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 :)


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