You can often catch me griping way more about an “Emacs API” than
Emacs Lisp, often without any explanation how the interfaces Emacs
provides could be possibly worse than the language used for this.
Here comes a not too obvious example of the issue at hand that will
not go away if you were to rewrite the code in question in <insert
favorite toy language>:
'(font-lock-string-face font-lock-comment-face font-lock-doc-face)
"Faces corresponding to text in programming-mode buffers.")
(defun flyspell-generic-progmode-verify ()
"Used for `flyspell-generic-check-word-predicate' in programming modes."
;; (point) is next char after the word. Must check one char before.
(let ((f (get-text-property (- (point) 1) 'face)))
(memq f flyspell-prog-text-faces)))
The above is helper code for Flyspell to detect whether point is in a
string or comment. While this is most certainly the easiest way to
achieve this goal, it isn’t what the manual recommends. What you’re
supposed to do instead is to use the syntax table parser by accessing
its state via syntax-ppss and checking slots 3 and 4. The
issue with that approach is that it’s not that simple, see the
sources of Smartparens for the ugly details, more specifically the
Let’s take a step back. Why exactly do we need to check the
syntactical state of Emacs? To understand what’s going on, you’ll
need to know that the first step when building a major mode for a
programming language is to set up the syntax tables so that Emacs can
recognize strings and comments properly. Once this has been done,
both syntax highlighting and movement work correctly. This hack has
been done back then to have that special case dealt with in a
performant manner, albeit it only works properly for languages looking
like C or Lisp. Single characters can be marked as having a special
syntax, with extra slots for two-character sequences. In Lisp, a
semicolon would start the comment and every newline would terminate
one, in C the comment starters and enders would be /* and */.
If you’re using a language with more complicated rules than that,
there is the crutch of writing a custom syntax-propertize-function
that walks over an arbitrary buffer region and applies syntax
properties manually. This is how triple-quoted strings are
implemented in python.el, the same technique is used to highlight
a S-expression comment in scheme.el which starts with #; and
extends to the following S-expression.
Unfortunately, not all modes play well. Some resort to just painting
over the thing to be a comment or string by applying the right face to
it and sacrifice proper movement over symbols and S-expressions. It
shouldn’t be surprising that given the choice to fix faulty modes or
circumvent a cumbersome API, plenty Emacs hackers pick the latter and
get more robust handling for free. After all, practicality beats
If you’ve hung out on #emacs for a while, chances are you’ve been
recommended ibuffer for advanced buffer management (like,
killing many buffers at once). There is a horror lurking beneath its
pretty interface though and it starts out with a customizable:
(defcustom ibuffer-always-compile-formats (featurep 'bytecomp)
"If non-nil, then use the byte-compiler to optimize `ibuffer-formats'.
This will increase the redisplay speed, at the cost of loading the
(defun ibuffer-compile-make-eliding-form (strvar elide from-end-p)
(let ((ellipsis (propertize ibuffer-eliding-string 'font-lock-face 'bold)))
(if (or elide (with-no-warnings ibuffer-elide-long-columns))
`(if (> strlen 5)
;; FIXME: this should probably also be using
;; `truncate-string-to-width' (Bug#24972)
,strvar (- strlen (string-width ,ellipsis)) nil ?.)
(defun ibuffer-compile-make-substring-form (strvar maxvar from-end-p)
;; FIXME: not sure if this case is correct (Bug#24972)
`(truncate-string-to-width str strlen (- strlen ,maxvar) nil ?\s)
`(truncate-string-to-width ,strvar ,maxvar nil ?\s)))
(defun ibuffer-compile-make-format-form (strvar widthform alignment)
(let* ((left `(make-string tmp2 ?\s))
(right `(make-string (- tmp1 tmp2) ?\s)))
(setq tmp1 ,widthform
tmp2 (/ tmp1 2))
(:right `(concat ,left ,right ,strvar))
(:center `(concat ,left ,strvar ,right))
(:left `(concat ,strvar ,left ,right))
(_ (error "Invalid alignment %s" alignment))))))
(defun ibuffer-compile-format (format)
(let ((result nil)
;; We use these variables to keep track of which variables
;; inside the generated function we need to bind, since
;; binding variables in Emacs takes time.
(dolist (form format)
;; Generate a form based on a particular format entry, like
;; " ", mark, or (mode 16 16 :right).
(if (stringp form)
;; It's a string; all we need to do is insert it.
