Oops; I meant to write those in CL, but I forgot about functions having their own namespace (been playing too long in a lisp-1). It should work in CL if you use FUNCALL.

If mquander's explanation isn't enough, you might look at it using terms from the OO world: CAR takes a pair object and calls its 'car method.

So you mean a meta-circular OOP implementation, where the hosting language already has car, cdr and cons. Alright.

I thought what you wrote was supposed to be a primitive implementation of the functions.

No, not really, unless you're running the code in a metacircular interpreter.

Imagine encoding the pair ('hello . 'world) into a function. You might say:

  (lambda (msg)
    (if (eq msg 'car) 'hello 'world))

You could get 'hello by applying that function to the 'car symbol, or 'world by applying it to anything else. Wrapping that in another function lets us generalise to any CAR/CDR.

  (lambda (car-value cdr-value)
    (lambda (msg)
      (if (eq msg 'car) car-value cdr-value)))

Those look correct to me. The function returned by cons is just a switch that holds two values in a closure, and interprets what you pass it as a signal to determine which to retrieve. You could just as easily rewrite it to pass it 0 or 1 instead of car and cdr.

If you evaluate (car (cons 1 2)) and (cdr (cons 1 2)) on paper using substitution, you'll see how it works.

Exactly. SICP shows some similar code, though I don't remember which part..