bug#59937: 28.2; Bad defcustom behavior

[Mauro Aranda]

Drew Adams <drew.adams@oracle.com> writes:

>>  > Expecting a defcustom definer to understand this
>>  > and figure out what a "valid default value for
>>  > the restricted-sexp widget" might be, is a bridge
>>  > too far, IMO.
>>
>> I don't think so.  The defcustom definer is specifying the matching
>> alternatives, he/she should be able to think of a valid default
>> value.
>
> How does s?he define/provide a default value for the
> `restricted-sexp' widget?  I see no way to do that.

It's the very first keyword mentioned under the Type Keywords node in
the ELisp manual.  Usage would be like this:


(defcustom foo nil
  "..."
  :type '(repeat (restricted-sexp
                  :value :key
                  :tag "Keyword"
                  :match-alternatives (keywordp))))

Clicking INS creates a restricted-sexp without trouble.

> Or did you mean that s?he should always provide, within
> the _option's_ default value, a value for each part of
> it that corresponds to a `restricted-sexp'?

I didn't mean that.  I meant something like the code above.

>> Maybe having some examples in the documentation could help here. I
>> could write one if you and others think it could be helpful.
>
> I _think_ I understand this now.  The problem is that for
> the Customize UI to present a field for inputting/defining
> the part of the option value that corresponds to a plist
> key (which is defined by a `restricted-sexp'), it needs to
> know just what kind of input/edit widget to build.  It
> needs to build an editable-field that also demands respect
> of the `restricted-sexp' predicates.

Yes.

> And that's the case whether or not the _option's_ default
> value has a part that corresponds to a plist key.  (If yes,
> the default value must match the `restricted-sexp', if no,
> you're prompted for the sexp type, so it knows what kind
> of field to make.)
>
> And yes, a simple example with `restricted-sexp' would
> help (maybe 2 examples: bad & good).

I'll work on a patch some day this week, then.

> The idea/problem isn't limited to `restricted-sexp', IIUC.
> But in other cases it's much less likely to be a gotcha,
> because the parts of the defcustom value that correspond
> to each field in the Customize UI will have types that
> correspond to existing widgets (they don't require
> additional input/prompting to know what kind of UI field
> to create).

Yes.

> The problem really stems, I guess, from the fact that
> `restricted-sexp' can involve any kinds of predicates,
> and depending on what those do, the UI field can be
> different.  Put differently, the UI field takes into
> account the `restricted-sexp' predicates.  But the
> prompting does not take them into account!

I'd say don't focus too much on the prompt.  It really shouldn't be
there, and I consider it a bug in the Widget code, but it's really an
implementation detail.  Without going into a lot of details, we want to
READ a string like this: (read var) where VAR is a string, the
representation of the value of the widget, whatever that is, but because
the widget's value didn't match, it is not a string that we read and VAR
is nil (Note: not "nil", but nil), so we end up calling (read nil) and
that's the unintended prompt.

> My thoughts about this - let me know what you think:
>
> 1. The warning(s) are not very helpful.  They will
> mainly confuse, I think.
>
> First, end users _will_ see them, as the defcustom
> author may not have tested every possibility well.
>
> Second, many defcustom authors also won't understand
> them.

I don't know if you're suggesting to improve it or to get rid of it.
I'd like to make it more helpful, as I think it should be there.

> 2. I think a big improvement could be to make use of
> any :tag that the defcustom author provides for the
> `restricted-sexp' field - using the :tag also as the
> prompt, instead of "Lisp expression: ".  When you see
> that generic prompt you have _no clue_ what it wants,
> or why.  The :tag should tell you what to enter.

I don't think that's an improvement because of what I said above. No
prompt should be there for starters.  At least that's what I understand
about the code.

Note that in Bug#25152 you ended up with a weird buffer state after
hitting C-g at that prompt.  That's because the Widget library is not
ready to take user input at that moment.

> 3. Don't show any warning when prompting.  Just try
> to have the inputting itself be clearer (#2).

Because of my response, I don't think #3 applies.  I hope you agree with
me after reading my response.

> With those changes, the manual could also be improved:
>
> (1) Tell defcustom definers that if they use
> `restricted-sexp' then good practice is to provide a
> :tag for the field.  And tell them that the :tag will
> also be used as a prompt for creating the appropriate
> editable field.

Here again, I don't think this is the path we want to follow.  And the
manual already emphasizes that providing a valid default value is
essential, when nil isn't it.

> (2) Explain that such prompting happens whenever the
> default value of the option doesn't provide a value
> for each of its parts that corresponds to the use of
> a `restricted-sexp'.

As I said, I consider the prompt a bug in the Widget code, so I don't
think we should mention it in docs.

>>  > I am curious whether you think there's actually
>>  > a bug or not.  It's hard for me to believe that
>>  > we should expect _anyone_ defining a defcustom
>>  > (let alone anyone using Customize) to understand
>>  > the `restricted-sexp' widget, what it requires
>>  > wrt its "default value", and how to adjust a
>>  > defcustom to give it what it needs, to DTRT.
>>
>> I think a better behavior would be to avoid the prompting altogether
>> (there should be no prompt at that moment, for starters).  But again,
>> this situation arises when there is a bug on the defcustom :type, so I'd
>> be happier if people can help with improving the warning message.
>
> See above.
>
> I don't think there's a bug in the defcustoms in
> the examples shown.  And I do think users should be
> prompted if Emacs needs to know what kind of input
> (UI) field to create.  And I don't think we should
> show any warnings.  We can raise an error if the
> user input is, in the end, invalid (and I think we
> already take care of that).

We'll have to disagree here, about two things.

1.  I still think there's a bug in the defcustoms.  Any widget needs a
valid default value, and it's up to the person that's defining the
widget to provide it.  If he/she doesn't, then that's a bug.

2.  I do think a warning (or an error) is in place.

> I may still be misunderstanding things.  Let me
> know.  But if so then I'm guessing others will also
> misunderstand.

I think you understand now, but the prompt being there is really
confusing (it was to me when I first read your bug report in
Bug#25152).  Hopefully I clarified a little more with my response.

> The current state is, I think, poor
> support for the flexible, powerful feature that is
> `restricted-sexp'.  Understandable, but we should
> somehow try to do better.
>
> I, for one, wish more definers of defcustoms spent
> more time defining tighter types.  And often that
> could mean using `restricted-sexp'.
>
> FWIW, this bug report came directly from a user
> question on emacs.SE, here:
>

> https://emacs.stackexchange.com/q/74913

I missed that.  I usually lurk on SE for questions with custom or widget
tags, but you do a better job than me :-).

> Thanks for your efforts with this.

You're welcome, I'm happy to help.