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Control.Monad.Reader

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 [email protected]
Stability experimental
Portability non-portable (multi-param classes, functional dependencies)
Safe Haskell Safe
Language Haskell2010

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.

MonadReader class

class Monad m => MonadReader r m | m -> r where Source

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 Source

Retrieves the monad environment.

local Source

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 Source

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 Source

local :: (r -> r) -> MaybeT m a -> MaybeT m a Source

reader :: (r -> a) -> MaybeT m a Source

MonadReader r m => MonadReader r (ListT m)
Instance details

Defined in Control.Monad.Reader.Class

Methods

ask :: ListT m r Source

local :: (r -> r) -> ListT m a -> ListT m a Source

reader :: (r -> a) -> ListT m a Source

(Monoid w, MonadReader r m) => MonadReader r (WriterT w m)
Instance details

Defined in Control.Monad.Reader.Class

Methods

ask :: WriterT w m r Source

local :: (r -> r) -> WriterT w m a -> WriterT w m a Source

reader :: (r -> a) -> WriterT w m a Source

(Monoid w, MonadReader r m) => MonadReader r (WriterT w m)
Instance details

Defined in Control.Monad.Reader.Class

Methods

ask :: WriterT w m r Source

local :: (r -> r) -> WriterT w m a -> WriterT w m a Source

reader :: (r -> a) -> WriterT w m a Source

MonadReader r m => MonadReader r (StateT s m)
Instance details

Defined in Control.Monad.Reader.Class

Methods

ask :: StateT s m r Source

local :: (r -> r) -> StateT s m a -> StateT s m a Source

reader :: (r -> a) -> StateT s m a Source

MonadReader r m => MonadReader r (StateT s m)
Instance details

Defined in Control.Monad.Reader.Class

Methods

ask :: StateT s m r Source

local :: (r -> r) -> StateT s m a -> StateT s m a Source

reader :: (r -> a) -> StateT s m a Source

MonadReader r m => MonadReader r (IdentityT m)
Instance details

Defined in Control.Monad.Reader.Class

Methods

ask :: IdentityT m r Source

local :: (r -> r) -> IdentityT m a -> IdentityT m a Source

reader :: (r -> a) -> IdentityT m a Source

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 Source

local :: (r -> r) -> ExceptT e m a -> ExceptT e m a Source

reader :: (r -> a) -> ExceptT e m a Source

(Error e, MonadReader r m) => MonadReader r (ErrorT e m)
Instance details

Defined in Control.Monad.Reader.Class

Methods

ask :: ErrorT e m r Source

local :: (r -> r) -> ErrorT e m a -> ErrorT e m a Source

reader :: (r -> a) -> ErrorT e m a Source

MonadReader r' m => MonadReader r' (ContT r m)
Instance details

Defined in Control.Monad.Reader.Class

Methods

ask :: ContT r m r' Source

local :: (r' -> r') -> ContT r m a -> ContT r m a Source

reader :: (r' -> a) -> ContT r m a Source

Monad m => MonadReader r (ReaderT r m)
Instance details

Defined in Control.Monad.Reader.Class

Methods

ask :: ReaderT r m r Source

local :: (r -> r) -> ReaderT r m a -> ReaderT r m a Source

reader :: (r -> a) -> ReaderT r m a Source

MonadReader r ((->) r :: Type -> Type)
Instance details

Defined in Control.Monad.Reader.Class

Methods

ask :: r -> r Source

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

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

(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 Source

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

reader :: (r -> a) -> RWST r w s m a Source

(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 Source

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

reader :: (r -> a) -> RWST r w s m a Source

asks Source

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.

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).

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 Source

catchError :: ReaderT r m a -> (e -> ReaderT r m a) -> ReaderT r m a Source

Monad m => MonadReader r (ReaderT r m)
Instance details

Defined in Control.Monad.Reader.Class

Methods

ask :: ReaderT r m r Source

local :: (r -> r) -> ReaderT r m a -> ReaderT r m a Source

reader :: (r -> a) -> ReaderT r m a Source

MonadState s m => MonadState s (ReaderT r m)
Instance details

Defined in Control.Monad.State.Class

Methods

get :: ReaderT r m s Source

put :: s -> ReaderT r m () Source

state :: (s -> (a, s)) -> ReaderT r m a Source

MonadWriter w m => MonadWriter w (ReaderT r m)
Instance details

Defined in Control.Monad.Writer.Class

Methods

writer :: (a, w) -> ReaderT r m a Source

tell :: w -> ReaderT r m () Source

listen :: ReaderT r m a -> ReaderT r m (a, w) Source

pass :: ReaderT r m (a, w -> w) -> ReaderT r m a Source

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 Source

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.

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).

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"

© The University of Glasgow and others
Licensed under a BSD-style license (see top of the page).
https://downloads.haskell.org/~ghc/8.6.1/docs/html/libraries/mtl-2.2.2/Control-Monad-Reader.html