Re: [Haskell-cafe] best practice for lifting of IO and could lifting

27 Oct 2015

      Hi Dimitri,

The implementation of `withCSVLifted` looks dodgy to me. If a
downstream consumer terminates early, then the file will never get
closed.

For pipes, the standard solution to resource management is the
pipes-safe[1] package. It handles early termination and IO exceptions
automatically. The example in the docs should fit your use case pretty
well.

[1] https://hackage.haskell.org/package/pipes-safe-2.2.3/docs/Pipes-Safe.html

On Wed, Oct 28, 2015 at 12:25 PM, Dimitri DeFigueiredo
 wrote:
...
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:
On Mon, Oct 26, 2015 at 11:36 PM, Dimitri DeFigueiredo
 wrote:
...
I might have over simplified the problem by using ReaderT in my example.
In my original problem this role is played by the Pipes library (and instead
of using 'ask', I wanted to 'yield' control to a downstream pipe).
Is there a way you could introduce just enough complexity to allow Oleg
another stab?
Also, there's always the fallback of showing your Pipes-based code although
that library doesn't enjoy universal familiarity.
-- Kim-Ee
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Chris Wong (https://lambda.xyz)

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