[Haskell-cafe] Re: C-like Haskell

[Duncan Coutts]

On Thu, 2009-01-29 at 10:35 +0000, John Lato wrote:
> > On Wed, 2009-01-28 at 20:11 -0500, sam lee wrote:
> >> Did you print it? I'm using same code with ghc --make -O2  and it
> >> takes forever to finish.
> >
> > Yes, you can see in the output that it prints the same answer in each
> > case. I was using r = 10^9 as you suggested.
> 
> I wouldn't call these answers the same.  Can this be ascribed to an
> overflow error?

Oh yes, you're quite right. I didn't notice because it only affects the
middle digits :-)

> >> > C version:
> >> > $ time ./circ
> >> > 3141592649589764829
>                       ^^
> >
> >> > Haskell version:
> >> > time ./circ2
> >> > 3141592653589764829

Compare:

circ2 r = (1+4*r) + 4 * (circ2' (rs+1) r 1 0)

vs

    # the main loop 
  }
  sum*=4;
  sum++;

Looks like the C version misses the +4*r at the end.

  3141592649589764829 + 4*10^9
= 3141592653589764829

Assuming that's right, the mistake only costs constant-time and doesn't
invalidate the timings.

Duncan