Emacs' byte-compiler can tell me if I call pure functions without saving the result:

(defun foo-calls-add ()
  ;; value returned from (+ 1 2) is unused:
  (+ 1 2)

However, I don't get any warnings when I define my own pure functions:

(defun foo-pure (x)
  (declare (pure t) (side-effect-free error-free))

(defun foo-calls-pure ()
  ;; No warning here:
  (foo-pure 1)

What do I need to do in order for the byte compiler to warn about foo-calls-pure?

I've compared (symbol-plist '+) with (symbol-plist 'foo-pure) and I can't see any obvious missing properties.

1 Answer 1


If you look in byte-optimize-form-code-walker, you can see that a warning is printed for side-effect-free functions, but error-free functions just get optimized out and logged (not warned) only. Logging means it only shows up if have byte-optimize-log set to non-nil (you'll want to set it to source otherwise there's a lot of noise).

      ((and for-effect (setq tmp (get fn 'side-effect-free))
            (or byte-compile-delete-errors
                (eq tmp 'error-free)
                  (byte-compile-warn "value returned from %s is unused"
                                     (prin1-to-string form))
       (byte-compile-log "  %s called for effect; deleted" fn)
       ;; appending a nil here might not be necessary, but it can't hurt.
        (cons 'progn (append (cdr form) '(nil))) t)

Additionally, the declaration doesn't take effect during compilation unless you use eval-when-compile, so you'll need something like:

  (defun foo-pure (x)
    (declare (pure t) (side-effect-free t))

Or to avoid putting the whole defun inside eval-and-compile:

(eval-and-compile (function-put 'foo-pure 'side-effect-free 't))
  • "the declaration doesn't take effect during compilation", aha, that's a subtlety that I completely missed. Is this documented, or did you just look at the code? Commented Nov 2, 2016 at 5:05
  • 1
    Actually I just guessed & checked that one. It doesn't seem like great behaviour, perhaps worth a bug report to change the declare expansion to use eval-and-compile.
    – npostavs
    Commented Nov 2, 2016 at 14:41
  • 1
    Awesome, so that works nicely. If foo-pure is in a separate library that's byte-compiled, then downstream consumers will get the error without me doing function-put. Commented Nov 3, 2016 at 0:19
  • 1
    I've opened an Emacs bug too. I've proposed a patch but changing defun-declarations-alist in my current Emacs instance doesn't fix the issue. Commented Nov 3, 2016 at 1:05

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