elisp help: Getting the current word position. (closure?)

elisp help: Getting the current word position. (closure?)

[cinsk]

Is there any clean way to get the position of the current word?

I implement my own function to do this, and it is working.  But I'm
afraid that my implementation is not a nice way to do what I want.

Could somebody give me some advice on this?


I wanted to get the position of the beginning/end of the current word
to insert some text around the current word. While experimenting
various functions, I found `current-word' that returns the word that
point is on.  After reading it's definition in simple.el, I decided
that the implementing something similar to `current-word' is beyond my
ability. So I want to use current-word to get the position of the
current word.

My `current-word' definition looks like (GNU Emacs 22.2.9999 on
Gentoo):

(defun current-word (&optional ...)
  (let* ((start (point))
         (end (point))
         ...)

    ;; Calculating the proper value of START and END.

    (unless (= start end)
       (buffer-substring-no-properties start end))))


My idea is to redefine `buffer-substring-no-properties' so that I can
save the START and END value passed to
`buffer-substring-no-properties' in the end of the `current-word'
definition.


(defun current-word-markers (&optional STRICT REALLY-WORD)
  "Return the position of the current word"
  (let ((old (symbol-function
                'buffer-substring-no-properties))
        (saved-start (make-marker))
        (saved-end (make-marker)) ret)

    (fset 'buffer-substring-no-properties
          (lambda (start end)
            (let (ret)
              (set-marker saved-start start)
              (set-marker saved-end end)
              (setq ret (funcall old start end))
              ret)))
    (setq ret (current-word STRICT REALLY-WORD))
    (fset 'buffer-substring-no-properties old)
    (if ret
        (cons saved-start saved-end)
      nil)))


`current-word-markers' returns a dotted pair with two markers; one for
the beginning of the current word, and another for the end of the
current word.

It redefines `buffer-substring-no-properties' so that new function can
save the passed START and END value somewhere in
`current-word-markers', and call the original function body using
`funcall'.  After getting what are needed, `current-word-markers'
restore the original function body of
`buffer-substring-no-properties'.

This function works.  With my short knowledge of LISP, I think the
(lambda ...) form in `current-word-markers' is a closure, since it
refers to the unbound variables (saved-start and saved-end).

Recently, I found that Emacs Lisp does not have "closures" according
to "GNU Emacs Lisp Reference Manual 2.8".

Here are a few questions:

1. Does the (lambda ...) in `current-word-markers' is a closure???

3. If not, enlighten me what is a closure.

2. If yes, is my function is safe?

3. Anyway, is there any better way (or predefined function) to do
this?


Thank you in advance.

Re: elisp help: Getting the current word position. (closure?)

[Johan Bockgård]

cinsk <cinsky@gmail.com> writes:

> 3. Anyway, is there any better way (or predefined function) to do
> this?

(bounds-of-thing-at-point 'word)

-- 
Johan Bockgård

Re: elisp help: Getting the current word position. (closure?)

[Tassilo Horn]

cinsk <cinsky@gmail.com> writes:

Hi!

> (defun current-word-markers (&optional STRICT REALLY-WORD)
>   "Return the position of the current word"
>   (let ((old (symbol-function
>                 'buffer-substring-no-properties))
>         (saved-start (make-marker))
>         (saved-end (make-marker)) ret)
>
>     (fset 'buffer-substring-no-properties
>           (lambda (start end)
>             (let (ret)
>               (set-marker saved-start start)
>               (set-marker saved-end end)
>               (setq ret (funcall old start end))
>               ret)))
>     (setq ret (current-word STRICT REALLY-WORD))
>     (fset 'buffer-substring-no-properties old)
>     (if ret
>         (cons saved-start saved-end)
>       nil)))
>
>
> This function works.  With my short knowledge of LISP, I think the
> (lambda ...) form in `current-word-markers' is a closure, since it
> refers to the unbound variables (saved-start and saved-end).

Due to emacs' dynamic scoping they're bound when this function is
called.  Dynamic scoping basically means that a function can see all
variables its caller defined (recursively).  Here's an example:

--8<---------------cut here---------------start------------->8---
(defun foo ()
  (let ((foo-val "I'm a local variable of foo!"))
    (bar)))

(defun bar ()
  foo-val)

(foo) ;; C-x C-e ==> "I'm a local variable of foo!"
--8<---------------cut here---------------end--------------->8---

> Recently, I found that Emacs Lisp does not have "closures" according
> to "GNU Emacs Lisp Reference Manual 2.8".

Yes, that's true.

> Here are a few questions:
>
> 1. Does the (lambda ...) in `current-word-markers' is a closure???

No.

> 3. If not, enlighten me what is a closure.

This real closure won't work in elisp:

--8<---------------cut here---------------start------------->8---
(defun make-adder (n)
  (lambda (m)
    (+ n m)))

(fset 'adder (make-adder 3))
(adder 3) ;; C-x C-e
==>
Debugger entered--Lisp error: (void-variable n)
  (+ n m)
  adder(3)
  eval((adder 3))
  eval-last-sexp-1(nil)
  eval-last-sexp(nil)
  call-interactively(eval-last-sexp nil nil)
--8<---------------cut here---------------end--------------->8---

Bye,
Tassilo
-- 
Richard Stallman can coerce meaningful data from /dev/null.