[Haskell-cafe] Re: Am I lazy enough?

3 May 2006

      ByteString's are strict in their contents, so when you do an
hGetContents you'll read the entire file into memory!  This negates
any laziness benefits right off the bat.  The trickiest part is the
lazy IO, you have to use unsafeInterleaveIO or something similar.

Below is a program that does approximately the same as yours.  Note
the getLinesLazily function.  I've only tested that it typechecks, I
haven't run it yet.

Spencer Janssen

-- Program begins here

import System.IO
import System.IO.Unsafe (unsafeInterleaveIO)

import qualified Data.ByteString.Char8 as B
import Data.ByteString.Char8 (ByteString)

main =
    getLinesLazily stdin >>= mapM B.putStrLn . relines 8

relines :: Int -> [ByteString] -> [ByteString]
relines n = go . map (\s -> (s, B.count ',' s))
 where
    go []       = []
    go [(s, _)] = [s]
    go ((s, x) : (t, y) : ss)
     | x + y > n = s : go ((t, y) : ss)
     | otherwise = go ((B.append s t, x + y) : ss)

getLinesLazily :: Handle -> IO [ByteString]
getLinesLazily h = do
    eof <- hIsEOF h
    if eof
        then return []
        else do
            l <- B.hGetLine h
            ls <- unsafeInterleaveIO $ getLinesLazily h
            return (l:ls)

-- Program ends here

On 5/3/06, Joel Reymont  wrote:
...
Folks,
I'm looking to use the following code to process a multi-GB text
file. I am using ByteStrings but there was a discussion today on IRC
about tail recursion, laziness and accumulators that made me wonder.
Is fixLines below lazy enough? Can it be made lazier?
Thanks, Joel
---
module Main where
import IO
import System
import Numeric
import Data.Char
import Data.Word
import qualified Data.Map as M
import qualified Data.ByteString.Char8 as B
import Prelude hiding (lines)
grabTableInfo x = (tableId', (tableType, tableStakes))
     where (tableId:tableType:_:tableStakes:_) =
               B.split ',' x
           Just (tableId', _) = B.readInt tableId
lines = B.split '\n'
--- My Oracle ascii dump is 80 characters wide so some lines
--- are split. I need to skip empty lines and join lines
--- containing less than the required number of commas.
fixLines 0 lines = lines
fixLines _ [] = []
fixLines n (line:lines) =
     fixLines' lines line []
         where fixLines' [] str acc
                   | B.count ',' str == n
                       = acc ++ [str]
                   | otherwise
                       = acc
               fixLines' (x:xs) str acc
                   | B.null str -- skip
                       = fixLines' xs x acc
                   | B.count ',' str < n -- join with next line
                       = fixLines' xs (B.append str x) acc
                   | otherwise
                       = fixLines' xs x (acc ++ [str])
mkMap = M.fromList . map grabTableInfo . fixLines 20
loadTableInfo = do
   bracket (openFile "game_info_tbl.csv" ReadMode)
           (hClose)
           (\h -> do
              c <- B.hGetContents h
              return $ mkMap $ lines c)
--
http://wagerlabs.com/
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