Copyright | (c) Andy Gill 2001 (c) Oregon Graduate Institute of Science and Technology 2001 |
---|---|
License | BSD-style (see the file LICENSE) |
Maintainer | [email protected] |
Stability | experimental |
Portability | non-portable (multi-param classes, functional dependencies) |
Safe Haskell | Safe |
Language | Haskell2010 |
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.
class Monad m => MonadState s m | m -> s where Source
Minimal definition is either both of get
and put
or just state
Return the state from the internals of the monad.
Replace the state inside the monad.
state :: (s -> (a, s)) -> m a Source
Embed a simple state action into the monad.
MonadState s m => MonadState s (MaybeT m) | |
MonadState s m => MonadState s (ListT m) | |
(Monoid w, MonadState s m) => MonadState s (WriterT w m) | |
(Monoid w, MonadState s m) => MonadState s (WriterT w m) | |
MonadState s m => MonadState s (IdentityT m) | |
MonadState s m => MonadState s (ExceptT e m) | Since: mtl-2.2 |
(Error e, MonadState s m) => MonadState s (ErrorT e m) | |
Monad m => MonadState s (StateT s m) | |
Monad m => MonadState s (StateT s m) | |
MonadState s m => MonadState s (ReaderT r m) | |
MonadState s m => MonadState s (ContT r m) | |
(Monad m, Monoid w) => MonadState s (RWST r w s m) | |
(Monad m, Monoid w) => MonadState s (RWST r w s m) | |
modify :: MonadState s m => (s -> s) -> m () Source
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.
modify' :: MonadState s m => (s -> s) -> m () Source
A variant of modify
in which the computation is strict in the new state.
Since: mtl-2.2
gets :: MonadState s m => (s -> a) -> m a Source
Gets specific component of the state, using a projection function supplied.
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.
:: 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
.
newtype StateT s (m :: Type -> Type) a Source
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.
StateT (s -> m (a, s)) |
MonadError e m => MonadError e (StateT s m) | |
Defined in Control.Monad.Error.Class MethodsthrowError :: e -> StateT s m a Source catchError :: StateT s m a -> (e -> StateT s m a) -> StateT s m a Source | |
MonadReader r m => MonadReader r (StateT s m) | |
Monad m => MonadState s (StateT s m) | |
MonadWriter w m => MonadWriter w (StateT s m) | |
MonadTrans (StateT s) | |
Defined in Control.Monad.Trans.State.Lazy | |
Monad m => Monad (StateT s m) | |
Functor m => Functor (StateT s m) | |
MonadFix m => MonadFix (StateT s m) | |
Defined in Control.Monad.Trans.State.Lazy | |
MonadFail m => MonadFail (StateT s m) | |
Defined in Control.Monad.Trans.State.Lazy | |
(Functor m, Monad m) => Applicative (StateT s m) | |
Contravariant m => Contravariant (StateT s m) | |
MonadIO m => MonadIO (StateT s m) | |
Defined in Control.Monad.Trans.State.Lazy | |
(Functor m, MonadPlus m) => Alternative (StateT s m) | |
MonadPlus m => MonadPlus (StateT s m) | |
MonadCont m => MonadCont (StateT s m) | |
runStateT :: StateT s m a -> s -> m (a, s) Source
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
module Control.Monad
module Control.Monad.Fix
module Control.Monad.Trans
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)
© The University of Glasgow and others
Licensed under a BSD-style license (see top of the page).
https://downloads.haskell.org/~ghc/8.6.1/docs/html/libraries/mtl-2.2.2/Control-Monad-State-Lazy.html