Re: Help with keybinding to delete between {}

[Mike Mattie <codermattie@gmail.com>]

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On Thu, 6 Dec 2007 09:14:12 -0800 (PST)
Xah Lee <xah@xahlee.org> wrote:

> for some reason my code in the previous post is completely non-
> functional. (i swear i used it for few months. Perhaps when i put on
> the website i got smart and edited it "for the better" without
> testing)
> 
> Here's the correct version:
> 
> (defun delete-enclosed-text ()
>   "Delete texts between any pair of delimiters.
> Note: if you have nested matching pairs, the cursor
> should be inside the inner most one. Else it gets confused.
> This code should to be fixed in the future."
>   (interactive)
>   (save-excursion
>     (let (p1 p2)
>       (skip-chars-backward "^(<["<<") (setq p1 (point))
>       (skip-chars-forward "^)>]">>") (setq p2 (point))
>       (delete-region p1 p2)
>     )
>   )
> )

I decided to fix it in regards to nested lists. My implementation works
however it is purely for elucidation. It is a recursive implementation
that is nth recursive where n is the number of nested delimiters in the list.

that means that if you use this on a list with deep nesting it *will* crash
after exhausting the recursion limit. So don't use it for real.

A reliable solution would likely be based off something from thingatpoint.el ?
there must be something like this in the code-motion code for elisp.

This sort of function/feature? is dangerous even when implemented procedurally
because it will go wild if your delimiters aren't matched correctly.

Other flaws:
  * error path in bounds scan marked by "error ?" not implemented because I have
    not studied elisp error handling yet.

  * does not handle multi-byte characters due to use of aref in bounds-scan-{forward,backward}

With that said it does illustrate the value of lexical-let, and solves the
problem in a general way. It would be quite easy to add variations that
kill instead of deleting, or whatever other features you want.

I implemented it recursively simply because It looked prettier to me, probably
since I started with scheme. anyways here goes.

one last big fat warning: this code is for fun, it will eat your children eventually.

;; --- start of elisp

(defun bounds-scan ( seek-bounds open-bound-p close-bound-p restart-position position level )
  "scan for the delimitation of a region. This is a general form of a
   simple algorithm that counts opening and closing delimiters to scan
   past nested delimited spans."
  (progn
    (goto-char position) ;; move to the starting position before scanning.
    (funcall seek-bounds)

    (cond
      ((funcall open-bound-p)
        (bounds-scan seek-bounds open-bound-p close-bound-p restart-position
          (funcall restart-position) (+ level 1)))

      ((funcall close-bound-p)
        (if (> level 0)
          ;; when we have a positive level to start with
          ;; scan again with a decremented level.
          (bounds-scan seek-bounds open-bound-p close-bound-p restart-position
            (funcall restart-position) (- level 1))

          ;; return point as we are done
          (point)
          ))
      ;; error ?
      )))

(defun bounds-scan-forward ( delimiters position )
  "entry point for bounds-scan forward. given delimiters: a
   string containing a pair of delimiting characters, which must
   be in \"open close\" order, scan forward for the bounding
   delimiter returning the position before the delimiter"

  (lexical-let
    ((open-delimiter (aref delimiters 0))
     (close-delimiter (aref delimiters 1)))

    (bounds-scan
      (lambda ()
        (skip-chars-forward (concat "^" delimiters)))

      (lambda ()
        (char-equal open-delimiter (char-after)))

      (lambda ()
        (char-equal close-delimiter (char-after)))

      (lambda ()
        (+ (point) 1))

      position 0)))

(defun bounds-scan-backward ( delimiters position)
  "entry point for bounds-scan backward. given delimiters: a
   string containing a pair of delimiting characters, which must
   be in \"open close\" order, scan backward for the bounding
   delimiter returning the position after the delimiter"

  (lexical-let
    ;; note the inversion of the order since we are looking backwards
    ((open-delimiter (aref delimiters 1))
     (close-delimiter (aref delimiters 0)))

    (bounds-scan
      (lambda ()
        (skip-chars-backward (concat "^" delimiters)))

      (lambda ()
        (char-equal open-delimiter (char-before)))

      (lambda ()
        (char-equal close-delimiter (char-before)))

      (lambda ()
        (- (point) 1))

      position 0)))

(defun scan-lisp-list-close ()
  "wrapper for bounds-scan that searches for the closing delimiter of a lisp list"
  (let*
    ((start-at (point))
      (close-at (bounds-scan-forward "()" start-at)))

    (if (> close-at start-at)
      (- close-at 1)
      start-at)
    ))

(defun scan-lisp-list-open ()
  "wrapper for bounds-scan that searches for the opening delimiter of a lisp list"
  (let*
    ((start-at (point))
      (open-at (bounds-scan-backward "()" start-at)))

    (if (< open-at start-at)
      (+ open-at 1)
      start-at)
    ))

(defun lisp-list-delete-body ()
  "delete the body of a lisp list including any nested lists"
  (interactive)
  (let
    ((open-pos (scan-lisp-list-open))
     (close-pos (scan-lisp-list-close)))

    (delete-backward-char (- close-pos open-pos))))

;;----- end of elisp

Cheers,
Mike Mattie

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