Previous: Differences Between Lexical and Dynamic Binding, Up: How let Binds Variables [Contents][Index]
In some cases, both lexical and dynamic binding behave identically. However, in other cases, they can change the meaning of your program. For example, see what happens in this code under lexical binding:
;;; -*- lexical-binding: t -*- (setq x 0) (defun getx () x) (setq x 1) (let ((x 2)) (getx)) ⇒ 1
Here, the result of (getx) is 1. Under lexical binding,
getx doesn’t see the value from our let expression.
That’s because the body of getx is outside of the body of our
let expression. Since getx is defined at the top,
global level of our code (i.e. not inside the body of any let
expression), it looks for and finds x at the global level as
well. When executing getx, the current global value of
x is 1, so that’s what getx returns.
If we use dynamic binding instead, the behavior is different:
;;; -*- lexical-binding: t -*- (defvar x) ;; Use dynamic binding for 'x'. (setq x 0) (defun getx () x) (setq x 1) (let ((x 2)) (getx)) ⇒ 2
Now, the result of (getx) is 2! That’s because under
dynamic binding, when executing getx, the current binding for
x at the top of our stack is the one from our let
binding. This time, getx doesn’t see the global value for
x, since its binding is below the one from our let
expression in the stack of bindings.
(The defvar declaration above is said to make the variable
“special”, which causes it to obey the dynamic binding rules instead of
the default binding rules. See Initializing a Variable with defvar.)
Previous: Differences Between Lexical and Dynamic Binding, Up: How let Binds Variables [Contents][Index]