Re: safe way to add contents to a file ?

[Jean-Christophe Helary <jean.christophe.helary@traduction-libre.org>]

> On Dec 22, 2019, at 12:14, Jean-Christophe Helary <jean.christophe.helary@traduction-libre.org> wrote:
> 
> 
> 
>> On Dec 21, 2019, at 1:00, Stefan Monnier <monnier@iro.umontreal.ca> wrote:
>> 
>>> (setq myText "<item>bla</item>")
>>> (setq myMarker "<!-- place new items before this comment -->")
>>> (setq myFile "/path/to/test.xml")
>> 
>> Being at top-level these aren't just setting the vars but defining them,
>> so should use `defvar` or `defconst`.
>> 
>>> (defun myInsert (myText myMarker myFile)
>>> (save-current-buffer
>>>   (set-buffer (find-file-noselect myFile))
>> 
>> `with-current-buffer` does the same, but shorter ;-)
> 
> How is it supposed to be shorter ?
> 
> (defun myInsert (myText myMarker myFile)
>  (save-current-buffer
>    (set-buffer (find-file-noselect myFile))
>    (goto-char (point-min))
>    (goto-char (- (search-forward myMarker) (length myMarker)))
>    (insert myText)
>    (indent-region (point-min) (point-max))
>    (save-buffer)
>    (kill-buffer)))
> 
> (defun myInsert2 (myText myMarker myFile)
>  (with-current-buffer
>      (set-buffer (find-file-noselect myFile))
>    (goto-char (point-min))
>    (goto-char (- (search-forward myMarker) (length myMarker)))
>    (insert myText)
>    (indent-region (point-min) (point-max))
>    (save-buffer)
>    (kill-buffer)))
> 
> 
> Honestly, as I read the respective doc strings and reference parts again, I'm not even sure why I chose save-current-buffer here anymore, but *because* there is no explanatory document on how to do proper file i/o in the Elisp Reference it is hard to make informed decisions, or even progress without a lot of searching outside the reference.
> 
>>>   (goto-char (point-min))
>>>   (goto-char (- (search-forward myMarker) (length myMarker)))
>> 
>> If the search fails, this will signal a "low-level" error, and it's
>> often useful to replace it with some other behavior (e.g. an error
>> message which the user is more likely to understand, or some other
>> behavior), so it's more idiomatic to do something like:
>> 
>>   (goto-char (point-min))
>>   (if (not (search-forward myMarker nil t))
>>       (user-error "Can't find foo bar in your fine file")
>>     (goto-char (match-beginning 0))
> 
> But here, the code would go on inserting the text in a position that's not correct, right ?
> 
> Isn't it possible to catch the error around the search ? The only language I know, AppleScript, has a
> 
> try
>  body
> on error
>  do something
> end try
> 
> block which seems equivalent to "condition-case" but it seems that I'd have to use a progn to have the equivalent:
> 
> (defun myInsert3 (myText myMarker myFile)
>  (save-current-buffer
>    (set-buffer (find-file-noselect myFile))
>    (goto-char (point-min))
>    (condition-case nil
> 	(progn
> 	  (goto-char (- (search-forward myMarker) (length myMarker)))
> 	  (insert myText)
> 	  (indent-region (point-min) (point-max))
> 	  (save-buffer))
>      (error (format "%s was not found" myMarker)))
>    (kill-buffer)))
> 
> And then the message is not displayed. I just get a "t" which I guess corresponds to the successful (kill-buffer)...
> 
> And if I put the kill-buffer inside the progn, then I'm left with an open buffer that's not relevant anymore...


Here is my latest version...


(defun myInsert4 (myText myMarker myFile)
  (save-current-buffer
    (set-buffer (find-file-noselect myFile))
    (goto-char (point-min))
    (if (not (search-forward myMarker nil t))
	(progn
	  (user-error (format "%s was not found" myMarker))
	  (kill-buffer))
      (progn
	(goto-char (point-min))
	(goto-char (- (search-forward myMarker) (length myMarker)))
	(insert myText)
	(indent-region (point-min) (point-max))
	(save-buffer)))
    (kill-buffer)))

Really not sure if I'm going in the right direction. Plus, for some reason, the buffer is not killed even after an error...


Jean-Christophe Helary
-----------------------------------------------
http://mac4translators.blogspot.com @brandelune