Control.Monad.Reader

monads-tf-0.1.0.3: Monad classes, using type families

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 | ross@soi.city.ac.uk
Stability | experimental
Portability | non-portable (type families)
Safe Haskell | Safe
Language | Haskell98

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 m where
  - type EnvType m
- asks :: MonadReader m => (EnvType m -> 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 k r m a :: forall k. * -> (k -> *) -> k -> * = ReaderT {
  - runReaderT :: r -> m a
  }
- mapReaderT :: (m a -> n b) -> ReaderT k r m a -> ReaderT k1 r n b
- withReaderT :: (r' -> r) -> ReaderT k r m a -> ReaderT k r' m a
- module Control.Monad
- module Control.Monad.Fix
- module Control.Monad.Trans

# 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"