Control.Monad.Reader

mtl-2.0.1.0: Monad classes, using functional dependencies

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

Control.Monad.Reader

Contents

- MonadReader class
- The Reader monad
- The ReaderT monad transformer
- Example 1: Simple Reader Usage
- Example 2: Modifying Reader Content With `local`
- Example 3: `ReaderT` Monad Transformer

Description

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.

Synopsis

- class Monad m => MonadReader r m | m -> r where
  - ask :: m r
  - local :: (r -> r) -> m a -> m a
- asks :: MonadReader r m => (r -> a) -> m a
- type Reader r = ReaderT r Identity
- reader :: (r -> a) -> Reader r a
- runReader :: Reader r a -> r -> a
- mapReader :: (a -> b) -> Reader r a -> Reader r b
- withReader :: (r' -> r) -> Reader r a -> Reader r' a
- newtype ReaderT r m a = ReaderT {
  - runReaderT :: r -> m a
  }
- mapReaderT :: (m a -> n b) -> ReaderT r m a -> ReaderT r n b
- withReaderT :: (r' -> r) -> ReaderT r m a -> ReaderT r' m a
- module Control.Monad
- module Control.Monad.Fix
- module Control.Monad.Trans

# MonadReader class

class Monad m => MonadReader r m | 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

Arguments

:: (r -> r) | The function to modify the environment.
-> m a | `Reader` to run in the modified environment.
-> m a |

Executes a computation in a modified environment.

Instances

MonadReader r m => MonadReader r (MaybeT m) |
MonadReader r m => MonadReader r (ListT m) |
MonadReader r m => MonadReader r (IdentityT m) |
MonadReader r ((->) r) |
(Monoid w, MonadReader r m) => MonadReader r (WriterT w m) |
(Monoid w, MonadReader r m) => MonadReader r (WriterT w m) |
MonadReader r m => MonadReader r (StateT s m) |
MonadReader r m => MonadReader r (StateT s m) |
(Error e, MonadReader r m) => MonadReader r (ErrorT e m) |
MonadReader r' m => MonadReader r' (ContT r m) |
Monad m => MonadReader r (ReaderT r m) |
(Monad m, Monoid w) => MonadReader r (RWST r w s m) |
(Monad m, Monoid w) => MonadReader r (RWST r w s m) |

asks

Arguments

:: MonadReader r m |
=> (r -> 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.

reader :: (r -> a) -> Reader r a

Constructor for computations in the reader monad.

runReader

Arguments

:: Reader r a | A `Reader` to run.
-> r | An initial environment.
-> a |

Runs a `Reader` and extracts the final value from it.

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

Transform the value returned by a `Reader`.

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`).

# The ReaderT monad transformer

newtype ReaderT r m a

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 The underlying computation, as a function of the environment.

Instances

MonadWriter w m => MonadWriter w (ReaderT r m) |
MonadState s m => MonadState s (ReaderT r m) |
Monad m => MonadReader r (ReaderT r m) |
MonadError e m => MonadError e (ReaderT r m) |
MonadTrans (ReaderT r) |
Monad m => Monad (ReaderT r m) |
Functor m => Functor (ReaderT r m) |
MonadFix m => MonadFix (ReaderT r m) |
MonadPlus m => MonadPlus (ReaderT r m) |
Applicative m => Applicative (ReaderT r m) |
Alternative m => Alternative (ReaderT r m) |
MonadIO m => MonadIO (ReaderT r m) |
MonadCont m => MonadCont (ReaderT r m) |

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

Transform the computation inside a `ReaderT`.

withReaderT

Arguments

:: (r' -> r) | The function to modify the environment.
-> ReaderT r m a | Computation to run in the modified environment.
-> ReaderT r' m a |

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

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"