| Portability | non-portable (multi-param classes, functional dependencies) |
|---|---|
| Stability | experimental |
| Maintainer | ross@soi.city.ac.uk |
Control.Monad.Reader
Contents
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.
- class Monad m => MonadReader r m | m -> r where
- 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 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.
Arguments
| :: (r -> r) | The function to modify the environment. |
| -> m a |
|
| -> 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) | |
| Monad m => MonadReader r (ReaderT r m) | |
| (Error e, MonadReader r m) => MonadReader r (ErrorT e m) | |
| MonadReader r' m => MonadReader r' (ContT r m) | |
| (Monad m, Monoid w) => MonadReader r (RWST r w s m) | |
| (Monad m, Monoid w) => MonadReader r (RWST r w s m) |
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.
Arguments
| :: Reader r a | A |
| -> r | An initial environment. |
| -> a |
Runs a Reader and extracts the final value from it.
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
| |
Instances
| MonadError e m => MonadError e (ReaderT r m) | |
| Monad m => MonadReader r (ReaderT r m) | |
| MonadState s m => MonadState s (ReaderT r m) | |
| MonadWriter w m => MonadWriter w (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.
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"