Re: [PATCH] lisp/files.el: Add `file-name-set-extension`

["Colin Woodbury" <colin@fosskers.ca>]

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Thanks Stefan, I've revised the patch according to your suggestions (see attached). I also revised the docstring.

Thanks!
Colin

On Tue, 25 May 2021, at 14:29, Stefan Monnier wrote:
> > That regex was borrowed directly from `string-trim`, to which I added
> > a `.`. You're right that the main motivation was to allow the caller to pass
> > either `.foo` or `foo` as the extension and have either case work (many
> > languages do this). Leaving the other characters in the regex seemed like
> > a sanitary thing to do, just in case they pass something bogus.
> 
> File names can be used in different ways for different purposes.
> We don't have any good reason to think that a space or a newline at the
> end of a file name (or beginning of an extension) is "bogus".
> 
> If such things occurred somewhat often and always in ways where they are
> indeed undesirable, we could consider removing them (the tradeoff
> between occasional harm and frequent error-avoidance would be
> favorable), but here this is just asking for trouble with no benefit.
> 
> > 1. Simplifying the regex to just include dots (and perhaps whitespace) (i.e. trust the caller), or;
> 
> Please do that.
> 
> > 2. Expanding the logic to sanitize both ends of both arguments, (i.e. don't trust the caller), or;
> 
> I don't think we have any reason to think we should "sanitize" it.
> 
> > 3. Adding error-throwing logic if malformed files or extensions are given (i.e. warn the user).
> 
> "Malformed" according to which standard?
> 
> 
>         Stefan
> 
> 

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diff --git a/lisp/files.el b/lisp/files.el
index 62e1702fdf..ab8be6ad9e 100644
--- a/lisp/files.el
+++ b/lisp/files.el
@@ -4889,6 +4889,21 @@ extension, the value is \"\"."
         (if period
             "")))))
 
+(defun file-name-set-extension (filename extension)
+  "Set the extension of a FILENAME to EXTENSION.
+Consolidates leading/trailing dots so that either `foo' or `.foo'
+can be passed as an EXTENSION.
+
+Returns nil if either of the FILENAME or EXTENSION are nil before
+dot consolidation, or empty afterwards."
+  (when (and filename extension)
+    (let* ((patt "[ .]+")
+           (filename (string-trim-right filename patt))
+           (extension (string-trim-left extension patt)))
+      (unless (or (string-empty-p filename)
+                  (string-empty-p extension))
+        (concat (file-name-sans-extension filename) "." extension)))))
+
 (defun file-name-base (&optional filename)
   "Return the base name of the FILENAME: no directory, no extension."
   (declare (advertised-calling-convention (filename) "27.1"))