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Subject: bug#79880: func-arity should take into account that advices change arity
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Hello,

Advising a function changes its arity.

(defun foo (a))

(func-arity 'foo)
  => (1 . 1)

(advice-add 'foo :around (lambda (fn a) (funcall fn a)))

(func-arity 'foo)
  => (0 . many)

I think that's problematic.  E.g., if a function gains a new argument
and a package would like to support both the old and the new version,
then it could use `func-arity' to determine whether to pass the new
argument or not, but this breaks if a third-party advises the same
function.

This could be dealt with by adding a new argument to `func-arity',
which controls whether any advice is peeled before determining the
arity.  I tend to think it should peel by default (i.e., the current
behavior looks like a bug to me).

I've been using a wrapper like this:

(defun my-func-arity (fn)
  (func-arity
   (advice--cd*r
    (if (symbolp fn) (symbol-function fn) fn))))

Thanks,
Jonas

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From: help-debbugs@HIDDEN (GNU bug Tracking System)
To: Jonas Bernoulli <jonas@HIDDEN>
Subject: bug#79880: Acknowledgement (func-arity should take into account
 that advices change arity)
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Subject: bug#79880: func-arity should take into account that advices change arity
Resent-From: Stefan Monnier <monnier@HIDDEN>
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Date: Sun, 23 Nov 2025 20:44:56 -0500
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> I think that's problematic.  E.g., if a function gains a new argument
> and a package would like to support both the old and the new version,
> then it could use `func-arity' to determine whether to pass the new
> argument or not, but this breaks if a third-party advises the same
> function.

    (condition-case nil
        (foo bar baz)
      (wrong-number-of-arguments
       (foo bar)))

is the standard way to deal with changes in API.
I recommend not to use `func-arity`, ever.


        Stefan

Last modified: Tue, 25 Nov 2025 20:45:01 UTC

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