lift-pipes-example.hs

import System.IO
import Control.Monad
import Control.Monad.IO.Class
import Control.Exception.Base (evaluate)

import Pipes
import Pipes.Prelude (stdoutLn)

withCSV :: FilePath -> (Handle -> IO r) -> IO r
withCSV path action = do
    putStrLn "opening file"
    h <- openFile path ReadMode
    r <- action h
    hClose h
    putStrLn "file closed"
    return r

getFileContents :: Producer String IO ()
getFileContents = undefined -- do I go about writing this?

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withCSVLifted :: MonadIO mIO => FilePath -> (Handle -> mIO r) -> mIO r
withCSVLifted path action = do
    liftIO $ putStrLn "opening file"
    h <- liftIO $ openFile path ReadMode
    r <- action h
    liftIO $ hClose h
    liftIO $ putStrLn "file closed"
    return r

getFileContentsLifted :: Producer String IO ()
getFileContentsLifted = withCSVLifted "data.csv" myReadFile
    where
        myReadFile :: Handle -> Producer String IO ()
        myReadFile handle = do
                            eof <- lift $ hIsEOF handle
                            unless eof $ do
                                str <- lift $ hGetLine handle
                                yield str
                                myReadFile handle


lineDoubler :: Pipe String String IO ()
lineDoubler = forever $ do
            s <- await
            yield s
            yield s

main = do
    runEffect $ getFileContentsLifted >-> lineDoubler >-> stdoutLn


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