Void Variables

A variable which has no value is said to be void, attempting to reference the value of such a symbol will result in an error. It is possible for the most recent binding of a variable to be void even though the inactive bindings may have values.

Function: boundp variable

Returns t if the symbol variable has a value, nil if its value is void.

Function: makunbound variable

This function makes the current binding of the symbol variable be void, then returns variable.

(setq foo 42)
    => 42
foo
    => 42
(boundp 'foo)
    => t
(makunbound 'foo)
    => foo
(boundp 'foo)
    => nil
foo
    error--> Value as variable is void: foo

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