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If you have never given a symbol any value as a global variable, we
say that that symbol’s global value is void. In other words, the
symbol’s value cell does not have any Lisp object in it. If you try to
evaluate the symbol, you get a void-variable
error rather than
a value.
Note that a value of nil
is not the same as void. The symbol
nil
is a Lisp object and can be the value of a variable just as any
other object can be; but it is a value. A void variable does not
have any value.
After you have given a variable a value, you can make it void once more
using makunbound
.
This function makes the current binding of symbol void.
Subsequent attempts to use this symbol’s value as a variable will signal
the error void-variable
, unless or until you set it again.
makunbound
returns symbol.
(makunbound 'x) ; Make the global value
; of x
void.
⇒ x
x error→ Symbol's value as variable is void: x
If symbol is locally bound, makunbound
affects the most
local existing binding. This is the only way a symbol can have a void
local binding, since all the constructs that create local bindings
create them with values. In this case, the voidness lasts at most as
long as the binding does; when the binding is removed due to exit from
the construct that made it, the previous or global binding is reexposed
as usual, and the variable is no longer void unless the newly reexposed
binding was void all along.
(setq x 1) ; Put a value in the global binding. ⇒ 1 (let ((x 2)) ; Locally bind it. (makunbound 'x) ; Void the local binding. x) error→ Symbol's value as variable is void: x
x ; The global binding is unchanged. ⇒ 1 (let ((x 2)) ; Locally bind it. (let ((x 3)) ; And again. (makunbound 'x) ; Void the innermost-local binding. x)) ; And refer: it’s void. error→ Symbol's value as variable is void: x
(let ((x 2))
(let ((x 3))
(makunbound 'x)) ; Void inner binding, then remove it.
x) ; Now outer let
binding is visible.
⇒ 2
A variable that has been made void with makunbound
is
indistinguishable from one that has never received a value and has
always been void.
You can use the function boundp
to test whether a variable is
currently void.
boundp
returns t
if variable (a symbol) is not void;
more precisely, if its current binding is not void. It returns
nil
otherwise.
(boundp 'abracadabra) ; Starts out void.
⇒ nil
(let ((abracadabra 5)) ; Locally bind it.
(boundp 'abracadabra))
⇒ t
(boundp 'abracadabra) ; Still globally void.
⇒ nil
(setq abracadabra 5) ; Make it globally nonvoid.
⇒ 5
(boundp 'abracadabra) ⇒ t
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