[Haskell-cafe] Why so slow?

[Donald Bruce Stewart]

lists:
> The code below is using way more RAM than it should. It seems to only 
> take so long when I build the 'programs' list - the actual 
> reading/parsing is fast. For a 5MB input file, it's using 50MB of RAM! 
> Any idea how to combat this?
> 
> Thanks,
> Lyle
> 
> {-# OPTIONS_GHC -fglasgow-exts #-}
> 
> -- linear_importer.hs
> 
> import Control.Monad (unless)
> import Data.ByteString.Char8 (ByteString)
> import qualified Data.ByteString.Char8 as BS
> import Text.Regex as RE
> import Data.Time.Calendar
> import Data.Time.LocalTime
> import System.Environment
> 
> -- Consider adding strictness as necessary for performance.
> 
> --                                 STB time      channel
> data ChannelChange = ChannelChange Int LocalTime Int
>    deriving Show
> 
> --                                     channel program_no start     
> end       network_no
> data ScheduleProgram = ScheduleProgram Int     Int        LocalTime 
> LocalTime (Maybe Int)
>    deriving Show
> 
> main = do
>    fileNames <- getArgs
>    ensure (length fileNames == 2) usageMessage
>    let [eventFileName,programFileName] = fileNames
>    putStrLn ("Reading program schedule file from '" ++ programFileName 
> ++ "'...")
>    text <- BS.readFile programFileName
>    programs <- sequence $ map parseScheduleProgram (BS.lines text)
>    print (take 20 programs)
>    return ()
> 
> usageMessage = "Usage: linear_importer <channel change file> <schedule 
> file>"
> 
> parseScheduleProgram :: ByteString -> IO ScheduleProgram
> parseScheduleProgram s = do
>    let fields = BS.split '|' s
>    ensure (length fields == 7) ("Wrong number of fields in schedule 
> program: " ++ BS.unpack s)
>    let 
> [_,channelNoText,programNoText,_,startTimeText,endTimeText,networkNoText] 
> = fields
>    let channelNo = read $ BS.unpack channelNoText
>        programNo = read $ BS.unpack programNoText
>    startTime <- parseProgramTime startTimeText
>    endTime <- parseProgramTime endTimeText
>    let networkNo = if BS.null networkNoText then Nothing else Just 
> (read (BS.unpack networkNoText))
>    return $ ScheduleProgram channelNo programNo startTime endTime networkNo
> 
> parseProgramTime :: ByteString -> IO LocalTime
> parseProgramTime s = do
>    let parts = BS.split 'T' s
>    ensure (length parts == 2)
>        ("Expected exactly one T in eventChannelChange time: " ++ 
> BS.unpack s)
>    let [datePart,timePart] = parts
>    ensure (BS.length datePart == 8)
>        ("Expected 8 digits in date part of eventChannelChange time: " 
> ++ BS.unpack s)
>    let (yearPart, monthDayPart) = BS.splitAt 4 datePart
>        (monthPart, dayPart) = BS.splitAt 2 monthDayPart
>        year = read $ BS.unpack yearPart
>    month = read $ BS.unpack monthPart
>    day = read $ BS.unpack dayPart
>    let date = fromGregorian year month day
>    ensure (BS.length timePart == 6)
>        ("Expected 6 digits in time part of eventChannelChange time: " 
> ++ BS.unpack s)
>    let (hoursPart,minutesSecondsPart) = BS.splitAt 2 timePart
>    (minutesPart,secondsPart) = BS.splitAt 2 minutesSecondsPart
>    hours = read $ BS.unpack hoursPart
>    minutes = read $ BS.unpack minutesPart
>    seconds = read $ BS.unpack secondsPart
>    let time = TimeOfDay hours minutes (fromInteger seconds)
>    return (LocalTime date time)

Argh, all those: read .unpacks are going to be painful.

Consider using Data.ByteString.Char8.readInt/Integer

-- Don