RE: defcustom keyword :require

[Drew Adams <drew.adams@oracle.com>]

> > `defcustom' keyword `:require' has the effect of invoking `require'
> > for the defining library whenever `custom-set-variables' sets the
> > option value.  The `require' is invoked before setting the option.
> >
> > This can be bothersome when a `custom-set-variables' sexp is inserted
> > automatically in an init file or `custom-file' by an Emacs session
> > where the option is defined, and the init file or `custom-file' is
> > subsequently used by an Emacs session where that library is not
> > available (e.g., cannot or should not be available).  A user might not
> > want Emacs to raise an error in such a case.
> >
> > Example in Emacs 26.1: I save bookmarks, some of which are to remote
> > files.  This causes my `custom-set-variables' to be updated to include
> > the following entries, because (only for Emacs 26.1) the defcustoms
> > for `tramp-default-method' and `tramp-verbose' use `:require 'tramp':
> >
> >  '(tramp-default-method "ftp" nil (tramp)) ; Require Tramp first
> >  '(tramp-verbose 9 nil (tramp))            ; Require Tramp first
> >
> > Prior to and after Emacs 26.1 those entries are just the following, so
> > no attempt is made to load Tramp when processing the
> > `custom-set-variables':
> >
> >  '(tramp-default-method "ftp")
> >  '(tramp-verbose 9)
> >
> > Using an Emacs release that does not include Tramp aborts
> > initialization when it encounters the requirement to load Tramp.  So
> > in a session with an old Emacs release those require-Tramp settings
> > can raise this error:
> >
> > Signaling: (file-error "Cannot open load file" "tramp")
> >   require(tramp)
> >   mapcar(require (tramp))
> >   custom-set-variables(...)
> >
> > The option setting hard-requires Tramp. It would be OK here if the
> > meaning of the REQUEST part of the `custom-set-variables' settings
> > were to just soft-require the library, i.e., if the REQUEST arg meant
> > (require 'tramp nil t) instead of (require 'tramp).
> >
> > OK, the Tramp example is problematic only for quite old Emacs releases
> > - but the point is general.
> >
> > FWIW, this is how I now work around the problem in my init file, but
> > it is not a good solution (better solutions are welcome):
> >
> >  (unless (require 'tramp nil t) (provide 'tramp))
> >  (load-file custom-file) ; Load only after faking providing Tramp
> >
> > For Emacs 26.2 and later I guess either it was realized that Tramp
> > need not really be loaded prior to setting these options or some
> > change was made to the Tramp code to obviate the load.
> >
> > The rationale given in the Elisp manual for :require is this:
> >
> >   The most common reason to use `:require' is when a variable enables
> >   a feature such as a minor mode, and just setting the variable won't
> >   have any effect unless the code which implements the mode is
> >   loaded.
> >
> > Seems like that common reason doesn't really call for a hard require
> > in general - a soft require might be sufficient.  The customized value
> > would not have the desired effect perhaps, but the world would not
> > end.
> >
> > We already point out, for `define-minor-mode', that "Except in unusual
> > circumstances" the mode variable "must" be initialized to `nil':
> >
> >   The initial value must be `nil' except in cases where (1) the mode
> >   is preloaded in Emacs, or (2) it is painless for loading to enable
> >   the mode even though the user did not request it.  For instance, if
> >   the mode has no effect unless something else is enabled, and will
> >   always be loaded by that time, enabling it by default is harmless.
> >   But these are unusual circumstances.  Normally, the initial value
> >   must be `nil'.
> >
> > The Elisp manual also says this about :require (in node `Defining
> > Minor Modes'):
> >
> >   One of the effects of making a minor mode global is that the MODE
> >   variable becomes a customization variable.  Toggling it through the
> >   Customize interface turns the mode on and off, and its value can be
> >   saved for future Emacs sessions (see (emacs)Saving Customizations).
> >   For the saved variable to work, you should ensure that the
> >   `define-minor-mode' form is evaluated each time Emacs starts; for
> >   packages that are not part of Emacs, the easiest way to do this is
> >   to specify a `:require' keyword.
> >
> > This also suggests that (1) :require is for minor-mode MODE variables,
> > and (2) it is for packages that are not part of Emacs.  Those things
> > are not made clear in the part of the manual where :require is
> > introduced, which is maybe why those Tramp variables obtained :require
> > for Emacs 26.1.  The description of :require should probably make
> > clear that its intended use is particularly narrow.
> >
> > Are there other, UNcommon reasons to use defcustom keyword :require?
> >
> > If `custom-set-variables' raises an error for this in an init file it
> > stops everything.  In such a case (the common use case for :require)
> > wouldn't a warning be more appropriate?
> >
> > Shouldn't :require really lead to a soft require and a warning, not a
> > hard require and an error?  Or should we perhaps add a :soft-require
> > keyword and promote its use (generally) over the use of :require?
> >
> > Should the doc also point out this potential problem with (hard)
> > :require?  Should it tell `defcustom' writers that user customization
> > when using an Emacs session where the option exists will lead to
> > failure-to-launch in a session where the option's library is not
> > available?
> >
> > FWIW, I have never run into this problem before - probably because (1)
> > few defcustoms actually use :require and (2) I always have the library
> > available, for any that do use :require.
> >
> > Seems like defcustom :require doesn't really fit too well with
> > automatic writing of `custom-set-variables' to a user's init file or
> > `custom-file' (the most common use of `custom-set-variables'.  That
> > code is typically used for Emacs sessions of all kinds (e.g. different
> > releases, different contexts).
> >
> > Maybe soft-requiring can handle its most common use cases?  Maybe
> > issuing a warning instead of erroring-out is generally more
> > appropriate?
> >
> > And perhaps there could be a user option that lets you override
> > :require, to make it act always like :soft-require?  That is, even if
> > a library chooses (hard) :require, maybe users need a way to override
> > that.
> >
> > What do you think?  I expect that few have actually run into this
> > problem, but I also expect that that is only because of the reasons
> > stated above: (1) :require is seldom used and (2) libraries that use
> > it are typically available across Emacs sessions.  (Most users
> > probably use only one Emacs release and use it with the same context
> > for each session.)
>
> FWIW, I also dislike this option.
>         Stefan

Great. (By "this option" I guess you mean using :require, or at least using it unnecessarily.)

 What do you think should be done about it?