Control.Monad.Reader

mtl-2.2.2: Monad classes, using functional dependencies

Copyright | (c) Andy Gill 2001 (c) Oregon Graduate Institute of Science and Technology 2001 (c) Jeff Newbern 2003-2007 (c) Andriy Palamarchuk 2007
License | BSD-style (see the file LICENSE)
Maintainer | libraries@haskell.org
Stability | experimental
Portability | non-portable (multi-param classes, functional dependencies)
Safe Haskell | Safe
Language | Haskell2010

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 `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
  - reader :: (r -> a) -> m a
- asks :: MonadReader r m => (r -> a) -> m a
- type Reader r = ReaderT r Identity
- 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 :: k -> Type) (a :: k) :: forall k. Type -> (k -> Type) -> k -> Type = ReaderT (r -> m a)
- runReaderT :: ReaderT r m a -> 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.

Minimal complete definition

(ask | reader), local

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.

reader #

Arguments

:: (r -> a) | The selector function to apply to the environment.
-> m a |

Retrieves a function of the current environment.

Instances

MonadReader r m => MonadReader r (MaybeT m) # |
Instance details Defined in Control.Monad.Reader.Class Methods ask :: MaybeT m r # local :: (r -> r) -> MaybeT m a -> MaybeT m a # reader :: (r -> a) -> MaybeT m a #
MonadReader r m => MonadReader r (ListT m) # |
Instance details Defined in Control.Monad.Reader.Class Methods ask :: ListT m r # local :: (r -> r) -> ListT m a -> ListT m a # reader :: (r -> a) -> ListT m a #
(Monoid w, MonadReader r m) => MonadReader r (WriterT w m) # |
Instance details Defined in Control.Monad.Reader.Class Methods ask :: WriterT w m r # local :: (r -> r) -> WriterT w m a -> WriterT w m a # reader :: (r -> a) -> WriterT w m a #
(Monoid w, MonadReader r m) => MonadReader r (WriterT w m) # |
Instance details Defined in Control.Monad.Reader.Class Methods ask :: WriterT w m r # local :: (r -> r) -> WriterT w m a -> WriterT w m a # reader :: (r -> a) -> WriterT w m a #
MonadReader r m => MonadReader r (StateT s m) # |
Instance details Defined in Control.Monad.Reader.Class Methods ask :: StateT s m r # local :: (r -> r) -> StateT s m a -> StateT s m a # reader :: (r -> a) -> StateT s m a #
MonadReader r m => MonadReader r (StateT s m) # |
Instance details Defined in Control.Monad.Reader.Class Methods ask :: StateT s m r # local :: (r -> r) -> StateT s m a -> StateT s m a # reader :: (r -> a) -> StateT s m a #
MonadReader r m => MonadReader r (IdentityT m) # |
Instance details Defined in Control.Monad.Reader.Class Methods ask :: IdentityT m r # local :: (r -> r) -> IdentityT m a -> IdentityT m a # reader :: (r -> a) -> IdentityT m a #
MonadReader r m => MonadReader r (ExceptT e m) # | Since: mtl-2.2
Instance details Defined in Control.Monad.Reader.Class Methods ask :: ExceptT e m r # local :: (r -> r) -> ExceptT e m a -> ExceptT e m a # reader :: (r -> a) -> ExceptT e m a #
(Error e, MonadReader r m) => MonadReader r (ErrorT e m) # |
Instance details Defined in Control.Monad.Reader.Class Methods ask :: ErrorT e m r # local :: (r -> r) -> ErrorT e m a -> ErrorT e m a # reader :: (r -> a) -> ErrorT e m a #
MonadReader r' m => MonadReader r' (ContT r m) # |
Instance details Defined in Control.Monad.Reader.Class Methods ask :: ContT r m r' # local :: (r' -> r') -> ContT r m a -> ContT r m a # reader :: (r' -> a) -> ContT r m a #
Monad m => MonadReader r (ReaderT r m) # |
Instance details Defined in Control.Monad.Reader.Class Methods ask :: ReaderT r m r # local :: (r -> r) -> ReaderT r m a -> ReaderT r m a # reader :: (r -> a) -> ReaderT r m a #
MonadReader r ((->) r :: Type -> Type) # |
Instance details Defined in Control.Monad.Reader.Class Methods ask :: r -> r # local :: (r -> r) -> (r -> a) -> r -> a # reader :: (r -> a) -> r -> a #
(Monad m, Monoid w) => MonadReader r (RWST r w s m) # |
Instance details Defined in Control.Monad.Reader.Class Methods ask :: RWST r w s m r # local :: (r -> r) -> RWST r w s m a -> RWST r w s m a # reader :: (r -> a) -> RWST r w s m a #
(Monad m, Monoid w) => MonadReader r (RWST r w s m) # |
Instance details Defined in Control.Monad.Reader.Class Methods ask :: RWST r w s m r # local :: (r -> r) -> RWST r w s m a -> RWST r w s m a # reader :: (r -> a) -> RWST r w s m a #

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 Source #

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 Source #

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 Source #

Transform the value returned by a `Reader`.

