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Control.Monad.Reader

Portability | non-portable (multi-param classes, functional dependencies)
Stability | experimental
Maintainer | libraries@haskell.org

Contents

Example 1: Simple Reader Usage
Example 2: Modifying Reader Content With `local`
Example 3: `ReaderT` Monad Transformer

Description

Computation type:
  Computations which read values from a shared environment.
Binding strategy:
  Monad values are functions from the environment to a value. The bound function is applied to the bound value, and both have access to the shared environment.
Useful for:
  Maintaining variable bindings, or other shared environment.
Zero and plus:
  None.
Example type:
  ``Reader` [(String,Value)] a`

The `Reader` monad (also called the Environment monad). Represents a computation, which can read values from a shared environment, pass values from function to function, and execute sub-computations in a modified environment. Using `Reader` monad for such computations is often clearer and easier than using the Control.Monad.State.State monad.

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.

Synopsis

module Control.Monad.Reader.Class

newtype Reader r a = Reader {

runReader :: r -> a

}

mapReader :: (a -> b) -> Reader r a -> Reader r b
withReader :: (r' -> r) -> Reader r a -> Reader r' a

newtype ReaderT r m a = ReaderT {

runReaderT :: r -> m a

}

mapReaderT :: (m a -> n b) -> ReaderT w m a -> ReaderT w n b
withReaderT :: (r' -> r) -> ReaderT r m a -> ReaderT r' m a
module Control.Monad
module Control.Monad.Fix
module Control.Monad.Trans

Documentation
module Control.Monad.Reader.Class

newtype Reader r a | Source

The parameterizable reader monad. The `return` function creates a `Reader` that ignores the environment, and produces the given value. The binding operator `>>=` produces a `Reader` that uses the environment to extract the value its left-hand side, and then applies the bound function to that value in the same environment.
Constructors

Reader |

runReader :: r -> a

Runs `Reader` and extracts the final value from it. To extract the value apply `(runReader reader)` to an environment value. Parameters:

- A `Reader` to run.
- An initial environment.

Instances

MonadReader r (Reader r)
Monad (Reader r)
Functor (Reader r)
MonadFix (Reader r)

mapReader :: (a -> b) -> Reader r a -> Reader r b | Source

withReader :: (r' -> r) -> Reader r a -> Reader r' a | Source

A more general version of `local`.

newtype ReaderT r m a | Source

The reader monad transformer. Can be used to add environment reading functionality to other monads.
Constructors

ReaderT |

runReaderT :: r -> m a |

Instances

MonadWriter w m => MonadWriter w (ReaderT r m)
MonadState s m => MonadState s (ReaderT r m)
Monad m => MonadReader r (ReaderT r m)
MonadError e m => MonadError e (ReaderT r m)
MonadTrans (ReaderT r)
Monad m => Monad (ReaderT r m)
Monad m => Functor (ReaderT r m)
MonadFix m => MonadFix (ReaderT r m)
MonadPlus m => MonadPlus (ReaderT r m)
MonadIO m => MonadIO (ReaderT r m)
MonadCont m => MonadCont (ReaderT r m)

mapReaderT :: (m a -> n b) -> ReaderT w m a -> ReaderT w n b | Source

withReaderT :: (r' -> r) -> ReaderT r m a -> ReaderT r' m a | Source

module Control.Monad
module Control.Monad.Fix
module Control.Monad.Trans
Example 1: Simple Reader Usage

In this example the `Reader` monad provides access to variable bindings. Bindings are a Map of integer variables. The variable `count` contains number of variables in the bindings. You can see how to run a Reader monad and retrieve data from it with `runReader`, how to access the Reader data with `ask` and `asks`.

     type Bindings = Map String Int;

    -- Returns True if the "count" variable contains correct bindings size.
    isCountCorrect :: Bindings -> Bool
    isCountCorrect bindings = runReader calc_isCountCorrect bindings

    -- The Reader monad, which implements this complicated check.
    calc_isCountCorrect :: Reader Bindings Bool
    calc_isCountCorrect = do
        count <- asks (lookupVar "count")
        bindings <- ask
        return (count == (Map.size bindings))

    -- The selector function to  use with 'asks'.
    -- Returns value of the variable with specified name.
    lookupVar :: String -> Bindings -> Int
    lookupVar name bindings = fromJust (Map.lookup name bindings)

    sampleBindings = Map.fromList [("count",3), ("1",1), ("b",2)]

    main = do
        putStr $ "Count is correct for bindings " ++ (show sampleBindings) ++ ": ";
        putStrLn $ show (isCountCorrect sampleBindings);

Example 2: Modifying Reader Content With `local`

Shows how to modify Reader content with `local`.

    calculateContentLen :: Reader String Int
    calculateContentLen = do
        content <- ask
        return (length content);

    -- Calls calculateContentLen after adding a prefix to the Reader content.
    calculateModifiedContentLen :: Reader String Int
    calculateModifiedContentLen = local ("Prefix " ++) calculateContentLen

    main = do
        let s = "12345";
        let modifiedLen = runReader calculateModifiedContentLen s
        let len = runReader calculateContentLen s
        putStrLn $ "Modified 's' length: " ++ (show modifiedLen)
        putStrLn $ "Original 's' length: " ++ (show len)

Example 3: `ReaderT` Monad Transformer

Now you are thinking: 'Wow, what a great monad! I wish I could use Reader functionality in MyFavoriteComplexMonad!'. Don't worry. This can be easy done with the `ReaderT` monad transformer. This example shows how to combine `ReaderT` with the IO monad.

    -- The Reader/IO combined monad, where Reader stores a string.
    printReaderContent :: ReaderT String IO ()
    printReaderContent = do
        content <- ask
        liftIO $ putStrLn ("The Reader Content: " ++ content)

    main = do
        runReaderT printReaderContent "Some Content"

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