# Periodically comparing the play count of two Last.FM users

There is some little program (using a Last.FM API wrapper) which gets the play count of 2 specific users every 5 minutes and appends the difference between these 2 numbers to log file.

Can you review that part of the code? I think that using the <- operator so many times decreases the readability and simplicity of the code.

main = forever $do diff <- user1 subtractCounts user2 timestamp <- formatTime defaultTimeLocale dateFormat <$> getZonedTime
let logMessage = printf "%s %d\n" timestamp diff :: String
homeDir <- getHomeDirectory
let logFilePath = homeDir </> logFile
appendFile logFilePath logMessage
threadDelay $10^6 * 60 * timeDelay -- in microseconds where subtractCounts = liftM2 ((-) on fromMaybe 0) on getPlayCount dateFormat = "[%s] %D %H:%M"  ## 1 Answer I don't see why you say <- "decreases readability", but you could line them up if you think that helps (this style is typical of Haskell programmers) main = forever$ do
diff      <- user1 subtractCounts user2
timestamp <- formatTime defaultTimeLocale dateFormat <$> getZonedTime homeDir <- getHomeDirectory let logMessage = printf "%s %d\n" timestamp diff logFilePath = homeDir </> logFile appendFile logFilePath logMessage threadDelay$ 10^6 * 60 * timeDelay -- in microseconds

subtractCounts = liftM2 ((-) on fromMaybe 0) on getPlayCount
dateFormat     = "[%s] %D %H:%M"


<- is a visual cue that the thing on the right might to perform an effect before producing a result and binding it to the name on the left.

Note that the type annotation on logMessage is unnecessary. Because it is used as the second argument to appendFile it will be inferred to be String.

I personally might make a few more refactorings, but they aren't really necessary:

logMessage :: String -> Integer -> String
logMessage  = printf "%s %d\n"

(f .: g) x y = f $g x y subtractCounts = liftM2 ((-) on fromMaybe 0) on getPlayCount main = forever$ do
homeDir <- getHomeDirectory
let logFilePath = homeDir </> logFile
logDiff     = appendFile logFilePath .: logMessage
forever $mainLoop logDiff mainLoop logDiff = do diff <- user1 subtractCounts user2 timestamp <- formatTime defaultTimeLocale "[%s] %D %H:%M" <$> getZonedTime
logDiff timestamp diff
threadDelay \$ 10^6 * 60 * timeDelay -- in microseconds


The type signature on logMessage is probably unnecessary, but imho it's a good idea to give it an explicit signature if you are using the result of printf as a function, rather than a String or IO ().

• Where is .: operator from? Mar 26, 2012 at 18:35
• Oh, sorry, I forgot to mention: (f .: g) x y = f (g x y). It's not in the standard libraries, but is a common trick for writing pointfree code. Mar 26, 2012 at 19:28
• Actually you forgot about threadDelay. Mar 26, 2012 at 20:01