(let* ((form (ibuffer-expand-format-entry form))
(sym (nth 0 form))
(min (nth 1 form))
(max (nth 2 form))
(align (nth 3 form))
(elide (nth 4 form)))
(let* ((from-end-p (when (cl-minusp min)
(setq min (- min))
min-used max-used strlen-used)
(when (or (not (integerp min)) (>= min 0))
;; This is a complex case; they want it limited to a
;; minimum size.
(setq min-used t)
(setq strlen-used t)
(setq vars-used '(str strlen tmp1 tmp2))
;; Generate code to limit the string to a minimum size.
(setq minform `(progn
`(- ,(if (integerp min)
(when (or (not (integerp max)) (> max 0))
(setq max-used t)
(cl-pushnew 'str vars-used)
;; Generate code to limit the string to a maximum size.
(setq maxform `(progn
(if (integerp max)
(setq strlen (string-width str))
'str elide from-end-p)))))
;; Now, put these forms together with the rest of the code.
;; Is this an "inline" column? This means we have
;; to get the code from the
;; `ibuffer-inline-columns' alist and insert it
;; into our generated code. Otherwise, we just
;; generate a call to the column function.
(ibuffer-aif (assq sym ibuffer-inline-columns)
(nth 1 it)
`(,sym buffer mark)))
;; You're not expected to understand this. Hell, I
;; don't even understand it, and I wrote it five
;; minutes ago.
(if (get sym 'ibuffer-column-summarizer)
;; I really, really wish Emacs Lisp had closures.
;; FIXME: Elisp does have them now.
(lambda (arg sym)
(let ((ret ,arg))
(put ',sym 'ibuffer-column-summary
(cons ret (get ',sym
(lambda (arg _sym)
(mincompform `(< strlen ,(if (integerp min)
(maxcompform `(> strlen ,(if (integerp max)
(if (or min-used max-used)
;; The complex case, where we have to limit the
;; form to a maximum or minimum size.
(when (and min-used (not (integerp min)))
(push `(min ,min) letbindings))
(when (and max-used (not (integerp max)))
(push `(max ,max) letbindings))
(if (and min-used max-used)
(push `(setq str ,callform
`(strlen (string-width str))))
(list (funcall insertgenfn 'str sym)))))
;; The simple case; just insert the string.
(push (funcall insertgenfn callform sym) outforms))
;; Finally, return a `let' form which binds the
;; variables in `letbindings', and contains all the
;; code in `outforms'.
;; We don't want to unconditionally load the byte-compiler.
(funcall (if (or ibuffer-always-compile-formats
;; Here, we actually create a lambda form which
;; inserts all the generated forms for each entry
;; in the format string.
`(lambda (buffer mark)
(defun ibuffer-recompile-formats ()
(mapcar #'ibuffer-compile-format ibuffer-formats))
(when (boundp 'ibuffer-filter-format-alist)
(mapcar (lambda (entry)
(cons (car entry)
(mapcar (lambda (formats)
(mapcar #'ibuffer-compile-format formats))
(defun ibuffer-clear-summary-columns (format)
(dolist (form format)
(when (and (consp form)
(get (car form) 'ibuffer-column-summarizer))
(put (car form) 'ibuffer-column-summary nil))))
(defun ibuffer-check-formats ()
(when (null ibuffer-formats)
(error "No formats!"))
(let ((ext-loaded (featurep 'ibuf-ext)))
(when (or (null ibuffer-compiled-formats)
(not (eq ibuffer-cached-formats ibuffer-formats))
(not (equal ibuffer-cached-eliding-string ibuffer-eliding-string))
(eql 0 ibuffer-cached-elide-long-columns)
(not (eql ibuffer-cached-elide-long-columns
(not (eq ibuffer-cached-filter-formats
(message "Formats have changed, recompiling...")
(setq ibuffer-cached-formats ibuffer-formats
ibuffer-cached-elide-long-columns (with-no-warnings ibuffer-elide-long-columns))
(setq ibuffer-cached-filter-formats ibuffer-filter-format-alist))
(message "Formats have changed, recompiling...done"))))
Another weird one is that the extracted autoloads for
ibuffer-ext.el reside in ibuffer.el, but that’s the lesser
evil of the two.
Credits go to holomorph for discovering that maintenance nightmare.
Update: I’ve been pointed to an emacs-devel discussion about
giving Emacs a proper GTK frontend, most comparable to the Win32 and
NS frontends (which are free from X11isms). This would be a better
approach than what’s been outlined below, with the Cairo code being
repurposed for the drawing bits.
