Package: emacs;
Reported by: Alan Mackenzie <acm <at> muc.de>
Date: Mon, 16 Jan 2017 21:24:01 UTC
Severity: wishlist
Done: Alan Mackenzie <acm <at> muc.de>
Bug is archived. No further changes may be made.
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From: Alan Mackenzie <acm <at> muc.de> To: Michael Heerdegen <michael_heerdegen <at> web.de> Cc: 25461 <at> debbugs.gnu.org Subject: bug#25461: [Patch]: Missing doc strings for "," and ",@". Date: Fri, 20 Jan 2017 16:58:56 +0000
Hello, Michael.
On Fri, Jan 20, 2017 at 00:12:18 UTC, Michael Heerdegen wrote:
> Alan Mackenzie <acm <at> muc.de> writes:
> > > > +;; Give `,' and `,@' documentation strings which can be examined by C-h f.
> > > > +(put '\, 'function-documentation
> > > > + "`,' signals that the next form should be evaluated and inserted.
> > > > +It can occur only in `\\=`' constructs.
> > > > +
> > > > +For example:
> > > > +
> > > > +b => (ba bb bc) ; assume b has this value
> > > > +\\=`(a ,b c) => (a (ba bb bc) c) ; insert the value of b
> > > > +
> > > > +See also `\\=`' and `,@'.
> > > I don't think this makes it easier for people to understand things.
> > > This suggests that "," has some kind of meaning per se, that it is a
> > > macro-like thing, etc, and that pcase somehow redefines it.
> > Yes. All these things are true, aren't they?
> That `,' "signals" something is too vague to be true or false. As an
> explanation of things, I don't find it good, and in the generality that
> this sentence is speaking, I would say it's wrong.
OK, I'll accept that. I can't see where it's wrong, though. I've
changed it to say "causes" rather than "signal". (See below.)
> That "," can only appear inside "`" is simply wrong.
That's poor wording on my part. What I really meant was that for , to
have the effect described, it must be in a ` construct. I've amended
this bit too (see below).
[ .... ]
> It is also not the case that `pcase' redefines "`".
pcase causes ` to be expanded by the macro \`--pcase-macroexpander in
place of the macro \`. What is that if it is not pcase redefining `?
> And yes, I've seen uses of "," outside of "`" in Lisp. And it's
> perfectly fine to use "," for anything anyone wants - like any other
> symbol.
I disagree with you, here. Using standard Elisp symbols to mean
something different is a recipe for confusion. I don't think it is in
any way fine to use `if', `and', `car', .... for non standard purposes.
I also don't think it's OK to do the same with ` or ,.
> You don't seem to be confused by the fact that `rx' "(re-)uses" `and',
> `or' etc. What makes "`" different? The fact that it is a "reader
> macro"?
I wasn't aware of rx. I don't think it was good to reuse `and' the way
it does, but (subjectively) I don't think it will have caused all that
much confusion.
> > > Of course there is a logic behind pcase's usage of ` and ,. The
> > > usage of these suggests a mental model for their "meaning". But we
> > > should not describe our mental models in docstrings. That's only
> > > useful for people sharing the same model.
> > I'm not sure I'm following you here. A high level description of a
> > function necessarily involves a mental model.
> Maybe. But you are describing properties of your mental model instead
> of properties of the language. Your mental model led you to confusion
> and wrong conclusions.
I disagree with you, here. I'm describing the simple straighforward
normal effect of ,. I don't understand why you're making such a big
deal out of it.
> It's not optimal to describe these things. It will confuse others.
I disagree with you here, too. , and ,@ have a simply stated meaning in
the typical case, far simpler than you're making it out to be.
> > ` has a specific meaning, and has had a high quality doc string for
> > ever.
> Because it has a `symbol-function'.
> > , likewise has a specific meaning,
> That's one part that led you to confusion.
> > but doesn't yet have a doc string.
> ... because it is undefined.
It is defined. I intend that definition to be in a doc string.
> > > So I think we maximally should describe what the reader does with `
> > > etc., so that people know what to search for in the manual or remember
> > > what they already had learned.
> > We don't do that for other functions. A function's doc string should be
> > a crisp summary of what a function _does_. A doc string which directs
> > people to a manual, or is so confusing or unspecific that the reader is
> > forced to open a manual, is a failed doc string.
> But that's exactly the point: "," is not a function or a macro.
No, but it needs a doc string.
> So how it is used is arbitrary.
If that were the case, Emacs wouldn't even build. ,'s use is no more
arbitrary than any other symbol in Lisp.
> It's traditionally used by "`", but it's not restricted to that.
> There may be even more use cases in future Elisp. And that is good.
I disagree with that last sentiment.
> > , has a definite precise function (disregarding its use in pcase for
> > now). Have you any specific suggestions on how to improve my wording of
> > its doc string?
> > > + ((get function 'reader-macro)
> > > + "a reader macro")
> > > We don't have reader macros in Emacs.
> > The reader performs macro-like actions. What is the correct alternative
> > term for what the reader does with ', `, ,, #', etc.?
> The manual seems to prefer the term "reader syntax". I think we could
> use this term.
No, that won't do. , has semantics as well as syntax.
> You can say that the reader expands `X to (` X), and that "`" is a
> defined macro. You can say that the reader expands ,X to (\, X), and
> that a frequent usage is in backquote forms.
That is describing the internal mechanism of , rather than the result.
It would be utterly useless and confusing for the people who need to
consult the meaning of , and ,@, namely new Elisp hackers.
> We should not say that "," "does" something, because this makes no
> sense and leads a person reading this to false assumptions.
It will enable them to understand code they are reading, and to start
writing ` constructs themselves.
Here is the latest proposed version of the doc string for ,,
incorporating some of the comments you've made.
"`,' causes the next form to be evaluated and inserted.
It occurs in `\\=`' constructs.
For example:
b => (ba bb bc) ; assume b has this value
\\=`(a ,b c) => (a (ba bb bc) c) ; insert the value of b
See also `\\=`' and `,@'.
(Note that ``' constructs (including `,'s) sometimes have different
semantics. This occurs, for example, with the macro `pcase' and other
macros with similar names.")
> Regards,
> Michael.
--
Alan Mackenzie (Nuremberg, Germany).
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