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Control.Monad.State.Lazy

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

Description

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.

MonadState class

class Monad m => MonadState s m | m -> s where Source

Minimal definition is either both of get and put or just state

Minimal complete definition

state | get, put

Methods

get :: m s Source

Return the state from the internals of the monad.

put :: s -> m () Source

Replace the state inside the monad.

state :: (s -> (a, s)) -> m a Source

Embed a simple state action into the monad.

Instances
MonadState s m => MonadState s (MaybeT m)
Instance details

Defined in Control.Monad.State.Class

Methods

get :: MaybeT m s Source

put :: s -> MaybeT m () Source

state :: (s -> (a, s)) -> MaybeT m a Source

MonadState s m => MonadState s (ListT m)
Instance details

Defined in Control.Monad.State.Class

Methods

get :: ListT m s Source

put :: s -> ListT m () Source

state :: (s -> (a, s)) -> ListT m a Source

(Monoid w, MonadState s m) => MonadState s (WriterT w m)
Instance details

Defined in Control.Monad.State.Class

Methods

get :: WriterT w m s Source

put :: s -> WriterT w m () Source

state :: (s -> (a, s)) -> WriterT w m a Source

(Monoid w, MonadState s m) => MonadState s (WriterT w m)
Instance details

Defined in Control.Monad.State.Class

Methods

get :: WriterT w m s Source

put :: s -> WriterT w m () Source

state :: (s -> (a, s)) -> WriterT w m a Source

MonadState s m => MonadState s (IdentityT m)
Instance details

Defined in Control.Monad.State.Class

Methods

get :: IdentityT m s Source

put :: s -> IdentityT m () Source

state :: (s -> (a, s)) -> IdentityT m a Source

MonadState s m => MonadState s (ExceptT e m)

Since: mtl-2.2

Instance details

Defined in Control.Monad.State.Class

Methods

get :: ExceptT e m s Source

put :: s -> ExceptT e m () Source

state :: (s -> (a, s)) -> ExceptT e m a Source

(Error e, MonadState s m) => MonadState s (ErrorT e m)
Instance details

Defined in Control.Monad.State.Class

Methods

get :: ErrorT e m s Source

put :: s -> ErrorT e m () Source

state :: (s -> (a, s)) -> ErrorT e m a Source

Monad m => MonadState s (StateT s m)
Instance details

Defined in Control.Monad.State.Class

Methods

get :: StateT s m s Source

put :: s -> StateT s m () Source

state :: (s -> (a, s)) -> StateT s m a Source

Monad m => MonadState s (StateT s m)
Instance details

Defined in Control.Monad.State.Class

Methods

get :: StateT s m s Source

put :: s -> StateT s m () Source

state :: (s -> (a, s)) -> StateT s m a Source

MonadState s m => MonadState s (ReaderT r m)
Instance details

Defined in Control.Monad.State.Class

Methods

get :: ReaderT r m s Source

put :: s -> ReaderT r m () Source

state :: (s -> (a, s)) -> ReaderT r m a Source

MonadState s m => MonadState s (ContT r m)
Instance details

Defined in Control.Monad.State.Class

Methods

get :: ContT r m s Source

put :: s -> ContT r m () Source

state :: (s -> (a, s)) -> ContT r m a Source

(Monad m, Monoid w) => MonadState s (RWST r w s m)
Instance details

Defined in Control.Monad.State.Class

Methods

get :: RWST r w s m s Source

put :: s -> RWST r w s m () Source

state :: (s -> (a, s)) -> RWST r w s m a Source

(Monad m, Monoid w) => MonadState s (RWST r w s m)
Instance details

Defined in Control.Monad.State.Class

Methods

get :: RWST r w s m s Source

put :: s -> RWST r w s m () Source

state :: (s -> (a, s)) -> RWST r w s m a Source

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.

The State monad

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.

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.)

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.

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.

The StateT monad transformer

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.

