Control/Monad/Reader.hs

    {- |
    Module      :  Control.Monad.Reader
    Copyright   :  (c) Andy Gill 2001,
                   (c) Oregon Graduate Institute of Science and Technology 2001,
                   (c) Jeff Newbern 2003-2007,
                   (c) Andriy Palamarchuk 2007
    License     :  BSD-style (see the file LICENSE)

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

    [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.
    -}

    module Control.Monad.Reader (
        -- * MonadReader class
        MonadReader(..),
        asks,
        -- * The Reader monad
        Reader,
        runReader,
        mapReader,
        withReader,
        -- * The ReaderT monad transformer
        ReaderT(ReaderT),
        runReaderT,
        mapReaderT,
        withReaderT,
        module Control.Monad,
        module Control.Monad.Fix,
        module Control.Monad.Trans,
        -- * Example 1: Simple Reader Usage
        -- $simpleReaderExample

        -- * Example 2: Modifying Reader Content With @local@
        -- $localExample

        -- * Example 3: @ReaderT@ Monad Transformer
        -- $ReaderTExample
        ) where

    import Control.Monad.Reader.Class

    import Control.Monad.Trans.Reader (
        Reader, runReader, mapReader, withReader,
        ReaderT(ReaderT), runReaderT, mapReaderT, withReaderT)
    import Control.Monad.Trans

    import Control.Monad
    import Control.Monad.Fix

    {- $simpleReaderExample

    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 = maybe 0 id (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);
    -}

    {- $localExample

    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)
    -}

    {- $ReaderTExample

    Now you are thinking: 'Wow, what a great monad! I wish I could use
    Reader functionality in MyFavoriteComplexMonad!'. Don't worry.
    This can be easily 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"
    -}