|Portability||non-portable (multi-param classes, functional dependencies)|
- 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:
- 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
Mark P Jones (http://web.cecs.pdx.edu/~mpj/)
Advanced School of Functional Programming, 1995.
The parameterizable reader monad.
The return function creates a Reader that ignores the environment,
and produces the given value.
The binding operator >>= produces a Reader that uses the environment
to extract the value its left-hand side,
and then applies the bound function to that value in the same environment.
|runReader :: r -> a|
Runs Reader and extracts the final value from it.
To extract the value apply (runReader reader) to an environment value.
- A Reader to run.
- An initial environment.
|A more general version of local.
|The reader monad transformer.
Can be used to add environment reading functionality to other monads.
|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"
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