From: Sean Whitton <spwhitton@spwhitton.name>
To: emacs-devel@gnu.org
Subject: [PATCH] Adding with-gensyms and once-only to subr-x
Date: Mon, 11 Apr 2022 09:26:36 -0700 [thread overview]
Message-ID: <87a6crr3gj.fsf@athena.silentflame.com> (raw)
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Hello,
Lately I have been finding myself wanting with-gensyms and once-only,
two classic macro-writing macros, available in core Elisp. (There is
already org-with-gensyms, but it's not in the least bit Org-specific.)
Here is my implementation for review.
--
Sean Whitton
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From 89e86db1ccb97ce3f91d5b4beb5d7b461311d196 Mon Sep 17 00:00:00 2001
From: Sean Whitton <spwhitton@spwhitton.name>
Date: Mon, 11 Apr 2022 09:20:35 -0700
Subject: [PATCH] * lisp/emacs-lisp/subr-x.el (with-gensyms, once-only): New
macros.
---
lisp/emacs-lisp/subr-x.el | 43 +++++++++++++++++++++++++++++++++++++++
1 file changed, 43 insertions(+)
diff --git a/lisp/emacs-lisp/subr-x.el b/lisp/emacs-lisp/subr-x.el
index abf85ab6c6..c74bf7f5f0 100644
--- a/lisp/emacs-lisp/subr-x.el
+++ b/lisp/emacs-lisp/subr-x.el
@@ -526,6 +526,49 @@ read-process-name
(error "No process selected"))
process)))
+(defmacro with-gensyms (names &rest body)
+ "Bind each of NAMES to an uninterned symbol and evaluate BODY."
+ (declare (debug (sexp body)) (indent 1))
+ `(let ,(cl-loop for name in names collect
+ `(,name (gensym (symbol-name ',name))))
+ ,@body))
+
+(defmacro once-only (names &rest body)
+ "Generate code to evaluate each of NAMES just once in BODY.
+
+This macro helps with writing other macros. Each of names is
+either (NAME FORM) or NAME, which latter means (NAME NAME).
+During macroexpansion, each NAME is bound to an uninterned
+symbol. The expansion evaluates each FORM and binds it to the
+corresponding uninterned symbol.
+
+For example, consider this macro:
+
+ (defmacro my-cons (x)
+ (once-only (x)
+ `(cons ,x ,x)))
+
+Consider the call (my-cons (pop y)). This will expand to
+something like this:
+
+ (let ((g1 (pop y)))
+ (cons g1 g1))
+
+This ensures that the pop is performed only once, as wanted."
+ (declare (debug (sexp body)) (indent 1))
+ (setq names (mapcar #'ensure-list names))
+ (let ((our-gensyms (cl-loop for name in names collect (gensym))))
+ ;; During macroexpansion, obtain a gensym for each NAME.
+ `(let ,(cl-loop for sym in our-gensyms collect `(,sym (gensym)))
+ ;; Evaluate each FORM and bind to the corresponding gensym.
+ `(let (,,@(cl-loop for name in names and gensym in our-gensyms
+ for to-eval = (or (cadr name) (car name))
+ collect ``(,,gensym ,,to-eval)))
+ ;; During macroexpansion, bind each NAME to its gensym.
+ ,(let ,(cl-loop for name in names and gensym in our-gensyms
+ collect `(,(car name) ,gensym))
+ ,@body)))))
+
(provide 'subr-x)
;;; subr-x.el ends here
--
2.30.2
next reply other threads:[~2022-04-11 16:26 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-04-11 16:26 Sean Whitton [this message]
2022-04-11 16:48 ` [PATCH] Adding with-gensyms and once-only to subr-x Stefan Monnier
2022-04-11 17:01 ` Sean Whitton
2022-04-11 17:26 ` Stefan Monnier
2022-04-11 18:41 ` Sean Whitton
2022-04-11 19:11 ` Stefan Monnier
2022-04-11 20:25 ` Sean Whitton
2022-04-11 21:11 ` Stefan Monnier
2022-04-11 23:05 ` Sean Whitton
2022-04-11 23:15 ` Sean Whitton
2022-04-12 0:06 ` Sean Whitton
2022-04-12 3:08 ` Stefan Monnier
2022-04-12 6:02 ` Sean Whitton
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