> andersmurphy and jwr have it exactly right about the REPL. Common Lisp and Smalltalk systems compile one function at a time, to native code, inside the running program, in milliseconds. Avi's headline brag, fixing a bug before the customer hangs up, was routine practice in Lisp shops in the 1980s. NASA once did it to a Lisp system running on a spacecraft a hundred million miles away: the Remote Agent on Deep Space 1 deadlocked in flight, and they diagnosed and fixed it through a REPL running on the spacecraft itself.

WOW.