Last active
August 29, 2015 14:04
-
-
Save hrldcpr/0b4b9e558f119b2cf7fa to your computer and use it in GitHub Desktop.
combination of Reader and Maybe monads, using the ReaderT monad transformer
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
import Control.Monad.Reader | |
import Control.Monad.Trans | |
import Data.List | |
import Data.Maybe | |
import Text.Printf | |
type Environment = [(String, String)] | |
get :: String -> ReaderT Environment Maybe String | |
get k = ask >>= lift . lookup k | |
-- here's the do-notation equivalent: | |
get' :: String -> ReaderT Environment Maybe String | |
get' k = do | |
env <- ask | |
lift $ lookup k env | |
format :: String -> String -> ReaderT Environment Maybe String | |
format fmt k = get k >>= return . printf fmt | |
business_card_lines :: Reader Environment [String] | |
business_card_lines = do | |
let lines = [format "Name: %s" "name", | |
format "Email: %s" "email", | |
format "Phone: %s" "phone"] | |
env <- ask | |
return $ catMaybes $ map (\r -> runReaderT r env) lines | |
business_card :: Reader Environment String | |
business_card = business_card_lines >>= return . intercalate "\n" | |
main = putStrLn $ runReader business_card [("name", "Harold"), ("email", "hrldcpr@gmail.com")] |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment