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[MUSIC PLAYING BY J.S. BACH] PROFESSOR: The last time we began having a look at how languages are constructed. Remember the main point that an evaluator for, LISP, say, has two main elements. There is EVAL, and EVAL's job is to take in an expression and an environment and turn that into a procedure and some arguments and pass that off to APPLY. And APPLY takes the procedure in the arguments, turns that back into, in a general case, another expression to be evaluated in another environment and passes that off to EVAL, which passes it to APPLY, and there's this whole big circle where things go around and around and around until you get either to some very primitive data or to a primitive procedure. See, what this cycle has to do with is unwinding the means of combination and the means of abstraction in the language. So for instance, you have a procedure in LISP-- a procedure is a general way of saying, I want to be able to evaluate this expression for any value of t...

[MUSIC PLAYING BY J.S. BACH] PROFESSOR: The last time we began having a look at how languages are constructed. Remember the main point that an evaluator for, LISP, say, has two main elements. There is EVAL, and EVAL's job is to take in an expression and an environment and turn that into a procedure and some arguments and pass that off to APPLY. And APPLY takes the procedure in the arguments, turns that back into, in a general case, another expression to be evaluated in another environment and passes that off to EVAL, which passes it to APPLY, and there's this whole big circle where things go around and around and around until you get either to some very primitive data or to a primitive procedure. See, what this cycle has to do with is unwinding the means of combination and the means of abstraction in the language. So for instance, you have a procedure in LISP-- a procedure is a general way of saying, I want to be able to evaluate this expression for any value of t...