When Data Becomes Code


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
elisp byte-compiler."
  :type 'boolean
  :group 'ibuffer)


(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)
             ,(if from-end-p
                  ;; FIXME: this should probably also be using
                  ;; `truncate-string-to-width' (Bug#24972)
                  `(concat ,ellipsis
                           (substring ,strvar
                                      (string-width ibuffer-eliding-string)))
                   ,strvar (- strlen (string-width ,ellipsis)) nil ?.)

(defun ibuffer-compile-make-substring-form (strvar maxvar from-end-p)
  (if 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))
       ,(pcase alignment
          (: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.
        (vars-used ()))
    (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.
           `(insert ,form)
         (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))
                  (letbindings nil)
                  (outforms nil)
                  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
                                (setq str
                                        `(- ,(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
                                (setq str
                                        (if (integerp max)
                                (setq strlen (string-width str))
                                (setq str
                                        'str elide from-end-p)))))
             ;; Now, put these forms together with the rest of the code.
             (let ((callform
                    ;; 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)
                        `(insert ,arg))))
                   (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)
                          `(if ,mincompform
                             (if ,maxcompform
                        (if min-used
                            `(when ,mincompform
                          `(when ,maxcompform
                     (push `(setq str ,callform
                                  ,@(when strlen-used
                                      `(strlen (string-width str))))
                     (setq outforms
                           (append outforms
                                   (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'.
               `(let ,letbindings
    ;; We don't want to unconditionally load the byte-compiler.
    (funcall (if (or ibuffer-always-compile-formats
                     (featurep 'bytecomp))
             ;; Here, we actually create a lambda form which
             ;; inserts all the generated forms for each entry
             ;; in the format string.
             `(lambda (buffer mark)
                (let ,vars-used
                  ,@(nreverse result))))))

(defun ibuffer-recompile-formats ()
  "Recompile `ibuffer-formats'."
  (setq ibuffer-compiled-formats
        (mapcar #'ibuffer-compile-format ibuffer-formats))
  (when (boundp 'ibuffer-filter-format-alist)
    (setq ibuffer-compiled-filter-formats
          (mapcar (lambda (entry)
                    (cons (car entry)
                          (mapcar (lambda (formats)
                                    (mapcar #'ibuffer-compile-format formats))
                                  (cdr entry))))

(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)
              (null ibuffer-cached-formats)
              (not (eq ibuffer-cached-formats ibuffer-formats))
              (null ibuffer-cached-eliding-string)
              (not (equal ibuffer-cached-eliding-string ibuffer-eliding-string))
              (eql 0 ibuffer-cached-elide-long-columns)
              (not (eql ibuffer-cached-elide-long-columns
                        (with-no-warnings ibuffer-elide-long-columns)))
              (and ext-loaded
                   (not (eq ibuffer-cached-filter-formats
                   (and ibuffer-filter-format-alist
                        (null ibuffer-compiled-filter-formats))))
      (message "Formats have changed, recompiling...")
      (setq ibuffer-cached-formats ibuffer-formats
            ibuffer-cached-eliding-string ibuffer-eliding-string
            ibuffer-cached-elide-long-columns (with-no-warnings ibuffer-elide-long-columns))
      (when ext-loaded
        (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.