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bug#25461: [Patch]: Missing doc strings for "," and ",@".

From: Alan Mackenzie
Subject: bug#25461: [Patch]: Missing doc strings for "," and ",@".
Date: Thu, 19 Jan 2017 17:58:01 +0000
User-agent: Mutt/1.5.24 (2015-08-30)

Hello, Michael.

On Thu, Jan 19, 2017 at 03:23:32AM +0100, Michael Heerdegen wrote:
> Hi Alan,

> > diff --git a/lisp/emacs-lisp/backquote.el b/lisp/emacs-lisp/backquote.el
> > index 94c561c..86ca010 100644
> > --- a/lisp/emacs-lisp/backquote.el
> > +++ b/lisp/emacs-lisp/backquote.el
> > @@ -247,4 +247,33 @@ backquote-listify
> >     tail))
> >       (t (cons 'list heads)))))

> > +
> > +;; 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?

> 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.  

> The bindings of variables depend on context.  The semantics of functions
> are fluent (advice, local functions).  And also the "meaning" of the
> sexps created by ` and , depend on the context - in Lisp, the "meaning"
> of all sexps depends on context.  (foo bar) can be a function call or a
> part of a let variable binding or a list or a pcase pattern.  It's not
> different for sexps involving `.

` has a specific meaning, and has had a high quality doc string for
ever.  , likewise has a specific meaning, but doesn't yet have a doc

In Emacs Lisp, functions generally have well defined context-free
semantics, though there are exceptions.

> 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.

, 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.?

> If we had them, we could implement ', ` etc. as reader macros.  But
> using this term in H-f is confusing, because it is not used in the
> manual.

What would be a less confusing alternative?

> Regards,

> Michael

Alan Mackenzie (Nuremberg, Germany).

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