[Haskell-cafe] Re: Very fast loops. Now!

[Eric Willigers]

Donald Bruce Stewart wrote:
> The following C program was described on #haskell
> 
>     #include <stdio.h>
> 
>     int main()
>     {
>         double x = 1.0/3.0;
>         double y = 3.0;
>         int i    = 1;
>         for (; i<=1000000000; i++) {
>             x = x*y/3.0;
>             y = x*9.0;
>         }
>         printf("%f\n", x+y);
>     }
> 
> 
> Which was translated to the following Haskell:
> 
>     {-# OPTIONS -fexcess-precision #-}
> 
>     import Text.Printf
> 
>     main = go (1/3) 3 1
> 
>     go :: Double -> Double -> Int -> IO ()
>     go !x !y !i
>         | i == 1000000000 = printf "%f\n" (x+y)
>         | otherwise       = go (x*y/3) (x*9) (i+1)
> 

Do the two programs implement the same algorithm? The C program updates 
x and y in sequence. The Haskell program updates x and y in parallel and 
can be easier for the compiler to optimize.