Skip to main content

makunbound

makunbound Function

Syntax:

makunbound symbol → symbol

Arguments and Values:

symbol—a symbol

Description:

Makes the symbol be unbound, regardless of whether it was previously bound.

Examples:

(setf (symbol-value ’a) 1) 
(boundp ’a) → true 
a → 1 
(makunbound ’a) → A 
(boundp ’a) → false

Side Effects:

The value cell of symbol is modified.

Exceptional Situations:

Should signal an error of type type-error if symbol is not a symbol.

See Also:

boundp, fmakunbound

Expanded Reference: makunbound​

Basic usage​

makunbound removes the value from a symbol's value cell, making it unbound. It returns the symbol.

(setf (symbol-value 'temp) 42)
(boundp 'temp)
=> T

(makunbound 'temp)
=> TEMP
(boundp 'temp)
=> NIL

Accessing an unbound symbol signals an error​

After makunbound, attempting to read the symbol's value signals an unbound-variable error.

(setf (symbol-value 'doomed) 99)
(makunbound 'doomed)

(handler-case (symbol-value 'doomed)
  (unbound-variable () :caught))
=> :CAUGHT

makunbound on an already unbound symbol​

Calling makunbound on a symbol that is already unbound is harmless.

(let ((sym (gensym)))
  (boundp sym)
  (makunbound sym)
  (boundp sym))
=> NIL

makunbound does not affect lexical bindings​

makunbound only affects the global dynamic binding, not any lexical bindings in scope.

(setf (symbol-value 'outer) :global)

(let ((outer :lexical))
  (makunbound 'outer)
  outer)
=> :LEXICAL

Round-trip with boundp​

makunbound and setf/symbol-value form a complementary pair for managing symbol bindings.

(let ((sym (gensym)))
  (values
    (boundp sym)
    (progn (setf (symbol-value sym) 1)
           (boundp sym))
    (progn (makunbound sym)
           (boundp sym))))
=> NIL
=> T
=> NIL