Re: Declaring Lisp function types

[Tomas Hlavaty <tom@logand.com>]

On Fri 15 Mar 2024 at 20:38, Eli Zaretskii <eliz@gnu.org> wrote:
>> all cases are grep-unfriendly
>
> What do you mean by that?
> Please tell more: what would you like to grep for and why?

Because good names help working with code bases.

(Almost like being given a direct link (like URL) instead of
instructions to go that page, click foo then click bar then search for
baz...)

In the context of "Declaring Lisp function types", grep gives too many
false positives when searching for the symbol 'function' or 'type'.
Some people suggested more complex regexp to match the declare sexp but
that breaks down pretty quickly.  A better, more unique symbol name
instead of 'function' or 'type' would be much easier to find.

Looking at

   (info "(elisp)Declare Form")

there are examples of good and bad names for declare spec properties.

Good; try searching for these:

   (rg "~/mr/emacs" "advertised-calling-convention")
   (rg "~/mr/emacs" "side-effect-free")
   (rg "~/mr/emacs" "no-font-lock-keyword")

Bad; try searching for these:

   (rg "~/mr/emacs" "pure")
   (rg "~/mr/emacs" "speed")

Interestingly, the info page above says that the speed property
"specifies the value of native-comp-speed in effect for native
compilation of this function".  If the property was called
native-comp-speed, it

- would be more correct,

- it would be easier to search for,

- I could even M-. on the symbol and jump automatically to the
  definition and see the documentation with minimal effort and
  distraction,

- I would even see helpful text as an eldoc message and see possible
  values just by placing cursor on the symbol name.

etc

Try this in elisp-mode:

(defun foo (a b)
  (declare (integer a b)
           (function a b)
           (type a b)
           (speed -1)
           (native-comp-speed -1)
           (advertised-calling-convention)
           (side-effect-free)
           (no-font-lock-keyword)
   )
  (+ a b 42))

Place your cursor on integer, function, type, speed, native-comp-speed,
advertised-calling-convention, side-effect-free, no-font-lock-keyword
and watch the difference in eldoc message, try M-. or rgrep emacs
codebase etc.

The symbol "function" already has specific meaning.  Using it as a
declare spec property name is just bad.  For example, eldoc or M-. shows
something unrelated to "Declaring Lisp function types".

The other symbols do not show or lead to anything, those are dead-ends
magically doing something to the codebase without me being able to use
usual tools to understand and navigate them.  This gap could be bridged
by defining dummy function/macro that would show the right arguments and
documentation and also display useful eldoc message.  For example like
this:

(defun side-effect-free (val)
  "If VAL is non-‘nil’, this function is free of side effects, so
   the byte compiler can ignore calls whose value is ignored.
   This is the same as the ‘side-effect-free’ property of the
   function’s symbol, *note Standard Properties::."
  (error "side-effect-free is not meant to be called"))

But maybe it would be possible to structure the code in such a way that
this dummy would not be needed, like the case of the symbol
"native-comp-speed" (if it was called native-comp-speed instead of
speed).

Searching for no-font-lock-keyword does not show any declare definition.
Is it not used for anything?

Elisp has great tools for navigating and understanding code, why work
against them?  Lisp symbols are a good concept supported by various
tools; use them, give them good names and all those tools will work as
usual and be very helpful in many contexts.