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11.3 Local Variables

Global variables have values that last until explicitly superseded with new values. Sometimes it is useful to give a variable a local value—a value that takes effect only within a certain part of a Lisp program. When a variable has a local value, we say that it is locally bound to that value, and that it is a local variable.

For example, when a function is called, its argument variables receive local values, which are the actual arguments supplied to the function call; these local bindings take effect within the body of the function. To take another example, the let special form explicitly establishes local bindings for specific variables, which take effect within the body of the let form.

We also speak of the global binding, which is where (conceptually) the global value is kept.

Establishing a local binding saves away the variable's previous value (or lack of one). We say that the previous value is shadowed. Both global and local values may be shadowed. If a local binding is in effect, using setq on the local variable stores the specified value in the local binding. When that local binding is no longer in effect, the previously shadowed value (or lack of one) comes back.

A variable can have more than one local binding at a time (e.g., if there are nested let forms that bind the variable). The current binding is the local binding that is actually in effect. It determines the value returned by evaluating the variable symbol, and it is the binding acted on by setq.

For most purposes, you can think of the current binding as the innermost local binding, or the global binding if there is no local binding. To be more precise, a rule called the scoping rule determines where in a program a local binding takes effect. The default scoping rule in Emacs Lisp is called dynamic scoping, which simply states that the current binding at any given point in the execution of a program is the most recently-created binding for that variable that still exists. For details about dynamic scoping, and an alternative scoping rule called lexical scoping, See Variable Scoping.

The special forms let and let* exist to create local bindings:

— Special Form: let (bindings...) forms...

This special form sets up local bindings for a certain set of variables, as specified by bindings, and then evaluates all of the forms in textual order. Its return value is the value of the last form in forms.

Each of the bindings is either (i) a symbol, in which case that symbol is locally bound to nil; or (ii) a list of the form (symbol value-form), in which case symbol is locally bound to the result of evaluating value-form. If value-form is omitted, nil is used.

All of the value-forms in bindings are evaluated in the order they appear and before binding any of the symbols to them. Here is an example of this: z is bound to the old value of y, which is 2, not the new value of y, which is 1.

          (setq y 2)
               ⇒ 2
          
          (let ((y 1)
                (z y))
            (list y z))
               ⇒ (1 2)

On the other hand, the order of bindings is unspecified: in the following example, either 1 or 2 might be printed.

          (let ((x 1)
                (x 2))
            (print x))

Therefore, avoid binding a variable more than once in a single let form.

— Special Form: let* (bindings...) forms...

This special form is like let, but it binds each variable right after computing its local value, before computing the local value for the next variable. Therefore, an expression in bindings can refer to the preceding symbols bound in this let* form. Compare the following example with the example above for let.

          (setq y 2)
               ⇒ 2
          
          (let* ((y 1)
                 (z y))    ; Use the just-established value of y.
            (list y z))
               ⇒ (1 1)

Here is a complete list of the other facilities that create local bindings:

Variables can also have buffer-local bindings (see Buffer-Local Variables); a few variables have terminal-local bindings (see Multiple Terminals). These kinds of bindings work somewhat like ordinary local bindings, but they are localized depending on where you are in Emacs.

— User Option: max-specpdl-size

This variable defines the limit on the total number of local variable bindings and unwind-protect cleanups (see Cleaning Up from Nonlocal Exits) that are allowed before Emacs signals an error (with data "Variable binding depth exceeds max-specpdl-size").

This limit, with the associated error when it is exceeded, is one way that Lisp avoids infinite recursion on an ill-defined function. max-lisp-eval-depth provides another limit on depth of nesting. See Eval.

The default value is 1300. Entry to the Lisp debugger increases the value, if there is little room left, to make sure the debugger itself has room to execute.