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I'm using the tco.el package, which provides tail-call optimisation for functions in elisp that call themselves in tail-position. The way such functions are identified is by using defun-tco instead of defun. But this means tail-call optimised functions no longer are styled like functions in terms of font-locking. How do I specify that defun-tco should be styled like defun?

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You need to set font-lock keywords, e.g.,

(font-lock-add-keywords
 'emacs-lisp-mode
 '(("(\\(defun-tco\\)\\_>\\s *\\(\\(?:\\sw\\|\\s_\\)+\\)?"
    (1 font-lock-keyword-face nil t)
    (2 font-lock-function-name-face nil t))))

the above is adapted from https://github.com/emacs-mirror/emacs/blob/8fe21b03/lisp/emacs-lisp/generator.el#L796. I suggest you to write an issue or pull request to the tco.el project, so you don't have to put the above to your own init file.

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  • Thanks for this - I figured it was font-lock-add-keywords, but I couldn't find where defun was configured, so wasn't sure what the content should look like. Unfortunately, while the above correctly styles the defun-tco keyword itself and the following function name, it does not change the description's font-locking. I'm not sure where defun's description font-locking is described.
    – emacsomancer
    Commented Jan 12, 2019 at 17:07
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    @emacsomancer I don't think the color of defun is from font-lock keywords (i.e., searching via regexp, so it's slow). The core syntax highlighting is from syntactic parsing, see (elisp) Font Lock Mode. I guess you mean docstring by "description", defun-tco doesn't support docstring at all, if it does, it will probably include (declare (doc-string 3)) then you'll get the highlight out-of-box.
    – xuchunyang
    Commented Jan 12, 2019 at 19:02
  • Yes, sorry I mean the docstring. It looks like that is font-lock-doc-face is applied in font-lock.el by the function font-lock-compile-keywords. Unfortunately, it's not exactly clear how to hack that to get it to apply after defun-tco as well as defun. I wonder if there's a different way to approach this - to redefine how font-locking applies to defun so that it also applies to defun-tco, so that other font-locking could essential just be 'inherited'? [Edit: Ah, I see what you mean - docstring support simply isn't defined for defun-tco.]
    – emacsomancer
    Commented Jan 12, 2019 at 19:40
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    @emacsomancer you only need to worry about font-lock-function-name-face. defun-tco is already colorized because it is defined as a macro. And if defun-tco supports docstring, the docstring will also be colorized because of (declare (doc-string ...)), the author will add it to follow the convention of defining macro.
    – xuchunyang
    Commented Jan 12, 2019 at 20:01

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