Composable handler for Reader effects. Handy for encapsulating an environment with immutable state for interpreters.

Using http://okmij.org/ftp/Haskell/extensible/Eff1.hs as a starting point.

In this example the Reader effect 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 effect and retrieve data from it with runReader, how to access the Reader data with ask and asks.

import Control.Monad.Freer
import Control.Monad.Freer.Reader
import Data.Map as Map
import Data.Maybe

type Bindings = Map String Int

-- Returns True if the "count" variable contains correct bindings size.
isCountCorrect :: Bindings -> Bool
isCountCorrect bindings = run $ runReader bindings calc_isCountCorrect

-- The Reader effect, which implements this complicated check.
calc_isCountCorrect :: Eff '[Reader Bindings] Bool
calc_isCountCorrect = do
  count <- asks (lookupVar "count")
  bindings <- (ask :: Eff '[Reader Bindings] Bindings)
  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.Map String Int
sampleBindings = Map.fromList [("count",3), ("1",1), ("b",2)]

main :: IO ()
main = putStrLn
  $ "Count is correct for bindings " ++ show sampleBindings ++ ": "
  ++ show (isCountCorrect sampleBindings)

Shows how to modify Reader content with local.

import Control.Monad.Freer
import Control.Monad.Freer.Reader

import Data.Map as Map
import Data.Maybe

type Bindings = Map String Int

calculateContentLen :: Eff '[Reader String] Int
calculateContentLen = do
  content <- (ask :: Eff '[Reader String] String)
  return (length content)

-- Calls calculateContentLen after adding a prefix to the Reader content.
calculateModifiedContentLen :: Eff '[Reader String] Int
calculateModifiedContentLen = local ("Prefix " ++) calculateContentLen

main :: IO ()
main = do
  let s = "12345"
  let modifiedLen = run $ runReader s calculateModifiedContentLen
  let len = run $ runReader s calculateContentLen
  putStrLn $ "Modified 's' length: " ++ (show modifiedLen)
  putStrLn $ "Original 's' length: " ++ (show len)