> m () | ||
| gets :: (MonadState s m) => (s -> a) -> m a | ||
newtype State s a = State {
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| 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 {
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| 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 | ||
| module Control.Monad | ||
| module Control.Monad.Fix | ||
| module Control.Monad.Trans |
get returns the state from the internals of the monad. put replaces the state inside the monad. | |||||||||
| Methods | |||||||||
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| Instances | |||||||||
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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.
| A parameterizable state monad where s is the type of the state to carry and a is the type of the return value. | ||||||
| Constructors | ||||||
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| Instances | ||||||
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| :: State s a | The state to evaluate |
| -> s | An initial value |
| -> a | The return value of the state application |
| Evaluate this state monad with the given initial state,throwing away the final state. Very much like fst composed with runstate. | |
| :: State s a | The state to evaluate |
| -> s | An initial value |
| -> s | The new state |
| Execute this state and return the new state, throwing away the return value. Very much like snd composed with runstate. | |
Map a stateful computation from one (return value, state) pair to another. For instance, to convert numberTree from a function that returns a tree to a function that returns the sum of the numbered tree (see the Examples section for numberTree and sumTree) you may write:
sumNumberedTree :: (Eq a) => Tree a -> State (Table a) Int sumNumberedTree = mapState (\ (t, tab) -> (sumTree t, tab)) . numberTree
A parameterizable state monad for encapsulating an inner monad. The StateT Monad structure is parameterized over two things:
Here are some examples of use: (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 Indentity 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)) | |||||||||||
| Constructors | |||||||||||
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| Instances | |||||||||||
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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 nAdd 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.ukc.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 tnumTree 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)