Add dlet to compat.el

lisp/compat.el

@@ -13,5 +13,22 @@
 (dolist (file (directory-files (locate-user-emacs-file "lisp/compat") :full "\\.el\\'"))
   (load file :noerror))
 
+;; Other stuff...
+
+(unless (fboundp 'dlet)
+  (defmacro dlet (binders &rest body)
+    "Like `let' but using dynamic scoping."
+    (declare (indent 1) (debug let))
+    ;; (defvar FOO) only affects the current scope, but in order for
+    ;; this not to affect code after the main `let' we need to create a new scope,
+    ;; which is what the surrounding `let' is for.
+    ;; FIXME: (let () ...) currently doesn't actually create a new scope,
+    ;; which is why we use (let (_) ...).
+    `(let (_)
+       ,@(mapcar (lambda (binder)
+                   `(defvar ,(if (consp binder) (car binder) binder)))
+                 binders)
+       (let ,binders ,@body))))
+
 (provide 'compat)
 ;;; compat.el ends here