Here's a final pipes example then. I don't think there's a way to
fix the problem as Oleg proposed because pipes are monad
transformers by design.
The Pipe monad transformer augments the base monad with two
operations:
- await: gets a result from an upstream pipe
- yield: sends a result to a downstream pipe
I have a producer (which is a pipe that can only 'yield') that
produces the lines of the .CSV file as Strings and returns () when
done:
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
I then have a simple pipeline that reads each line and prints it
twice:
lineDoubler :: Pipe String String IO ()
lineDoubler = forever $ do
s <- await
yield s
yield s
main = do
runEffect $ getFileContentsLifted >-> lineDoubler
>-> stdoutLn
The problem as before is that this code does not work with the
original version of withCSV:
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
only with the lifted (i.e. generalized) one.
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
And I have the same question: Should I always "generalize" my
monadic actions that take callbacks as parameters?
I hope this version is still clear. Thanks for everyone for their
input. I thought this was an easier problem than it now appears to
be.
Dimitri
PS. Full code is here
https://gist.github.com/dimitri-xyz/f1f5bd4c0f7f2bf85379
On 10/26/15 10:47 AM, Kim-Ee Yeoh
wrote: