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
vec
⇒ [nil nil nil foo nil nil nil nil nil nil]
Can someone explain what is going on here?
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)
print-gensym and print-circle to t produces a printed representation which can read back to something equivalent.
vsymbols are not the same and(let* ((form (macroexpand '(setf (aref vec i) val))) (symb1 (caar (cadr form))) (symb2 (caar (cdadr form)))) (equal symb1 symb2))returnsnil.setfin source filegv.elseems to create thevsymbols with a vanilla use of(gensym "v")and this should append a counter value after the "v" prefix, creating uninterned symbolsv0,v1,v2, etc.print-gensymto better see what's going on.print-gensymAFAICT :-( Does anyone have an explanation why the(gensym "v")in source filegv.eldoes not appendgensym-counter?!print-gensym, you likely just looked at the wrong place (tryC-h oinstead ofC-h f). Thelet*in your expanded code is likely generated bymacroexp-let2which usesmake-symbolrather thangensym.