implementing a reader macro

implementing a reader macro

[Ag Ibragimov]

How difficult is it to implement a new reader macro for Elisp?
The one I particularly wish to be ported from Clojure is the "discard
macro" - `#_`. What it does is that it ignores the next form completely.

e.g., 
#+begin_src clojure
    #_(list 1 2 3)

    (list 1 #_2 3)
#+end_src

And it can be stacked to omit multiple forms:

#+begin_src clojure
  (foo 1
     #_#_(bar)       ; both, (bar) and 
     (zap "zop"))    ;       (zap "zop") will be ignored
#+end_src

It is a very nice way of (usually temporarily) commenting out sexps
without disrupting the structure too much. Of course, we can employ nice
tricks such as one described by Malabarba:
http://endlessparentheses.com/a-comment-or-uncomment-sexp-command.html
But wouldn't it be nice to have something even better?

Re: implementing a reader macro

[Stefan Monnier]

> How difficult is it to implement a new reader macro for Elisp?
> The one I particularly wish to be ported from Clojure is the "discard
> macro" - `#_`. What it does is that it ignores the next form completely.

Does `clojure-mode` support that correctly?
Scheme has a similar construct and I found it difficult to make
`scheme-mode` handle it correctly (so: it doesn't).

I don't think we want to add to ELisp a feature that our support tools
can't handle well.

> It is a very nice way of (usually temporarily) commenting out sexps
> without disrupting the structure too much. Of course, we can employ nice
> tricks such as one described by Malabarba:
> http://endlessparentheses.com/a-comment-or-uncomment-sexp-command.html
> But wouldn't it be nice to have something even better?

I don't think this feature pays for itself, personally.
There are plenty of alternative solutions that don't require changing
the syntax of the language.


        Stefan

Re: implementing a reader macro

[Jose A. Ortega Ruiz]

On Fri, Jul 22 2022, Stefan Monnier wrote:

>> How difficult is it to implement a new reader macro for Elisp?
>> The one I particularly wish to be ported from Clojure is the "discard
>> macro" - `#_`. What it does is that it ignores the next form completely.
>
> Does `clojure-mode` support that correctly?

in my experience (i edit clojure daily), it does (FSVO "correctly",
defined by my usage patterns, of course).  at the language level, i find
this feature extremely useful, and i'm a very happy user inside
clojure-mode.

cheers,
jao
-- 
Poetry is the art of creating imaginary gardens with real
toads. -Marianne Moore, poet (1887-1972)

Re: implementing a reader macro

[Ag Ibragimov]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

>> How difficult is it to implement a new reader macro for Elisp?
>> The one I particularly wish to be ported from Clojure is the "discard
>> macro" - `#_`. What it does is that it ignores the next form completely.
>
> Does `clojure-mode` support that correctly?

AFAIK it works very well in clojure-mode.

> I don't think this feature pays for itself, personally.

Well, to be honest, initially, it does look like a gimmick that doesn't add much
value. But once you get used to having it, it really feels nice.

> There are plenty of alternative solutions that don't require changing
> the syntax of the language.

Can you show some examples? I think most of them still require you to
change the structure - you either have to wrap the form or prepend every line, etc.

Re: implementing a reader macro

[Stefan Monnier]

>>> How difficult is it to implement a new reader macro for Elisp?
>>> The one I particularly wish to be ported from Clojure is the "discard
>>> macro" - `#_`. What it does is that it ignores the next form completely.
>> Does `clojure-mode` support that correctly?
> AFAIK it works very well in clojure-mode.

That's actually not quite an answer to my question.

AFAICT clojure-mode just "manually" highlights those comments with
`font-lock-comment-face`, so sexp navigation will not handle them
properly w.r.t `parse-sexp-ignore-comments`, and `forward-comment` won't
skip them either.

Also this relies on the `font-lock-multiline` variable, which is a kind
of "best effort", so if the beginning of the comment is never displayed
(because it's never between `window-start` and `window-end`) the rest of
the comment may end up displayed without the `font-lock-comment-face`.

> Can you show some examples? I think most of them still require you to
> change the structure - you either have to wrap the form or prepend
> every line, etc.

My go-to key sequence for that is:

    C-M-SPC M-;

but in most circumstances there are other approaches that depend on the
specific situation.  E.g. often just adding a ' does the trick (tho
I usually prefer `M-;`).


        Stefan

Re: implementing a reader macro

[Lars Ingebrigtsen]

Ag Ibragimov <agzam.ibragimov@gmail.com> writes:

> How difficult is it to implement a new reader macro for Elisp?
> The one I particularly wish to be ported from Clojure is the "discard
> macro" - `#_`. What it does is that it ignores the next form completely.

This has already been implemented more than a couple of times (but using
the Common Lisp versions of this, i.e.,

  #+nil(this is commented out)

and

  #| this too |#

Adjusting the reader to allow these forms is pretty trivial, but the
reason this hasn't been added to Emacs is that nobody has stepped up to
adjust the tooling (font locking, sexp movement, etc), too.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

Re: implementing a reader macro

[Ag Ibragimov]

Lars Ingebrigtsen <larsi@gnus.org> writes:

> Adjusting the reader to allow these forms is pretty trivial, but the
> reason this hasn't been added to Emacs is that nobody has stepped up to
> adjust the tooling (font locking, sexp movement, etc), too.

Since we have those implemented in clojure-mode, maybe it can be
adapted for elisp? Reader macro itself of course is implemented in clojure
codebase, but font-locking, et al, [I think] is all in clojure-mode.el.