Global variables have values that last until explicitly superseded with new values. Sometimes it is useful to create variable values that exist temporarily--only until a certain part of the program finishes. These values are called local, and the variables so used are called local variables.
For example, when a function is called, its argument variables receive
new local values that last until the function exits. The let
special form explicitly establishes new local values for specified
variables; these last until exit from the let form.
Establishing a local value saves away the previous value (or lack of one) of the variable. When the life span of the local value is over, the previous value is restored. In the mean time, we say that the previous value is shadowed and not visible. Both global and local values may be shadowed (see section Scope).
If you set a variable (such as with setq) while it is local,
this replaces the local value; it does not alter the global value, or
previous local values, that are shadowed. To model this behavior, we
speak of a local binding of the variable as well as a local value.
The local binding is a conceptual place that holds a local value.
Entry to a function, or a special form such as let, creates the
local binding; exit from the function or from the let removes the
local binding. As long as the local binding lasts, the variable's value
is stored within it. Use of setq or set while there is a
local binding stores a different value into the local binding; it does
not create a new binding.
We also speak of the global binding, which is where (conceptually) the global value is kept.
A variable can have more than one local binding at a time (for
example, if there are nested let forms that bind it). In such a
case, the most recently created local binding that still exists is the
current binding of the variable. (This rule is called
dynamic scoping; see section Scoping Rules for Variable Bindings.) If there are no
local bindings, the variable's global binding is its current binding.
We sometimes call the current binding the most-local existing
binding, for emphasis. Ordinary evaluation of a symbol always returns
the value of its current binding.
The special forms let and let* exist to create
local bindings.
let-form
returns the value of the last form in forms.
Each of the bindings is either (i) a symbol, in which case
that symbol is bound to nil; or (ii) a list of the form
(symbol value-form), in which case symbol is
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)
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
reasonably 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:
condition-case (see section Errors).
Variables can also have buffer-local bindings (see section Buffer-Local Variables) and frame-local bindings (see section Frame-Local Variables); a few variables have terminal-local bindings (see section Multiple Displays). These kinds of bindings work somewhat like ordinary local bindings, but they are localized depending on "where" you are in Emacs, rather than localized in time.
unwind-protect cleanups (see section Nonlocal Exits)
that are allowed before signaling 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 section Eval.
The default value is 600. Entry to the Lisp debugger increases the value, if there is little room left, to make sure the debugger itself has room to execute.
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