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Haskell: Monad Transformers -- Combine ReaderT and WriterT
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| -- | Main entry point to the application. | |
| module Main where | |
| import Control.Monad.Reader | |
| import Control.Monad.Writer | |
| {- | |
| The ReaderT transformer is used to retrieve a read-only value from some environment. | |
| The WriterT transformer will log the result of each retrieval. | |
| Running the two transformers together yields a log of each step along with the actual results. | |
| -} | |
| data Person = Person { name :: String } | |
| deriving (Show) | |
| alex = Person "Alex Fontaine" | |
| philip = Person "Philip Carpenter" | |
| kim = Person "Kim Lynch" | |
| peopleDb = [alex, philip, kim] | |
| -- Get person from environment, log the person, and print name | |
| process :: ReaderT Person (WriterT String IO) () | |
| process = do | |
| tell "Looking up a person. " | |
| Person p <- ask | |
| tell $ "Found person: " ++ p ++ ". " | |
| liftIO $ putStrLn p | |
| -- Get person from environment, log the person, and return the name | |
| process' :: ReaderT Person (WriterT String IO) String | |
| process' = do | |
| tell "Looking up a person... " | |
| Person p <- ask | |
| tell $ "Found person: " ++ p ++ ". " | |
| return p | |
| main :: IO () | |
| main = do | |
| -- Print name from monad transformer | |
| result1 <- runWriterT (runReaderT process alex) -- :: ((), String) | |
| putStrLn $ snd result1 | |
| -- Extract the name from monad transfomer, then print it | |
| result2 <- runWriterT (runReaderT process' alex) -- :: (String, String) | |
| putStrLn $ fst result2 | |
| putStrLn $ snd result2 | |
| -- Now do the same thing for a list of people using mapM | |
| result3 <- runWriterT (mapM (runReaderT process') peopleDb) -- :: ([String], String) | |
| let people = fst result3 | |
| log = snd result3 | |
| putStrLn "\n\nReaderT values:\n" | |
| mapM_ putStrLn people | |
| putStrLn "\nWriterT log:\n" | |
| putStrLn $ log | |
| return () | |
| ---OUTPUT--- | |
| {- | |
| Alex Fontaine | |
| Looking up a person. Found person: Alex Fontaine. | |
| Alex Fontaine | |
| Looking up a person... Found person: Alex Fontaine. | |
| ReaderT values: | |
| Alex Fontaine | |
| Philip Carpenter | |
| Kim Lynch | |
| WriterT log: | |
| Looking up a person... Found person: Alex Fontaine. Looking up a person... Found person: Philip Carpenter. Looking up a person... Found person: Kim Lynch. | |
| -} |
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