[Haskell-cafe] best practice for lifting of IO and could lifting be automated?

Dimitri DeFigueiredo defigueiredo at ucdavis.edu
Mon Oct 26 07:27:37 UTC 2015

Hi Kim-Ee,

Sorry for not making the problem clear enough! Here's an example. It is 
somewhat contrived, but I think it captures the essence of the problem.

Imagine I need to read a .CSV file which may or may not contain column 
titles on its first line. I'm not interested in the column titles, I 
just want the rest of the file. I am provided a library function to read 
the contents of the file (using a "callback"). The library author 
provided this function in the IO monad.

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

The problem arises because I also want to use the ReaderT monad 
transformer. My environment information will tell me
whether or not to disregard the first (i.e. column title) line. Here's a 
*failed* attempt at writing this next step:

data ColumnHeaders = FirstLine | None

getFileContents :: ReaderT ColumnHeaders IO String
getFileContents = liftIO $ withCSV "data.csv" myReadFile
         myReadFile :: Handle -> IO String
         myReadFile handle = do
             header <- ask --- OOOPPSss!!! FAIL! Can't ask.
             case header of
                 None      -> return ""
                 FirstLine -> hGetLine handle -- skip first line
             text <- hGetContents handle
             evaluate (length text) -- force lazy IO
             return text

main = do
     cs <- runReaderT getFileContents FirstLine
     print cs

Unfortunately, I can't write getFileContents as described above because 
myReadFile is an IO action and cannot access the configuration 
information available through the Reader. If I could rewrite withCSV I 
could fix this issue:

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

The difference between withCSV and withCSVLifted is just a bunch of 
liftIO operations and a more flexible type signature. The crucial change 
is that the lifted version allows any function of type (MonadIO mIO => 
Handle -> mIO r) rather than just (Handle -> IO r). This is general 
enough to allow me to re-write my configuration step and call ask (from 
within the callback).

getFileContentsLifted :: ReaderT ColumnHeaders IO String
getFileContentsLifted = withCSVLifted "data.csv" myReadFile
         myReadFile :: Handle -> ReaderT ColumnHeaders IO String
         myReadFile handle = do
             header <- ask
             case header of
                 None      -> return ""
                 FirstLine -> liftIO $ hGetLine handle -- skip first 
line then
             text <- liftIO $ hGetContents handle
             liftIO $ evaluate (length text) -- force lazy IO
             return text

Other than calling the respective lifted version of withCSV the only 
difference between getFileContentsLifted and getFileContents are the 
extra liftIO calls.

It can be very cumbersome to write a working version of getFileContents 
in the IO monad  without easy access to ReaderT's ask. So, my questions 

1. Should library authors always provide lifted versions of functions 
that take callbacks? In other words, is
withCSVLifted :: MonadIO mIO => FilePath -> (Handle -> mIO r) -> mIO r
always better than
withCSV :: FilePath -> (Handle -> IO r) -> IO r
? If not, what's the best practice?

2. Once we define the MonadIO class, shouldn't the compiler be able to 
withCSV :: FilePath -> (Handle -> IO r) -> IO r
withCSVLifted :: MonadIO mIO => FilePath -> (Handle -> mIO r) -> mIO r
by adding a number of liftIO calls to that class upon request? It seems 
like the kind of change we would like to automate.

This email turned out to be longer than I expected. I hope it is clearer.
You can find all the code here:




On 10/23/15 7:48 PM, Kim-Ee Yeoh wrote:
> On Sat, Oct 24, 2015 at 5:25 AM, Dimitri DeFigueiredo 
> <defigueiredo at ucdavis.edu <mailto:defigueiredo at ucdavis.edu>> wrote:
>     Unfortunately, I am using the pipes library, so I cannot avoid
>     using a monad transformer. Because of the functionality pipes
>     provides, it does make sense for it to be a monad transformer.
> Hi Dimitri,
> This is a very interesting topic, thank you for bringing it up.
> Unfortunately because of the very generalized way it's presented, it's 
> very hard for anyone else aside from Yuras to give it the attention it 
> deserves.
> Do you have a concrete example with sample code that you could 
> simplify and present instead?
> E.g. instead of the multiply-stacked monad transformer embedded in 200 
> lines that you're facing, can you present an example with 2 monadic 
> layers (the base being IO) in say, 20 lines?
> -- Kim-Ee

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