Control.Monad.Reader

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 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.

MonadReader class

class Monad m => MonadReader m where

See examples in Control.Monad.Reader. Note, the partially applied function type (->) r is a simple reader monad. See the instance declaration below.

Minimal complete definition

ask, local

Associated Types

type EnvType m

Methods

ask :: m (EnvType m)

Retrieves the monad environment.

local :: (EnvType m -> EnvType m) -> m a -> m a

Executes a computation in a modified environment.

Instances

MonadReader ((->) r)

Associated Types

type EnvType ((->) r :: * -> *) :: *

Methods

ask :: r -> EnvType ((->) r)

local :: (EnvType ((->) r) -> EnvType ((->) r)) -> (r -> a) -> r -> a

MonadReader m => MonadReader (MaybeT m)

Associated Types

type EnvType (MaybeT m :: * -> *) :: *

Methods

ask :: MaybeT m (EnvType (MaybeT m))

local :: (EnvType (MaybeT m) -> EnvType (MaybeT m)) -> MaybeT m a -> MaybeT m a

MonadReader m => MonadReader (ListT m)

Associated Types

type EnvType (ListT m :: * -> *) :: *

Methods

ask :: ListT m (EnvType (ListT m))

local :: (EnvType (ListT m) -> EnvType (ListT m)) -> ListT m a -> ListT m a

(Monoid w, MonadReader m) => MonadReader (WriterT w m)

Associated Types

type EnvType (WriterT w m :: * -> *) :: *

Methods

ask :: WriterT w m (EnvType (WriterT w m))

local :: (EnvType (WriterT w m) -> EnvType (WriterT w m)) -> WriterT w m a -> WriterT w m a

(Monoid w, MonadReader m) => MonadReader (WriterT w m)

Associated Types

type EnvType (WriterT w m :: * -> *) :: *

Methods

ask :: WriterT w m (EnvType (WriterT w m))

local :: (EnvType (WriterT w m) -> EnvType (WriterT w m)) -> WriterT w m a -> WriterT w m a

MonadReader m => MonadReader (StateT s m)

Associated Types

type EnvType (StateT s m :: * -> *) :: *

Methods

ask :: StateT s m (EnvType (StateT s m))

local :: (EnvType (StateT s m) -> EnvType (StateT s m)) -> StateT s m a -> StateT s m a

MonadReader m => MonadReader (StateT s m)

Associated Types

type EnvType (StateT s m :: * -> *) :: *

Methods

ask :: StateT s m (EnvType (StateT s m))

local :: (EnvType (StateT s m) -> EnvType (StateT s m)) -> StateT s m a -> StateT s m a

MonadReader m => MonadReader (IdentityT * m)

Associated Types

type EnvType (IdentityT * m :: * -> *) :: *

Methods

ask :: IdentityT * m (EnvType (IdentityT * m))

local :: (EnvType (IdentityT * m) -> EnvType (IdentityT * m)) -> IdentityT * m a -> IdentityT * m a

(Error e, MonadReader m) => MonadReader (ErrorT e m)

Associated Types

type EnvType (ErrorT e m :: * -> *) :: *

Methods

ask :: ErrorT e m (EnvType (ErrorT e m))

local :: (EnvType (ErrorT e m) -> EnvType (ErrorT e m)) -> ErrorT e m a -> ErrorT e m a

Monad m => MonadReader (ReaderT * r m)

Associated Types

type EnvType (ReaderT * r m :: * -> *) :: *

Methods

ask :: ReaderT * r m (EnvType (ReaderT * r m))

local :: (EnvType (ReaderT * r m) -> EnvType (ReaderT * r m)) -> ReaderT * r m a -> ReaderT * r m a

MonadReader m => MonadReader (ContT * r m)

Associated Types

type EnvType (ContT * r m :: * -> *) :: *

Methods

ask :: ContT * r m (EnvType (ContT * r m))

local :: (EnvType (ContT * r m) -> EnvType (ContT * r m)) -> ContT * r m a -> ContT * r m a

(Monoid w, Monad m) => MonadReader (RWST r w s m)

Associated Types

type EnvType (RWST r w s m :: * -> *) :: *

Methods

ask :: RWST r w s m (EnvType (RWST r w s m))

local :: (EnvType (RWST r w s m) -> EnvType (RWST r w s m)) -> RWST r w s m a -> RWST r w s m a

(Monoid w, Monad m) => MonadReader (RWST r w s m)

Associated Types

type EnvType (RWST r w s m :: * -> *) :: *

Methods

ask :: RWST r w s m (EnvType (RWST r w s m))

local :: (EnvType (RWST r w s m) -> EnvType (RWST r w s m)) -> RWST r w s m a -> RWST r w s m a

asks

Arguments

:: MonadReader m

=> (EnvType m -> a)

The selector function to apply to the environment.

-> m a

Retrieves a function of the current environment.

The Reader monad

type Reader r = ReaderT * r Identity

The parameterizable reader monad.

Computations are functions of a shared environment.

The return function ignores the environment, while >>= passes the inherited environment to both subcomputations.

runReader

Arguments

:: Reader r a

A Reader to run.

-> r

An initial environment.

-> a

Runs a Reader and extracts the final value from it. (The inverse of reader.)

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

Transform the value returned by a Reader.

• runReader (mapReader f m) = f . runReader m

withReader

Arguments

:: (r' -> r)

The function to modify the environment.

