Erlang seems to pay a small performance penalty for that ability though. The VM is not allowed to run more than a few thousand instructions without checking for

Erlang seems to pay a small performance penalty for that ability though. The VM is not allowed to run more than a few thousand instructions without checking for a signal to task switch. But if that kind of guaranteed cooperation isn't needed, I'd expect that lightweight threads could be added to a language like this in a straightforward way.