Contents

• MonadReader class
• The Reader monad
• The ReaderT monad transformer
• Example 1: Simple Reader Usage
• Example 2: Modifying Reader Content Withlocal
• Example 3:ReaderT Monad Transformer

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 accessto the shared environment.

Useful for:

Maintaining variable bindings, or other shared environment.

Zero and plus:

None.

Example type:

Reader [(String,Value)] a

TheReader monad (also called the Environment monad).Represents a computation, which can read values froma shared environment, pass values from function to function,and execute sub-computations in a modified environment.UsingReader monad for such computations is often clearer and easierthan using theState monad.

Inspired by the paperFunctional Programming with Overloading and Higher-Order Polymorphism, Mark P Jones (http://web.cecs.pdx.edu/~mpj/) Advanced School of Functional Programming, 1995.

classMonad m =>MonadReader r m | m -> rwhereSource#

asksSource#

typeReader r =ReaderT rIdentitySource#

runReaderSource#

mapReader :: (a -> b) ->Reader r a ->Reader r bSource#

withReaderSource#

newtypeReaderT r (m ::Type ->Type) aSource#

runReaderT ::ReaderT r m a -> r -> m aSource#

mapReaderT :: (m a -> n b) ->ReaderT r m a ->ReaderT r n bSource#

withReaderTSource#

moduleControl.Monad

moduleControl.Monad.Fix

moduleControl.Monad.Trans

In this example theReader monad provides access to variable bindings.Bindings are aMap of integer variables.The variablecount contains number of variables in the bindings.You can see how to run a Reader monad and retrieve data from itwithrunReader, how to access the Reader data withask andasks.

 type Bindings = Map String Int;-- Returns True if the "count" variable contains correct bindings size.isCountCorrect :: Bindings -> BoolisCountCorrect bindings = runReader calc_isCountCorrect bindings-- The Reader monad, which implements this complicated check.calc_isCountCorrect :: Reader Bindings Boolcalc_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 -> IntlookupVar 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);

Shows how to modify Reader content withlocal.

calculateContentLen :: Reader String IntcalculateContentLen = do    content <- ask    return (length content);-- Calls calculateContentLen after adding a prefix to the Reader content.calculateModifiedContentLen :: Reader String IntcalculateModifiedContentLen = local ("Prefix " ++) calculateContentLenmain = 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)

Now you are thinking: 'Wow, what a great monad! I wish I could useReader functionality in MyFavoriteComplexMonad!'. Don't worry.This can be easily done with theReaderT monad transformer.This example shows how to combineReaderT 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"