face-attribute function returning strange cons with "quote", causing htmlfontify-buffer bug: 'invalid face: quote'

Question

I've been debugging an error when I call htmlfontify-buffer on my *shell* buffer or SLIME.

I found out the problem seems to be a value returned by the face-attribute function that seems to be a cons whose first element is just quote. That gets passed to the hfy-face-to-style-i function which then breaks with error invalid face: quote.

Here's a demonstration of the problematic value:

(face-attribute 'ansi-color-bold :inherit)
'bold

(type-of (face-attribute 'ansi-color-bold :inherit))
cons

(princ (face-attribute 'ansi-color-bold :inherit))
'bold'bold

(car (face-attribute 'ansi-color-bold :inherit))
quote

I don't understand enough elisp to know if this is "ok" and the bug is in the other parts of the code.

Looking with the debugger, I see that this weird value comes into play when hfy-face-attr-for-class is called with the FACE argument with value 'ansi-color-bold...

In the Customization buffer, I see that this face has a List Expression:

((t
  (:inherit 'bold)))

When FACE gets passed into (face-attr-construct face), it returns (:inherit 'bold).

This gets into hfy-face-to-style-i as fn which does:

    (let ((key  (car  fn))
          (val  (cadr fn))

Now, val has the weird value, it runs this:

      (if (eq key :inherit)
        (let ((vs (if (listp val) val (list val))))

As listp val returns t, as the weird value is a cons, now vs has the value 'bold and it goes on to dolist on it:

          (dolist (v vs)
            (setq parent
                  (append
                   parent
                   (hfy-face-to-style-i
                    (hfy-face-attr-for-class v hfy-display-class))))))

In the first iteration, v is just quote. (type-of v) returns symbol.

And that's how quote gets passed to hfy-face-to-style-i which errors out.

So, I belive I have all information to know how to fix the bug, but still can't do it as I fail to understand this one weird cons with quote and symbol bold in it.

Is that a "proper" value? If so, should I edit the dolist call above to handle the quote specially? Or do I need to fix this value being returned by face-attribute in the first place?

EDIT:

I found out that ansi-color-bold is defined like this at /Applications/Emacs.app/Contents/Resources/lisp/ansi-color.el.gz:

(defface ansi-color-bold
  '((t :inherit 'bold))
  "Face used to render bold text."
  :group 'ansi-colors
  :version "28.1")

Is the 'bold incorrectly quoted as @db48x says? I found this file on GitHub here but it does not seem to have a quote... This file seems to be part of my installation, which came from brew.

Answer 1

You just have a typo in your customization of the ansi-color-bold face. Remove the apostrophe and the problem will go away.

The apostrophe tells the Lisp interpreter to quote the next thing that it reads, so '(+ 2 2) evaluates to the list of three things, a symbol + and two 2s while (+ 2 2) evaluates to 4 (because it is a call to the addition function).

Internally this quoting is done by consing the value with the symbol quote. Thus '(+ 2 2) is read as (quote (+ 2 2)) and 'bold is read as (quote bold).

Your problem happens when you use a quote inside of something that is already quoted. Something like '(+ 2 'bold) is read as (quote (+ 2 (quote bold))) (this is a nonsense expression, but it is illustrative).