Environment Concepts (MIT/GNU Scheme 12.1)

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#### 1.2.2 Environment Concepts

An environment is a set of variable bindings. If an environment has no binding for a variable, that variable is said to be unbound in that environment. Referencing an unbound variable signals a condition of type `condition-type:unbound-variable`.

A new environment can be created by extending an existing environment with a set of new bindings. Note that “extending an environment” does not modify the environment; rather, it creates a new environment that contains the new bindings and the old ones. The new bindings shadow the old ones; that is, if an environment that contains a binding for `x` is extended with a new binding for `x`, then only the new binding is seen when `x` is looked up in the extended environment. Sometimes we say that the original environment is the parent of the new one, or that the new environment is a child of the old one, or that the new environment inherits the bindings in the old one.

Procedure calls extend an environment, as do `let`, `let*`, `letrec`, and `do` expressions. Internal definitions (see Internal Definitions) also extend an environment. (Actually, all the constructs that extend environments can be expressed in terms of procedure calls, so there is really just one fundamental mechanism for environment extension.) A top-level definition (see Top-Level Definitions) may add a binding to an existing environment.