Come on, when people are talking about a code without side effects they meant so called pure functions, which always produce the same output for the same input.

Come on, when people are talking about a code without side effects they meant so called pure functions, which always produce the same output for the same input. Message passing is a way of communication which is about "causing effects" so let's not take it to absurd. On the other hand, non-networking procedures in Erlang are pure functions, so they could be safely evaluated in parallel. Its spawned processes are isolated, they share nothing, not even a common stack, so they could fail and being restarted without affecting any other code or data. It is much better to read Armstrong's thesis where he gives outline of the design of Erlang, hows and whys. It might has a bit ugly syntax, but semantics are fine.