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)