Hi Andreas,

`nil` seemed more appropriate than returning either an empty string or throwing an error. The intent is to signal a failure in general, as it looks like other related functions in that file do.

On Tue, 25 May 2021, at 10:48, Andreas Schwab wrote:
> On Mai 25 2021, Colin Woodbury wrote:
> 
> > diff --git a/lisp/files.el b/lisp/files.el
> > index 62e1702fdf..f8aefa7930 100644
> > --- a/lisp/files.el
> > +++ b/lisp/files.el
> > @@ -4889,6 +4889,20 @@ extension, the value is \"\"."
> >          (if period
> >              "")))))
> >  
> > +(defun file-name-set-extension (filename extension)
> > +  "Change the extension of a FILENAME to EXTENSION.
> > +Sanitizes the input to consolidate leading/trailing dots.
> > +
> > +Returns `nil' if either of the FILENAME or EXTENSION are `nil'
> > +before sanitizing, or empty afterwards."
> > +  (when (and filename extension)
> 
> What is the use-case for nil?
> 
> Andreas.
> 
> -- 
> Andreas Schwab, schwab@linux-m68k.org <mailto:schwab%40linux-m68k.org>
> GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510  2552 DF73 E780 A9DA AEC1
> "And now for something completely different."
>