-> Reader r a

Computation to run in the modified environment.

-> Reader r' a

Execute a computation in a modified environment (a specialization of withReaderT).

• runReader (withReader f m) = runReader m . f

The ReaderT monad transformer

newtype ReaderT k r m a :: forall k. * -> (k -> *) -> k -> *

The reader monad transformer, which adds a read-only environment to the given monad.

The return function ignores the environment, while >>= passes the inherited environment to both subcomputations.

Constructors

ReaderT

Fields

• runReaderT :: r -> m a

Instances

MonadTrans (ReaderT * r)

Methods

lift :: Monad m => m a -> ReaderT * r m a

Monad m => Monad (ReaderT * r m)

Methods

(>>=) :: ReaderT * r m a -> (a -> ReaderT * r m b) -> ReaderT * r m b

(>>) :: ReaderT * r m a -> ReaderT * r m b -> ReaderT * r m b

return :: a -> ReaderT * r m a

fail :: String -> ReaderT * r m a

Functor m => Functor (ReaderT * r m)

Methods

fmap :: (a -> b) -> ReaderT * r m a -> ReaderT * r m b

(<$) :: a -> ReaderT * r m b -> ReaderT * r m a

MonadFix m => MonadFix (ReaderT * r m)

Methods

mfix :: (a -> ReaderT * r m a) -> ReaderT * r m a

MonadFail m => MonadFail (ReaderT * r m)

Methods

fail :: String -> ReaderT * r m a

Applicative m => Applicative (ReaderT * r m)

Methods

pure :: a -> ReaderT * r m a

(<*>) :: ReaderT * r m (a -> b) -> ReaderT * r m a -> ReaderT * r m b

(*>) :: ReaderT * r m a -> ReaderT * r m b -> ReaderT * r m b

(<*) :: ReaderT * r m a -> ReaderT * r m b -> ReaderT * r m a

MonadZip m => MonadZip (ReaderT * r m)

Methods

mzip :: ReaderT * r m a -> ReaderT * r m b -> ReaderT * r m (a, b)

mzipWith :: (a -> b -> c) -> ReaderT * r m a -> ReaderT * r m b -> ReaderT * r m c

munzip :: ReaderT * r m (a, b) -> (ReaderT * r m a, ReaderT * r m b)

MonadIO m => MonadIO (ReaderT * r m)

Methods

liftIO :: IO a -> ReaderT * r m a

Alternative m => Alternative (ReaderT * r m)

Methods

empty :: ReaderT * r m a

(<|>) :: ReaderT * r m a -> ReaderT * r m a -> ReaderT * r m a

some :: ReaderT * r m a -> ReaderT * r m [a]

many :: ReaderT * r m a -> ReaderT * r m [a]

MonadPlus m => MonadPlus (ReaderT * r m)

Methods

mzero :: ReaderT * r m a

mplus :: ReaderT * r m a -> ReaderT * r m a -> ReaderT * r m a

MonadCont m => MonadCont (ReaderT * r m)

Methods

callCC :: ((a -> ReaderT * r m b) -> ReaderT * r m a) -> ReaderT * r m a

MonadError m => MonadError (ReaderT * r m)

Associated Types

type ErrorType (ReaderT * r m :: * -> *) :: *

Methods

throwError :: ErrorType (ReaderT * r m) -> ReaderT * r m a

catchError :: ReaderT * r m a -> (ErrorType (ReaderT * r m) -> ReaderT * r m a) -> ReaderT * r m a

Monad m => MonadReader (ReaderT * r m)

Associated Types

type EnvType (ReaderT * r m :: * -> *) :: *

Methods

ask :: ReaderT * r m (EnvType (ReaderT * r m))

local :: (EnvType (ReaderT * r m) -> EnvType (ReaderT * r m)) -> ReaderT * r m a -> ReaderT * r m a

MonadState m => MonadState (ReaderT * r m)

Associated Types

type StateType (ReaderT * r m :: * -> *) :: *

Methods

get :: ReaderT * r m (StateType (ReaderT * r m))

put :: StateType (ReaderT * r m) -> ReaderT * r m ()

MonadWriter m => MonadWriter (ReaderT * r m)

Associated Types

type WriterType (ReaderT * r m :: * -> *) :: *

Methods

tell :: WriterType (ReaderT * r m) -> ReaderT * r m ()

listen :: ReaderT * r m a -> ReaderT * r m (a, WriterType (ReaderT * r m))

pass :: ReaderT * r m (a, WriterType (ReaderT * r m) -> WriterType (ReaderT * r m)) -> ReaderT * r m a

type ErrorType (ReaderT * r m)

type ErrorType (ReaderT * r m) = ErrorType m

type EnvType (ReaderT * r m)

type EnvType (ReaderT * r m) = r

type StateType (ReaderT * r m)

type StateType (ReaderT * r m) = StateType m

type WriterType (ReaderT * r m)

type WriterType (ReaderT * r m) = WriterType m

mapReaderT :: (m a -> n b) -> ReaderT k r m a -> ReaderT k1 r n b

Transform the computation inside a ReaderT.

• runReaderT (mapReaderT f m) = f . runReaderT m

withReaderT

Arguments

:: (r' -> r)

The function to modify the environment.

-> ReaderT k r m a

Computation to run in the modified environment.

-> ReaderT k r' m a

Execute a computation in a modified environment (a more general version of local).

• runReaderT (withReaderT f m) = runReaderT m . f

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"