3.1.1 Introduction to Environments

A binding is an association between a name and that which the name denotes. Bindings are established in a lexical environment or a dynamic environment by particular special operators.

An environment is a set of bindings and other information used during evaluation (e.g., to associate meanings with names).

Bindings in an environment are partitioned into namespaces. A single name can simultaneously have more than one associated binding per environment, but can have only one associated binding per namespace.

3.1.1.1 The Global Environment

The global environment is that part of an environment that contains bindings with both indefinite scope and indefinite extent. The global environment contains, among other things, the following:

• bindings of dynamic variables and constant variables.
• bindings of functions, macros, and special operators.
• bindings of compiler macros.
• bindings of type and class names
• information about proclamations.

3.1.1.2 Dynamic Environments

A dynamic environment for evaluation is that part of an environment that contains bindings whose duration is bounded by points of establishment and disestablishment within the execution of the form that established the binding. A dynamic environment contains, among other things, the following:

• bindings for dynamic variables.
• information about active catch tags.
• information about exit points established by unwind-protect.
• information about active handlers and restarts.

The dynamic environment that is active at any given point in the execution of a program is referred to by definite reference as “the current dynamic environment,” or sometimes as just “the dynamic environment.”

Within a given namespace, a name is said to be bound in a dynamic environment if there is a binding associated with its name in the dynamic environment or, if not, there is a binding associated with its name in the global environment.

3.1.1.3 Lexical Environments

A lexical environment for evaluation at some position in a program is that part of the environment that contains information having lexical scope within the forms containing that position. A lexical environment contains, among other things, the following:

• bindings of lexical variables and symbol macros.
• bindings of functions and macros. (Implicit in this is information about those compiler macros that are locally disabled.)
• bindings of block tags.
• bindings of go tags.
• information about declarations.

The lexical environment that is active at any given position in a program being semantically processed is referred to by definite reference as “the current lexical environment,” or sometimes as just “the lexical environment.”

Within a given namespace, a name is said to be bound in a lexical environment if there is a binding associated with its name in the lexical environment or, if not, there is a binding associated with its name in the global environment.

3.1.1.3.1 The Null Lexical Environment

The null lexical environment is equivalent to the global environment.

Although in general the representation of an environment object is implementation-dependent, nil can be used in any situation where an environment object is called for in order to denote the null lexical environment.

3.1.1.4 Environment Objects

Some operators make use of an object, called an environment object, that represents the set of lexical bindings needed to perform semantic analysis on a form in a given lexical environment. The set of bindings in an environment object may be a subset of the bindings that would be needed to actually perform an evaluation; for example, values associated with variable names and function names in the corresponding lexical environment might not be available in an environment object.

The type and nature of an environment object is implementation-dependent. The values of environment parameters to macro functions are examples of environment objects.

The object nil when used as an environment object denotes the null lexical environment; see Section 3.1.1.3.1 (The Null Lexical Environment).