Daniel Colascione’s excellent write-up on bringing double-buffered
rendering to Emacs has prompted me to do the same on a set of
questions that can be occasionally spotted on #emacs:
- Which GUI build of Emacs shall I choose?
- What’s the difference between the GTK build and the other builds of
Emacs on Linux?
- Does the GTK build run on Wayland?
- Does the GTK build run on Broadway?
- Why does Emacs not render as nicely as <insert more modern text editor>?
If you’ve ever programmed a GUI application with a modern/popular GUI
toolkit, you’ll have noticed that while it gives you loads of
desirable features, it forces you to structure your application around
its idea of an event loop. In other words, your application is forced
to react asynchronously to user events. Games are pretty much the
only major kind of graphical application that can get away with doing
their own event loop, but end up doing their own GUI instead.
Now, the issue with Emacs is that it does its own event loop,
pretending that the frontend is a textual or graphical terminal. It’s
pretty much the graphical equivalent of a REPL and at the time it only
had a text terminal frontend, this way of doing things worked out
fairly well. However, by the time users demanded having pretty GTK
widgets in Emacs, it became clear that more involved hacks were needed
to make that happen. This is why Emacs runs the GTK event loop one
iteration at a time, pushes its own user events into it (to make
widgets react) and a plethora of more hacks to reconcile their
rendering with everything done by X11.
In other words, Emacs is more of a X11 application plus your favorite
widgets. The choice of GUI toolkit to build it with is mostly
irrelevant, save an infamous bug with the GTK3 frontend that can crash
the daemon. Emacs will therefore not run on a pure Wayland system or
under Broadway in the browser. If anyone would want to make that
happen, either the GTK frontend would need to yank out everything X11
(unlikely as it’s deeply entrenched) or to create a new frontend doing
platform-agnostic drawing (the Cairo feature being a prime candidate
Further reading material:
;;; Please do not try to understand this code unless you have a VERY
;;; good reason to do so. I gave up trying to figure it out well
;;; enough to explain it, long ago.
This precedes paredit-forward-sexps-to-kill which appears to be
code that deals with the problem that commands operating on
S-expressions may fail mysteriously if there’s no trailing newline
Credits go to contrapunctus.
Today I took another stab at my abandoned Emacs Lisp IRC bot and
thought to myself that it would be nice if it were able to notify me
on RSS and Atom feed updates. Now, I’m not terribly fond of
reinventing the wheel, so like every good programmer I dared taking a
look at existing solutions, like News Ticker:
(defun newsticker--do-xml-workarounds ()
"Fix all issues which `xml-parse-region' could be choking on."
;; a very very dirty workaround to overcome the
;; problems with the newest (20030621) xml.el:
;; remove all unnecessary whitespace
(while (re-search-forward ">[ \t\r\n]+<" nil t)
(replace-match "><" nil t))
;; and another brutal workaround (20031105)! For some
;; reason the xml parser does not like the colon in the
;; doctype name "rdf:RDF"
(if (re-search-forward "<!DOCTYPE[ \t\n]+rdf:RDF" nil t)
(replace-match "<!DOCTYPE rdfColonRDF" nil t))
;; finally.... ~##^°!!!!!
(while (search-forward "\r\n" nil t)
(replace-match "\n" nil t))
;; still more brutal workarounds (20040309)! The xml
;; parser does not like doctype rss
(if (re-search-forward "<!DOCTYPE[ \t\n]+rss[ \t\n]*>" nil t)
(replace-match "" nil t))
;; And another one (20050618)! (Fixed in GNU Emacs 220.127.116.11)
;; Remove comments to avoid this xml-parsing bug:
;; "XML files can have only one toplevel tag"
(while (search-forward "<!--" nil t)
(let ((start (match-beginning 0)))
(unless (search-forward "-->" nil t)
(error "Can't find end of comment"))
(delete-region start (point))))
;; And another one (20050702)! If description is HTML
;; encoded and starts with a `<', wrap the whole
;; description in a CDATA expression. This happened for
"<description>\\(<img.*?\\)</description>" nil t)
"<description><![CDATA[ \\1 ]]></description>"))
;; And another one (20051123)! XML parser does not
;; like this: <yweather:location city="Frankfurt/Main"
;; region="" country="GM" />
;; try to "fix" empty attributes
;; This happened for
(while (re-search-forward "\\(<[^>]*\\)=\"\"" nil t)
(replace-match "\\1=\" \""))
I guess I’ll not be using this and go for a special-purpose solution
instead without over a decade old workarounds. After all, I’m not
stuck in 2005 with Emacs 21…