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Re: defcustom keyword :require
Re: defcustom keyword :require
Fri, 31 Aug 2018 13:35:02 -0400
Gnus/5.13 (Gnus v5.13) Emacs/27.0.50 (gnu/linux)
FWIW, I also dislike this option.
Drew Adams <address@hidden> writes:
> `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
> '(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")
> mapcar(require (tramp))
> 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
> 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
> 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
> 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
> 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
> 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.)