Re: file-exists-p on empty string

[Michael Albinus <michael.albinus@gmx.de>]

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Stefan Monnier <monnier@iro.umontreal.ca> writes:

>>> So the (not (string-empty-p fn)) test just reflects the tool's semantics
>>> and is not an artifact of some suboptimal behavior of file-exists-p.
>> I gree, but there is not a line about empty strings in doc-string.
>> It is great that this is mentioned in documentation, but anyway
>> behaviour was quite unexpected
>
> The problem is that docstrings describe the behavior of a specific
> function, so they usually don't mention the more general aspects that
> affect all functions of a given subsystem, such as here the general
> treatment of the empty string when used as a file name.
>
> Otherwise, every file-name-manipulating function would have to repeat
> this information in its docstring.

What about the appended patch? It does not touch the docstrings, but it
explains the situation more verbose in the Lisp manual.

>         Stefan

Best regards, Michael.


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  names.  A file name is a string.  Most of these functions expand file
  name arguments using the function @code{expand-file-name}, so that
  @file{~} is handled correctly, as are relative file names (including
! @file{../}).  @xref{File Name Expansion}.
  
    In addition, certain @dfn{magic} file names are handled specially.
  For example, when a remote file name is specified, Emacs accesses the
--- 16,22 ----
  names.  A file name is a string.  Most of these functions expand file
  name arguments using the function @code{expand-file-name}, so that
  @file{~} is handled correctly, as are relative file names (including
! @file{../} and the empty string).  @xref{File Name Expansion}.
  
    In addition, certain @dfn{magic} file names are handled specially.
  For example, when a remote file name is specified, Emacs accesses the
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  superroot above the root directory @file{/}.  On other filesystems,
  @file{/../} is interpreted exactly the same as @file{/}.
  
+ Expanding @file{.} or the empty string returns the default directory:
+ 
+ @example
+ @group
+ (expand-file-name "." "/usr/spool/")
+      @result{} "/usr/spool"
+ (expand-file-name "" "/usr/spool/")
+      @result{} "/usr/spool"
+ @end group
+ @end example
+ 
  Note that @code{expand-file-name} does @emph{not} expand environment
  variables; only @code{substitute-in-file-name} does that: