Components - asdf Manual
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### 3.2 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 string or symbol and behaves just like any other component name (including with regard to the case conversion rules for component names).

— 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. The resulting pathname is loaded if either of the following conditions is true:

> - there is no system of that name in memory
> - the file's last-modified time exceeds the last-modified time of the system in memory

> 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 function that 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. Hence, it is strongly advised to define a system foo in the corresponding file foo.asd.