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

withReader Source #

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 r (m :: k -> Type) (a :: k) :: forall k. Type -> (k -> Type) -> k -> Type Source #

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 (r -> m a) |

Instances

MonadError e m => MonadError e (ReaderT r m) # |
Instance details Defined in Control.Monad.Error.Class Methods throwError :: e -> ReaderT r m a # catchError :: ReaderT r m a -> (e -> ReaderT r m a) -> ReaderT r m a #
Monad m => MonadReader r (ReaderT r m) # |
Instance details Defined in Control.Monad.Reader.Class Methods ask :: ReaderT r m r # local :: (r -> r) -> ReaderT r m a -> ReaderT r m a # reader :: (r -> a) -> ReaderT r m a #
MonadState s m => MonadState s (ReaderT r m) # |
Instance details Defined in Control.Monad.State.Class Methods get :: ReaderT r m s # put :: s -> ReaderT r m () # state :: (s -> (a, s)) -> ReaderT r m a #
MonadWriter w m => MonadWriter w (ReaderT r m) # |
Instance details Defined in Control.Monad.Writer.Class Methods writer :: (a, w) -> ReaderT r m a # tell :: w -> ReaderT r m () # listen :: ReaderT r m a -> ReaderT r m (a, w) # pass :: ReaderT r m (a, w -> w) -> ReaderT r m a #
MonadTrans (ReaderT r :: (Type -> Type) -> Type -> Type) |
Instance details Defined in Control.Monad.Trans.Reader Methods lift :: Monad m => m a -> ReaderT r m a Source #
Monad m => Monad (ReaderT r m) |
Instance details Defined in Control.Monad.Trans.Reader Methods (>>=) :: ReaderT r m a -> (a -> ReaderT r m b) -> ReaderT r m b Source # (>>) :: ReaderT r m a -> ReaderT r m b -> ReaderT r m b Source # return :: a -> ReaderT r m a Source # fail :: String -> ReaderT r m a Source #
Functor m => Functor (ReaderT r m) |
Instance details Defined in Control.Monad.Trans.Reader Methods fmap :: (a -> b) -> ReaderT r m a -> ReaderT r m b Source # (<$) :: a -> ReaderT r m b -> ReaderT r m a Source #
MonadFix m => MonadFix (ReaderT r m) |
Instance details Defined in Control.Monad.Trans.Reader Methods mfix :: (a -> ReaderT r m a) -> ReaderT r m a Source #
MonadFail m => MonadFail (ReaderT r m) |
Instance details Defined in Control.Monad.Trans.Reader Methods fail :: String -> ReaderT r m a Source #
Applicative m => Applicative (ReaderT r m) |
Instance details Defined in Control.Monad.Trans.Reader Methods pure :: a -> ReaderT r m a Source # (<*>) :: ReaderT r m (a -> b) -> ReaderT r m a -> ReaderT r m b Source # liftA2 :: (a -> b -> c) -> ReaderT r m a -> ReaderT r m b -> ReaderT r m c Source # (*>) :: ReaderT r m a -> ReaderT r m b -> ReaderT r m b Source # (<*) :: ReaderT r m a -> ReaderT r m b -> ReaderT r m a Source #
Contravariant m => Contravariant (ReaderT r m) |
Instance details Defined in Control.Monad.Trans.Reader Methods contramap :: (a -> b) -> ReaderT r m b -> ReaderT r m a Source # (>$) :: b -> ReaderT r m b -> ReaderT r m a Source #
MonadZip m => MonadZip (ReaderT r m) |
Instance details Defined in Control.Monad.Trans.Reader Methods mzip :: ReaderT r m a -> ReaderT r m b -> ReaderT r m (a, b) Source # mzipWith :: (a -> b -> c) -> ReaderT r m a -> ReaderT r m b -> ReaderT r m c Source # munzip :: ReaderT r m (a, b) -> (ReaderT r m a, ReaderT r m b) Source #
MonadIO m => MonadIO (ReaderT r m) |
Instance details Defined in Control.Monad.Trans.Reader Methods liftIO :: IO a -> ReaderT r m a Source #
Alternative m => Alternative (ReaderT r m) |
Instance details Defined in Control.Monad.Trans.Reader Methods empty :: ReaderT r m a Source # (<|>) :: ReaderT r m a -> ReaderT r m a -> ReaderT r m a Source # some :: ReaderT r m a -> ReaderT r m [a] Source # many :: ReaderT r m a -> ReaderT r m [a] Source #
MonadPlus m => MonadPlus (ReaderT r m) |
Instance details Defined in Control.Monad.Trans.Reader Methods mzero :: ReaderT r m a Source # mplus :: ReaderT r m a -> ReaderT r m a -> ReaderT r m a Source #
MonadCont m => MonadCont (ReaderT r m) # |
Instance details Defined in Control.Monad.Cont.Class Methods callCC :: ((a -> ReaderT r m b) -> ReaderT r m a) -> ReaderT r m a #

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

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

Transform the computation inside a `ReaderT`.

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

withReaderT Source #

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`).

-     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 = 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);

# 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 easily 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"