Q: are there breaking changes to eieio in Emacs 27?

I just upgraded to the Emacs 27 snapshot, and I'm getting a peculiar break in code that works in previous versions of Emacs.


Here's a toy class and a toy :after method:

(defclass simple-class ()
    :initarg :value
    :initform nil
    :documentation "Test slot"))
  "Test class.")

This works:

(simple-class :value "some value") ;; ==> #s(simple-class "some value")

Now I'd like to define an :after method on initialize-instance:

(cl-defmethod initialize-instance :after ((sc simple-class) &key)
  (with-slots (value) sc
    (setf value (upcase value))))

Now it's broken:

(simple-class :value "some value") ;; ==> (error "Keyword argument nil not one of nil")

NB: it's nothing about the content of the :after method: I can define an empty method and I get the same error.

(cl-defmethod initialize-instance :after ((sc simple-class) &key))

what happened?

So: have there been changes to eieio (specifically, cl-defmethod) in Emacs 27 that break previous code? Or is there something bizarre about the snapshot?

  • Might be incorrect use of &key with no following arg name, I think Emacs got stricter about that. By the way, in 25.3, I get Lisp error: (wrong-type-argument char-or-string-p nil) after the cl-defmethod, did you mean (simple-class :value "some value")? – npostavs Jun 9 at 19:58
  • Fixed the typos; sorry. Not sure how else one would define an after method without &keys here. – Dan Jun 9 at 20:09
  • what do you think &key means in an expression like (cl-defun foo (x &key) nil)? – npostavs Jun 9 at 20:22
  • We need the &key to maintain the same number of arguments. Removing &key throws an error to that effect. – Dan Jun 9 at 20:31
  • 1
    C-h n (view-emacs-news) is usually the starting point for "What's new?". – Drew Jun 10 at 2:41

So: have there been changes to eieio (specifically, cl-defmethod) in Emacs 27 that break previous code?

It's not a change in eieio or cl-defmethod directly; what's changed is the handling of the &key symbol in cl-def* macros.

(cl-defmethod initialize-instance :after ((sc simple-class) &key))

In Emacs 26 and earlier this produces a function which accepts any number of arguments (this is a bug: there is only one argument in the arglist, named sc). In Emacs 27 it produces a function which accepts only one argument (sc). The correct argument list for initialize-instance (in all versions of Emacs) would be like this:

(cl-defmethod initialize-instance :after ((sc simple-class) &optional slots))

As told by <f1> f initialize-instance RET:

initialize-instance is a compiled Lisp function in ‘eieio.el’.

(initialize-instance THIS &optional SLOTS)
  • Thank you! NB: the ... &optional slots version also appears to work with previous versions of Emacs. Where did you find the information about changes to the cl-def* macros? I didn't see it in the release news. – Dan Jun 10 at 17:04
  • @Dan I found it by experiment :) It's not in NEWS because it's a bug fix. – npostavs Jun 10 at 20:49

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