mtl- Monad transformer library

Portabilitynon-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:
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 ( Advanced School of Functional Programming, 1995.



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.




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.


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

withReader :: (r' -> r) -> Reader r a -> Reader r' aSource

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.




runReaderT :: r -> m a


MonadWriter w m => MonadWriter w (ReaderT r m) 
MonadError e m => MonadError e (ReaderT r m) 
MonadState s m => MonadState s (ReaderT r m) 
Monad m => MonadReader r (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 bSource

withReaderT :: (r' -> r) -> ReaderT r m a -> ReaderT r' m aSource

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"