Re: substitute-in-file-name and "$"

[Michael Albinus]

Luc Teirlinck <address@hidden> writes:

> None of the examples in the Elisp manual, (elisp)File Name Expansion,
> involves Elisp variables.  Even when working with variables, it would
> seem that you can call (substitute-in-file-name filename) 1001 times

That's right.

> without danger (it has no side effects).  Danger arises from stuff
> like:
>
> 1. (setq filename (substitute-in-file-name filename))
>
> or
>
> 2. (my-favorite-file-function (substitute-in-file-name filename))
>
> The Elisp manual could point out that (1) should only be done if the
> user really knows what he is doing.  Usually, using the return value
> of (substitute-in-file-name filename) only at the time it is actually
> needed (and _not_ changing filename itself at that time) or doing:
>
> (1b) (setq substituted-filename (substitute-in-file-name filename))
>
> will be safer.

That's right, too.

> In as far as (2) is concerned, the documentation string of
> `my-favorite-file-function' should clearly point out what operations
> it is going to perform on filename.  If not, that is a bug in the
> documentation string.  In that case, the Elisp user could try to
> figure it out by experimentation and file a bug report.  If
> `my-favorite-file-function' is going to pass filename on to other
> functions it calls, it should exercise a similar care itself.

That wouldn't be possible. You know what `my-favorite-file-function'
is doing the default case. You don't know what's going on with an
alternative implementation within another file name handler. Would you
like to say for every file name operation "substitute-in file-name is
called" (or not)? And even if you do it, you cannot control, because
it could be called indirectly, within another file name function you
use.

>    And, btw, if substitute-in-file-name can be called only once, how do I
>    know it where to perform this call? And what shall the others do, if
>    they would like to perform substitute-in-file-name?  Requote "$"? Or
>    apply an optional parameter (to be introduced), which forces this?
>
> Do you have examples where there is a real need to do (1) instead of
> (1b)?  I have a hard time thinking of examples in the abstract, let
> alone of examples occurring in actual situations.  Unquoting and then
> requoting seems like a senseless circular operation to me.

`PC-do-completion' must do it, because the result is shown in the
minibuffer.

Or a file name like this: "$NEXT_HOP:/share$$", where $NEXT_HOP has
the value "/smb:next.hop.com". At least case (2) would be needed,
because Tramp file name handler could be activated only after applying
substitute-in-file-name to that file name.

> Sincerely,
>
> Luc.

Best regards, Michael.