While I fully agree that the immutability-by-default mindset helps with concurrency, having single-assignment variables within the local scope of functions (wha

While I fully agree that the immutability-by-default mindset helps with concurrency, having single-assignment variables within the local scope of functions (what zephj is complaining about) is strictly due to Erlang being originally implemented in Prolog. Strictly speaking, letting you destructively modify local variables in Erlang wouldn't have any impact on concurrency. In Prolog, it's because there's only unification, not assignment. Semantically, it's as if "X always had that value, it just wasn't known until now" - you can pass around unbound values and retroactively bind them for all places of use, but you can't ever change them. (Difference lists are probably the best example of this in action.)