Why are functional languages considered a boon for multi threaded environments?

Question

I hear a lot about functional languages, and how they scale well because there is no state around a function; and therefore that function can be massively parallelized.

However, this makes little sense to me because almost all real-world practical programs need/have state to take care of. I also find it interesting that most major scaling libraries, i.e. MapReduce, are typically written in imperative languages like C or C++.

I'd like to hear from the functional camp where this hype I'm hearing is coming from..

Answer 1

It's important to add one word: "there's no shared state".

Any meaningful program (in any language) changes the state of the world. But (some) functional languages make it impossible to access the same resource from multiple threads simultaneously. The absence of shared state makes multithreading safe.