Copyright | (c) Andy Gill 2001, (c) Oregon Graduate Institute of Science and Technology, 2001 |
---|---|
License | BSD-style (see the file LICENSE) |
Maintainer | ross@soi.city.ac.uk |
Stability | experimental |
Portability | portable |
Safe Haskell | Safe |
Language | Haskell98 |
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.
type State s = StateT s Identity Source
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.
:: 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
.)
:: 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
.)
:: 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.
:: 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.
mapState :: ((a, s) -> (b, s)) -> State s a -> State s b Source
Map both the return value and final state of a computation using the given function.
withState :: (s -> s) -> State s a -> State s a Source
withState f m
executes action m
on a state modified by applying f
.
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.
evalStateT :: Monad m => StateT s m a -> s -> m a Source
Evaluate a state computation with the given initial state and return the final value, discarding the final state.
evalStateT
m s =liftM
fst
(runStateT
m s)
execStateT :: Monad m => StateT s m a -> s -> m s Source
Evaluate a state computation with the given initial state and return the final state, discarding the final value.
execStateT
m s =liftM
snd
(runStateT
m s)
mapStateT :: (m (a, s) -> n (b, s)) -> StateT s m a -> StateT s n b Source
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 Source
withStateT f m
executes action m
on a state modified by applying f
.
withStateT
f m =modify
f >> m
get :: Monad m => StateT s m s Source
Fetch the current value of the state within the monad.
put :: Monad m => s -> StateT s m () Source
put s
sets the state within the monad to s
.
modify :: Monad m => (s -> s) -> StateT s m () Source
modify f
is an action that updates the state to the result of applying f
to the current state.
modify' :: Monad m => (s -> s) -> StateT s m () Source
A variant of modify
in which the computation is strict in the new state.
gets :: Monad m => (s -> a) -> StateT s m a Source
Get a specific component of the state, using a projection function supplied.
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.
liftCallCC' :: CallCC m (a, s) (b, s) -> CallCC (StateT s m) a b Source
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 :: Catch e m (a, s) -> Catch e (StateT s m) a Source
Lift a catchE
operation to the new monad.
liftListen :: Monad m => Listen w m (a, s) -> Listen w (StateT s m) a Source
Lift a listen
operation to the new monad.
liftPass :: Monad m => Pass w m (a, s) -> Pass w (StateT s m) a Source
Lift a pass
operation to the new monad.
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))
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
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
© The University of Glasgow and others
Licensed under a BSD-style license (see top of the page).
https://downloads.haskell.org/~ghc/7.10.3/docs/html/libraries/transformers-0.4.2.0/Control-Monad-Trans-State-Lazy.html