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GHC 9.10.3 · lts/ghc-9.10.x · 248f8f0 · 2026-10-05

Modulemtl-2.3.1Haskell2010

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 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.

  • 2 types
  • 1 class
  • 7 values
  • Packagemtl-2.3.1
  • Exports10
  • LanguageHaskell2010
  • LicenceBSD-3-Clause
  • SourceReader.hs

MonadReader class

2 declarations
classclass Monad m => MonadReader r (m :: Type -> Type) | m -> r where
#

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

Methods

  • ask :: m r

    Retrieves the monad environment.

  • local :: (r -> r) -> m a -> m a

    Executes a computation in a modified environment.

  • reader :: (r -> a) -> m a

    Retrieves a function of the current environment.

Instances16MonadReader, …
valueasks
  1. :: MonadReader r m
  2. => (r -> a)

    The selector function to apply to the environment.

  3. -> m a
#

Retrieves a function of the current environment.

The Reader monad

4 declarations
valuerunReader
  1. :: Reader r a

    A Reader to run.

  2. -> r

    An initial environment.

  3. -> a
#

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

The ReaderT monad transformer

4 declarations
newtypenewtype ReaderT r (m :: Type -> Type) a
#

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

The return function ignores the environment, while m >>= k passes the inherited environment to both subcomputations:

image: images/bind-ReaderT.svg

Constructors

Instances22Generic1, MonadAccum, MonadError, MonadReader, MonadState, MonadWriter, …

module Control.Monad.Trans

Example 1: Simple Reader Usage

0 declarations

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.

import           Control.Monad.Reader
import           Data.Map (Map)
import qualified Data.Map as Map

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

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

Example 2: Modifying Reader Content With local

0 declarations

Shows how to modify Reader content with local.

import Control.Monad.Reader

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 :: IO ()
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

0 declarations

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.

import Control.Monad.Reader

-- 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 :: IO ()
main = runReaderT printReaderContent "Some Content"