Common Lisp Double-Backquote, Unquote, Quote, Unquote sequence?

Question

I\'m reading Let Over Lambda, which deals with some pretty deeply layered macro authoring. It\'s fascinating and I\'m mostly managing to keep up with it.

In Chapter 4 Ho

Answer 1

The ,',X trick is used to shield the X from another evaluation.

See how:

     (setq a 'fn)
     (let ((x 'a)) ``(,,x ,',x)) ==>  `(,a a) ==> (fn a)

     ;; ``,',X ==> `,(quote "the value of X") ==> "the value of X"

     ;; ``,,X  ==> `,"the value of X" ==> "the value of the value of X"