Expand a macro form completely

Question

I'd like to learn the internals of Lisp, so I want to see how everything is implemented.

For example,

(macroexpand '(loop for i upto 10 collect i))

gives me (in SBCL)

(BLOCK NIL
  (LET ((I 0))
    (DECLARE (TYPE (AND NUMBER REAL) I))
    (SB-LOOP::WITH-LOOP-LIST-COLLECTION-HEAD (#:LOOP-LIST-HEAD-1026
                                              #:LOOP-LIST-TAIL-1027)
      (SB-LOOP::LOOP-BODY NIL
                          (NIL NIL (WHEN (> I '10) (GO SB-LOOP::END-LOOP)) NIL)
                          ((SB-LOOP::LOOP-COLLECT-RPLACD
                            (#:LOOP-LIST-HEAD-1026 #:LOOP-LIST-TAIL-1027)
                            (LIST I)))
                          (NIL (SB-LOOP::LOOP-REALLY-DESETQ I (1+ I))
                           (WHEN (> I '10) (GO SB-LOOP::END-LOOP)) NIL)
                          ((RETURN-FROM NIL
                             (SB-LOOP::LOOP-COLLECT-ANSWER
                              #:LOOP-LIST-HEAD-1026)))))))

But LOOP-BODY, WITH-LOOP-LIST-COLLECTION-HEAD, etc. are still macros. How can I expand a macro form completely?

Answer 1

To see the full expansion one needs to walk the Lisp form on all levels and expand them. For this it is necessary that this so-called code walker understands Lisp syntax (and not just s-expression syntax). For example in (lambda (a b) (setf a b)), the list (a b) is a parameter list and should not be macro expanded.

Various Common Lisp implementations provide such a tool. The answer of 6502 mentions MACROEXPAND-ALL which is provided by SBCL.

If you use a development environment, it is usually provided as a command:

• SLIME: M-x slime-macroexpand-all with C-c M-m

• LispWorks: menu Expression > Walk or M-x Walk Form, shorter M-Sh-m.

Answer 2

The other answers are excellent for you question but you say you want to see how everything is implemented.

Many macros (as you know already) are implemented using macros and whilst macroexpand-all is very useful but you can lose the context of what macro was responsible for what change.

One nice middle ground (if you are using slime) is to use slime-expand-1 (C-c Enter) which shows the expansion is another buffer. You can then useslime-expand-1 inside this new buffer to expand macros in-place. This allows you to walk the tree expanding as you read and also to use undo to close the expansions again.

For me this has been a god-send in understanding other people's macros. Hope this helps you too, have fun!