Dynamic Variables (ANSI and GNU Common Lisp Document)

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#### 3.1.2.4 Dynamic Variables

A variable is a dynamic variable if one of the following conditions hold:

`*`

  It is locally declared or globally proclaimed special.

`*`

  It occurs textually within a form that creates a dynamic binding for a variable of the same name, and the binding is not shadowed_2 by a form that creates a lexical binding of the same variable name.

A dynamic variable can be referenced at any time in any program; there is no textual limitation on references to dynamic variables. At any given time, all dynamic variables with a given name refer to exactly one binding, either in the dynamic environment or in the global environment.

The value part of the binding for a dynamic variable might be empty; in this case, the dynamic variable is said to have no value, or to be unbound. A dynamic variable can be made unbound by using makunbound.

The effect of binding a dynamic variable is to create a new binding to which all references to that dynamic variable in any program refer for the duration of the evaluation of the form that creates the dynamic binding.

A dynamic variable can be referenced outside the dynamic extent of a form that binds it. Such a variable is sometimes called a “global variable” but is still in all respects just a dynamic variable whose binding happens to exist in the global environment rather than in some dynamic environment.

A dynamic variable is unbound unless and until explicitly assigned a value, except for those variables whose initial value is defined in this specification or by an implementation.

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