Scaling in functional programming

Question

I heard that programs written in a functional language tend to scale better. Is this true and if so then what are the differences from non functional languages that cause this?

Answer 1

Whoever you heard this from was most likely referring to the fact that "Disallowing side effects provides for referential transparency, which makes it easier to verify, optimize, and parallelize programs, and easier to write automated tools to perform those tasks."

In other words, functional programs tend to not have side effects (modifying global state) so running many instances of a functional program in parallel should produce the same output. For example, consider the difference between

int a;
void increment_a() {
    a++;
}

and

int increment(int a) {
    return a+1;
}

The second has no side-effects and can be run in parallel, provided you structure your code so that you provide all the necessary inputs.