Trying to understand what setf can do, I called

(macroexpand '(setf (aref vec i) val))
⇒ (let* ((v vec) (v i)) (aset v v val))

This seems obviously wrong.

However I couldn't create an actual instance where (setf (aref .. fails. E.g.

 (setq vec (make-vector 10 nil) i 3 val 'foo)
 ⇒ foo
 (setf (aref vec i) val)
 ⇒ foo
 ⇒ [nil nil nil foo nil nil nil nil nil nil]

Can someone explain what is going on here?

  • 2
    OK. I understand now. The two v symbols are not the same and (let* ((form (macroexpand '(setf (aref vec i) val))) (symb1 (caar (cadr form))) (symb2 (caar (cdadr form)))) (equal symb1 symb2)) returns nil.
    – phs
    Mar 11 '19 at 9:58
  • 1
    Strangely, the code for setf in source file gv.el seems to create the v symbols with a vanilla use of (gensym "v") and this should append a counter value after the "v" prefix, creating uninterned symbols v0, v1, v2, etc.
    – phs
    Mar 11 '19 at 10:05
  • 2
    You might like to play with print-gensym to better see what's going on.
    – Stefan
    Mar 11 '19 at 12:35
  • @stefan: I have emacs-26.1 and it has no print-gensym AFAICT :-( Does anyone have an explanation why the (gensym "v") in source file gv.el does not append gensym-counter ?!
    – phs
    Mar 11 '19 at 13:59
  • 1
    I'm pretty sure you do have print-gensym, you likely just looked at the wrong place (try C-h o instead of C-h f). The let* in your expanded code is likely generated by macroexp-let2 which uses make-symbol rather than gensym.
    – Stefan
    Mar 11 '19 at 19:01

From your comment you've figured this out for yourself, but...

In the macro expansion you're seeing the printed representation of two independent symbols with the same name. Most likely both of those symbols are uninterned.

A printed representation like this, if passed back to the lisp reader, would not be equivalent to the original, as the lisp reader would intern the symbols.

This is similar to:

(list (make-symbol "v") (make-symbol "v"))
(v v)
  • 3
    Worth noting that setting print-gensym and print-circle to t produces a printed representation which can read back to something equivalent.
    – npostavs
    Mar 11 '19 at 22:31

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