From 754bbf7a25a8dda49b5d08ef0d0443bbf5af0e36 Mon Sep 17 00:00:00 2001 From: Craig Jennings Date: Sun, 7 Apr 2024 13:41:34 -0500 Subject: new repository --- devdocs/elisp/dynamic-binding.html | 37 +++++++++++++++++++++++++++++++++++++ 1 file changed, 37 insertions(+) create mode 100644 devdocs/elisp/dynamic-binding.html (limited to 'devdocs/elisp/dynamic-binding.html') diff --git a/devdocs/elisp/dynamic-binding.html b/devdocs/elisp/dynamic-binding.html new file mode 100644 index 00000000..a34a0b35 --- /dev/null +++ b/devdocs/elisp/dynamic-binding.html @@ -0,0 +1,37 @@ +

Dynamic Binding

By default, the local variable bindings made by Emacs are dynamic bindings. When a variable is dynamically bound, its current binding at any point in the execution of the Lisp program is simply the most recently-created dynamic local binding for that symbol, or the global binding if there is no such local binding.

Dynamic bindings have dynamic scope and extent, as shown by the following example: 

(defvar x -99)  ; x receives an initial value of -99.
+
+(defun getx ()
+  x)            ; x is used free in this function.
+
+(let ((x 1))    ; x is dynamically bound.
+  (getx))
+     ⇒ 1
+
+;; After the let form finishes, x reverts to its
+;; previous value, which is -99.
+
+(getx)
+     ⇒ -99
+
+

The function getx refers to x. This is a free reference, in the sense that there is no binding for x within that defun construct itself. When we call getx from within a let form in which x is (dynamically) bound, it retrieves the local value (i.e., 1). But when we call getx outside the let form, it retrieves the global value (i.e., -99).

Here is another example, which illustrates setting a dynamically bound variable using setq: 

(defvar x -99)      ; x receives an initial value of -99.
+
+(defun addx ()
+  (setq x (1+ x)))  ; Add 1 to x and return its new value.
+
+(let ((x 1))
+  (addx)
+  (addx))
+     ⇒ 3           ; The two addx calls add to x twice.
+
+;; After the let form finishes, x reverts to its
+;; previous value, which is -99.
+
+(addx)
+     ⇒ -98
+
+

Dynamic binding is implemented in Emacs Lisp in a simple way. Each symbol has a value cell, which specifies its current dynamic value (or absence of value). See Symbol Components. When a symbol is given a dynamic local binding, Emacs records the contents of the value cell (or absence thereof) in a stack, and stores the new local value in the value cell. When the binding construct finishes executing, Emacs pops the old value off the stack, and puts it in the value cell.

Note that when code using Dynamic Binding is native compiled the native compiler will not perform any Lisp specific optimization.

+

+ Copyright © 1990-1996, 1998-2022 Free Software Foundation, Inc.
Licensed under the GNU GPL license.
+ https://www.gnu.org/software/emacs/manual/html_node/elisp/Dynamic-Binding.html +

+
-- cgit v1.2.3