[PATCH] Adding with-gensyms and once-only to subr-x

[Sean Whitton <spwhitton@spwhitton.name>]

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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