Lisp unroll/partial eval function

Question

Is there a way to show the evaluation steps in Common Lisp like follows:

> (defun fac (n) (if (= n 0) 0 (if (= n 1) 1 (* n (fac (- n 1))))))
FAC
> (step-by-step (fac 3))
0: (FAC 3)
1: (* 3 (FAC 2))
3: (* 3 (* 2 (FAC 1)))
4: (* 3 (* 2 (1)))
5: (* 3 2)
6: 6
Result: 6

Looking for a way to visualize recursion and return values in general for a small course. I know of (step fn) and (optimize (debug 3))) unfortunately this does not produce the desired output, or I don't know how to tell it to.

NOTE: non-emacs/slime solution preferred

Answer 1

It's not everything you asked for, and the specific output is implementation dependent, but you may get some good milage out of the standard trace. It won't show the expansion like you showed, but it's definitely a way to meet some of your requirements

[to] visualize recursion and return values in general … to see the whole expression on each step. Like having a debug print of each function call.

Many implementations include additional arguments that can customize how things get traced, what gets printed, etc. Here's an example of the default behavior with SBCL:

CL-USER> (defun fac (n) (if (= n 0) 0 (if (= n 1) 1 (* n (fac (- n 1))))))
FAC
CL-USER> (trace fac)
(FAC)
CL-USER> (fac 3)
  0: (FAC 3)
    1: (FAC 2)
      2: (FAC 1)
      2: FAC returned 1
    1: FAC returned 2
  0: FAC returned 6
;=> 6

In CLISP:

CL-USER> (fac 3)
1. Trace: (FAC '3)
2. Trace: (FAC '2)
3. Trace: (FAC '1)
3. Trace: FAC ==> 1
2. Trace: FAC ==> 2
1. Trace: FAC ==> 6
;=> 6