>There is the family of Lisps, Smalltalks and friends, where the system is live and you can set variables to values whenever you wish. The problem there tends t

>There is the family of Lisps, Smalltalks and friends, where the system is live and you can set variables to values whenever you wish. The problem there tends to be isolation of speculation. I don't want my what-ifs to turn into has-becomes. "What if X is 33?"... OK done and now the computer shall never forget. I want all speculation to fly in a transient layer above the existing stable codebase. Is this a real issue? Don’t you just need 2 copies of the code base, and terminate your 2nd copy to make it “transient”?