bug#58602: 29.0.50; Please document (:documentation FORM) spec for closures

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Michael Heerdegen [2022-10-18 02:46:00] wrote:
> I think we need to document this new feature described in etc/NEWS.25:
>
> | ** Lexical closures can use '(:documentation FORM)' to build their docstring.
> | It should be placed right where the docstring would be, and FORM is then
> | evaluated (and should return a string) when the closure is built.
>
> at least in the ELisp reference manual (dunno, maybe also in the
> docstring of `lambda' and `defun'?).

[ Our emails crossed: I just sent the following to emacs-devel, but it's
  probably better to discuss it here.  ]

Someone pointed out that I forgot to document the `:documentation`
thingy for functions.

I came up with the text below, but I find it too terse.  I'm wondering
whether maybe it should be expanded to a subsubsection maybe, where
I could more easily put some examples.

Tho, maybe there's a way to make it understandable without an example.
I don't know.  I need help.


        Stefan


diff --git a/doc/lispref/functions.texi b/doc/lispref/functions.texi
index 8b858e0aa01..f85e0498851 100644
--- a/doc/lispref/functions.texi
+++ b/doc/lispref/functions.texi
@@ -533,6 +533,20 @@ Function Documentation
 compiler emit a warning message when it compiles Lisp programs which
 use the deprecated calling convention.
 
+@cindex Computed documentation string
+@kindex{:documentation}
+Documentation strings are usually completely static, but occasionally
+it can be necessary to generate them dynamically.  In some cases this
+can be done by writing a macro which will generate at compile time the
+code of the function, including the desired documentation string.
+But you can also generate the docstring at run-time when the function
+is defined by writing @code{(:documentation @var{form})} instead of
+the documentation string@footnote{This only works in code using
+@code{lexical-binding}.}.  Furthermore, you can also compute the
+documentation string on the fly when it is requested by setting
+the @code{function-documentation} property of the function's symbol to
+a Lisp form that should evaluate to a string.
+
 @node Function Names
 @section Naming a Function
 @cindex function definition