Constructors

StateT (s -> m (a, s))
Instances
MonadError e m => MonadError e (StateT s m)
Instance details

Defined in Control.Monad.Error.Class

Methods

throwError :: 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)
Instance details

Defined in Control.Monad.Reader.Class

Methods

ask :: StateT s m r Source

local :: (r -> r) -> StateT s m a -> StateT s m a Source

reader :: (r -> a) -> StateT s m a Source

Monad m => MonadState s (StateT s m)
Instance details

Defined in Control.Monad.State.Class

Methods

get :: StateT s m s Source

put :: s -> StateT s m () Source

state :: (s -> (a, s)) -> StateT s m a Source

MonadWriter w m => MonadWriter w (StateT s m)
Instance details

Defined in Control.Monad.Writer.Class

Methods

writer :: (a, w) -> StateT s m a Source

tell :: w -> StateT s m () Source

listen :: StateT s m a -> StateT s m (a, w) Source

pass :: StateT s m (a, w -> w) -> StateT s m a Source

MonadTrans (StateT s)
Instance details

Defined in Control.Monad.Trans.State.Lazy

Methods

lift :: Monad m => m a -> StateT s m a Source

Monad m => Monad (StateT s m)
Instance details

Defined in Control.Monad.Trans.State.Lazy

Methods

(>>=) :: StateT s m a -> (a -> StateT s m b) -> StateT s m b Source

(>>) :: StateT s m a -> StateT s m b -> StateT s m b Source

return :: a -> StateT s m a Source

fail :: String -> StateT s m a Source

Functor m => Functor (StateT s m)
Instance details

Defined in Control.Monad.Trans.State.Lazy

Methods

fmap :: (a -> b) -> StateT s m a -> StateT s m b Source

(<$) :: a -> StateT s m b -> StateT s m a Source

MonadFix m => MonadFix (StateT s m)
Instance details

Defined in Control.Monad.Trans.State.Lazy

Methods

mfix :: (a -> StateT s m a) -> StateT s m a Source

MonadFail m => MonadFail (StateT s m)
Instance details

Defined in Control.Monad.Trans.State.Lazy

Methods

fail :: String -> StateT s m a Source

(Functor m, Monad m) => Applicative (StateT s m)
Instance details

Defined in Control.Monad.Trans.State.Lazy

Methods

pure :: a -> StateT s m a Source

(<*>) :: StateT s m (a -> b) -> StateT s m a -> StateT s m b Source

liftA2 :: (a -> b -> c) -> StateT s m a -> StateT s m b -> StateT s m c Source

(*>) :: StateT s m a -> StateT s m b -> StateT s m b Source

(<*) :: StateT s m a -> StateT s m b -> StateT s m a Source

Contravariant m => Contravariant (StateT s m)
Instance details

Defined in Control.Monad.Trans.State.Lazy

Methods

contramap :: (a -> b) -> StateT s m b -> StateT s m a Source

(>$) :: b -> StateT s m b -> StateT s m a Source

MonadIO m => MonadIO (StateT s m)
Instance details

Defined in Control.Monad.Trans.State.Lazy

Methods

liftIO :: IO a -> StateT s m a Source

(Functor m, MonadPlus m) => Alternative (StateT s m)
Instance details

Defined in Control.Monad.Trans.State.Lazy

Methods

empty :: StateT s m a Source

(<|>) :: StateT s m a -> StateT s m a -> StateT s m a Source

some :: StateT s m a -> StateT s m [a] Source

many :: StateT s m a -> StateT s m [a] Source

MonadPlus m => MonadPlus (StateT s m)
Instance details

Defined in Control.Monad.Trans.State.Lazy

Methods

mzero :: StateT s m a Source

mplus :: StateT s m a -> StateT s m a -> StateT s m a Source

MonadCont m => MonadCont (StateT s m)
Instance details

Defined in Control.Monad.Cont.Class

Methods

callCC :: ((a -> StateT s m b) -> StateT s m a) -> StateT s m a Source

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.

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.

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.

module Control.Monad

module Control.Monad.Fix

module Control.Monad.Trans

Examples

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