inpackage

In Common Lisp, in-package is a special operator that changes the current package for subsequent evaluation. The current package acts as the namespace for symbols; switching packages affects which symbols are read, created, and exported by default. The common usage is (in-package :my-package) or (in-package "MY-PACKAGE"), and the named package must already exist; if the package does not exist, a package-error is signaled.

When in-package is evaluated, the reader and evaluator treat unqualified symbols as belonging to the new current package. This means that definitions such as defun, defvar, and other top-level forms will intern their symbols in the current package unless a package prefix is used. In this way, in-package provides a convenient way to organize code into namespaces and avoid naming conflicts across large projects.

Packages themselves are defined and configured with defpackage, which specifies the package name, its nicknames, and which packages it can use or export from. Importantly, in-package does not create packages or modify their exports; it only sets the active package context for subsequent code. Developers typically place an in-package form at the beginning of a source file to establish the intended namespace for all following definitions, or insert it in sections where a namespace switch is desired.

Notes on usage: the package name can be given as a symbol or a string; the exact syntax varies by implementation, but the standard behavior is to switch to the existing package. Care should be taken to ensure the target package exists and is properly defined before using in-package.

See also: defpackage.