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7.2 Components

A component represents a source file or (recursively) a collection of components. A system is (roughly speaking) a top-level component that can be found via find-system.

A system designator is a system itself, or a string or symbol that behaves just like any other component name (including with regard to the case conversion rules for component names).

A component designator, relative to a base component, is either a component itself, or a string or symbol, or a list of designators.

— Function: find-system system-designator &optional (error-p t)

Given a system designator, find-system finds and returns a system. If no system is found, an error of type missing-component is thrown, or nil is returned if error-p is false.

To find and update systems, find-system funcalls each element in the *system-definition-search-functions* list, expecting a pathname to be returned, or a system object, from which a pathname may be extracted, and that will be registered. The resulting pathname (if any) is loaded if one of the following conditions is true:

When system definitions are loaded from .asd files, a new scratch package is created for them to load into, so that different systems do not overwrite each others operations. The user may also wish to (and is recommended to) include defpackage and in-package forms in his system definition files, however, so that they can be loaded manually if need be.

The default value of *system-definition-search-functions* is a list of two functions. The first function looks in each of the directories given by evaluating members of *central-registry* for a file whose name is the name of the system and whose type is asd. The first such file is returned, whether or not it turns out to actually define the appropriate system. The second function does something similar, for the directories specified in the source-registry. Hence, it is strongly advised to define a system foo in the corresponding file foo.asd.

— Function: find-component base path

Given a base component (or designator for such), and a path, find the component designated by the path starting from the base.

If path is a component object, it designates itself, independently from the base.

If path is a string, or symbol denoting a string via coerce-name, then base is resolved to a component object, which must be a system or module, and the designated component is the child named by the path.

If path is a cons cell, find-component with the base and the car of the path, and the resulting object is used as the base for a tail call to find-component with the car of the path.

If base is a component object, it designates itself.

If base is null, then path is used as the base, with nil as the path.

If base is a string, or symbol denoting a string via coerce-name, it designates a system as per find-system.

If base is a cons cell, it designates the component found by find-component with its car as base and cdr as path.