Adding a method to ‘make-instance’ fails every time I try

Question

So, I’ve learned enough about generic functions, CLOS, and EIEIO to believe that this should work:

(defclass foo () nil)

(cl-defmethod make-instance ((x foo) &rest args)
  (with-temp-message "Is this thing on?"
    (sit-for 1))
  (cl-call-next-method))

(make-instance 'foo)

But, that just returns a new “foo” object, it doesn’t stop and display a message like I thought it would. Is there something I’m doing wrong here? (Bonus points if there’s a way I can investigate visually what’s going on—eieio-display-method-list doesn’t seem to work anymore, and I’ve tried to look into the internals of how methods are selected but I just get lost in a maze of structs and functions with names like cl--generic-generalizer-specializers-function.)

I’m using Emacs 26.2 on Ubuntu 18.04 LTS.

Tina Russell · Accepted Answer · 2019-04-26T06:14:03.107

0

So, the problem works something like this: make-instance is a class-level method, not an instance-level method (after all, when make-instance is run, the object itself hasn’t been created yet), and that means the format for the specializer is different. This actually works:

(defclass foo () nil)

(cl-defmethod make-instance ((cls (subclass foo)) &rest args)
  (with-temp-message "Is this thing on?"
    (sit-for 1))
  (cl-call-next-method))

(make-instance 'foo)

Now, I can’t tell you whether or not arguments are properly passed to the next method or anything like that, but my basic question is now answered so I will mark it as such.

Thanks to Eric Abrahamsen for his help with this!

edited Apr 26 '19 at 06:14

answered Apr 24 '19 at 00:21

Tina Russell

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Works without the last `nil` on Emacs 26.2 for me. – npostavs Apr 24 '19 at 11:06
Huh! Thanks for letting me know. I’m on the Git version of Emacs, branch “emacs-26”, pulled and built on April 16, 2019. – Tina Russell Apr 25 '19 at 20:25
…Ah, whoops, that /is/ Emacs 26.2. I hadn’t noticed the version bump. Anyway, I just updated and rebuilt Emacs and the example works without the final `nil`, just like you said. I’ll update the question and answer accordingly. – Tina Russell Apr 26 '19 at 06:11

Adding a method to ‘make-instance’ fails every time I try

1 Answers1