[Haskell-cafe] Streaming bytes and performance

18 Mar 2013

      Hi All!

I tune my toy project for performance and hit the wall on simple, in 
imperative world, task. Here is the code that model what I'm trying to 
achieve

import qualified Data.ByteString.Lazy as L
import Data.Word8(isSpace)
import Data.Word
import Control.Monad.State

type Stream = State L.ByteString

get_byte :: Stream (Maybe Word8)
get_byte = do
     s <- get
     case L.uncons s of
         Nothing -> return Nothing
         Just (x, xs) -> put xs >> return (Just x)

main = do
     f <- L.readFile "test.txt"
     let r = evalState count_spaces f
     print r
   where
     count_spaces = go 0
       where
         go a = do
             x <- get_byte
             case x of
                 Just x' ->  if isSpace x' then go (a + 1) else go a
                 Nothing -> return a

It takes the file and count spaces, in imperative way, consuming bytes 
one by one. The problem is: How to rewrite this to get rid of constant 
allocation of state but still working with stream of bytes? I can 
rewrite this as one-liner L.foldl, but that doesn't help me in any way 
to optimize my toy project where all algorithms build upon consuming 
stream of bytes.

PS. My main lang is C++ over 10 years and I only learn Haskell :)

[Haskell-cafe] Streaming bytes and performance

Konstantin Litvinenko