transformers-0.5.2.0: Concrete functor and monad transformers

Copyright (c) Andy Gill 2001(c) Oregon Graduate Institute of Science and Technology 2001 BSD-style (see the file LICENSE) R.Paterson@city.ac.uk experimental portable Safe Haskell98

Description

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.

Synopsis

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.

Arguments

 :: 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.)

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.)

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.

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.

newtype StateT s m 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 FieldsrunStateT :: s -> m (a, s)

Instances

## 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
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