challenge #3 - Parallel for loop

[Benji Smith]

janderson wrote:
> I would like to be able to run a for loop in parallel, using syntax like:
> 
> //example 1
> int sum = 0;
> foreach_parallel(int a; array)
> {
>   sum += array[a];   //This could be anything
> }

Isn't this really just a specialized case of Map/Reduce?

It's trivial, as long as the operations inside the loop are completely 
associative. In other words:

A(B + C) == A(B) + A(C)

Since a summation satisfies the associativity requirements, then this 
one is pretty trivial to implement.

But, not all operations are associative, and those ones won't be 
trivially parallizable, without changing the algorithm.

Rather than focusing on a foreach_parallel construct, I'd be interested 
in seeing a competition to implement the most elegant (syntax/semantics) 
version of Map/Reduce. It'd be especially cool if a library 
implementation of Map/Reduce could abstract away the details of whether 
the function was mapped across multiples cores, cpus, or entire machines.

More details here:

http://labs.google.com/papers/mapreduce.html
http://en.wikipedia.org/wiki/MapReduce

--benji