GNU bug report logs - #76132
Clojure-style auto-gensyms for macros

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Tassilo Horn <tsdh <at> gnu.org>
Subject: bug#76132: closed (Re: bug#76132: Clojure-style auto-gensyms for
 macros)
Date: Sun, 23 Feb 2025 09:12:02 +0000

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Your bug report

#76132: Clojure-style auto-gensyms for macros

which was filed against the emacs package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to 76132 <at> debbugs.gnu.org.

-- 
76132: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=76132
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

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From: Tassilo Horn <tsdh <at> gnu.org>
To: Stefan Kangas <stefankangas <at> gmail.com>
Cc: 76132-done <at> debbugs.gnu.org, Richard Stallman <rms <at> gnu.org>,
 Stefan Monnier <monnier <at> iro.umontreal.ca>
Subject: Re: bug#76132: Clojure-style auto-gensyms for macros
Date: Sun, 23 Feb 2025 10:11:08 +0100

Stefan Kangas <stefankangas <at> gmail.com> writes:

> So what's the conclusion here?  

Macros (in the presence of eager macro expansion) are hard. :-)

> Should this bug be closed, or is there more left to discuss?

No, I'm closing it now.

Bye,
Tassilo

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From: Tassilo Horn <tsdh <at> gnu.org>
To: bug-gnu-emacs <at> gnu.org
Subject: Clojure-style auto-gensyms for macros
Date: Fri, 07 Feb 2025 22:12:14 +0100

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Tags: patch

Hi all,

in a recent bug report the topic macro hygiene came up, i.e., that a
macro which introduces local bindings in its expansion better uses
uninterned symbols for those in order not to clash with code passed as
macro arguments which are spliced into the expansion.

Clojure has a very convenient feature to make that easy.  While you can
write such macros traditionally like

(defmacro foo [x y]
  (let [xv (gensym "x")
        yv (gensym "y")]
    `(let [,xv ,x
           ,yv ,y]
       (do-stuff (* ,xv ,xv) (* ,yv ,yv)))))

you can also write much more concise and convenient

(defmacro foo [x y]
  `(let [xv# ,x
         yv# ,y]
     (do-stuff (* xv# xv#) (* yv# yv#))))

where each symbol ending in # will be replaced by a unique gensymed
symbol (per name).  The expansion of the two macros is the same.

Would there be interest in adding something like that to Elisp?

I've attached a proof-of-concept implementation where the feature is
provided by a macro with-uninterned-symbols [1] which you simply wrap
around your backquoted form.

[1] In Clojure, it's a feature of the reader triggered by backquote.

In GNU Emacs 31.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version
 3.24.48, cairo version 1.18.2) of 2025-02-07 built on thinkpad-t440p
Repository revision: 1751739152149608d28853782ce53b0b9a749bb2
Repository branch: master
System Description: Arch Linux

Configured using:
 'configure --without-native-compilation --with-modules --with-pgtk'

[clojure-macro.el (text/patch, attachment)]

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