> To: haskell-cafe@haskell.org
> From: to.darkangel@gmail.com
> Date: Tue, 19 Mar 2013 23:27:09 +0200
> Subject: Re: [Haskell-cafe] Streaming bytes and performance
>
> On 03/19/2013 10:49 PM, Konstantin Litvinenko wrote:
> > {-# LANGUAGE BangPatterns #-}
> >
> > import Control.Monad.State.Strict
> >
> > data S6 = S6 !Int !Int
> >
> > main_6 = do
> > let r = evalState go (S6 10000 0)
> > print r
> > where
> > go = do
> > (S6 i a) <- get
> > if (i == 0) then return a else (put (S6 (i - 1) (a + i))) >> go
> >
> > main_7 = do
> > let r = go (S6 10000 0)
> > print r
> > where
> > go (S6 i a)
> > | i == 0 = a
> > | otherwise = go $ S6 (i - 1) (a + i)
> >
> > main = main_6
> >
> > main_6 doing constant allocations while main_7 run in constant space.
> > Can you suggest something that improve situation? I don't want to
> > manually unfold all my code that I want to be fast :(.

Your problem is that main_6 thunks 'i' and 'a' .

If you write (S6 !i !a) <- get

than there is no problem any more...

>
> Correction - they both run in constant space, that's not a problem. The
> problem is main_6 doing constant allocation/destroying and main_7 doesn't.

No main_6 does not runs in constant space if you dont use bang patterns...