It was originally in the original Erlang interpreter written in Prolog but you could use it for back-doors in the communication so we removed them. If you sent

It was originally in the original Erlang interpreter written in Prolog but you could use it for back-doors in the communication so we removed them. If you sent an unbound variable in a message then the receiving process could reply by binding the variable. We wanted all communication to be explicit. I agree though that unification is cool. And I still like Prolog and other logic languages, and have implemented Prolog in Erlang, of course. We also gave some thought to laziness but felt it just didn't fit as for the type problems in which we were interested. For those problems when things are done is critical, not just that they are eventually done. That would have meant a lot explicit evaluation which defeated the point of laziness.