17.2 Libraries and Collections

A library is module declaration for use by multiple programs. Racket further groups libraries into collections that can be easily distributed and added to a local Racket installation.

Some libraries are distributed via PLaneT packages. Such libraries are referenced through a planet module path (see require) and are downloaded by Racket on demand.

Other collections are distributed with Racket, in which case each collection is a directory that is located in a "collects" directory relative to the Racket executable. A collection can also be installed in a user-specific directory. More generally, the search path for installed collections can be configured through the current-library-collection-paths parameter. Finally, the location of collections can be specified through the collection links files; see Collection Links for more information. In all of these cases, the collections are referenced through lib paths (see require) or symbolic shorthands.

For example, the following module uses the "getinfo.rkt" library module from the "setup" collection, and the "cards.rkt" library module from the "games" collection’s "cards" subcollection:

#lang racket

(require (lib "setup/getinfo.rkt")

(lib "games/cards/cards.rkt"))

....

This example is more compactly and more commonly written using symbolic shorthands:

#lang racket

(require setup/getinfo

games/cards/cards)

....

When an identifier id is used in a require form, it is converted to (lib rel-string) where rel-string is the string form of id.

A rel-string in (lib rel-string) consists of one or more path elements that name collections, and then a final path element that names a library file; the path elements are separated by /. If rel-string contains no /s, then then /main.rkt is implicitly appended to the path. If rel-string contains / but does not end with a file suffix, then .rkt is implicitly appended to the path.

The translation of a planet or lib path to a module declaration is determined by the module name resolver, as specified by the current-module-name-resolver parameter.

For the default module name resolver, the search path for collections is determined by the content of (find-system-path 'links-file) (if it exists) and the current-library-collection-paths parameter. The collection links and then list of paths in current-library-collection-paths is searched from first to last to locate the first that contains rel-string. In other words, the filesystem tree for each element in the link table and search path is spliced together with the filesystem trees of other path elements. Some Racket tools rely on unique resolution of module path names, so an installation and current-library-collection-paths configuration should not allow multiple files to match the same collection and file name.

The value of the current-library-collection-paths parameter is initialized in the Racket executable to the result of (find-library-collection-paths).

• The path produced by (build-path (find-system-path 'addon-dir) (version) "collects") is the first element of the default collection path list, unless the value of the use-user-specific-search-paths parameter is #f.

• Extra directories provided in pre-extras are included next to the default collection path list, converted to complete paths relative to the executable.

• If the directory specified by (find-system-path 'collects-dir) is absolute, or if it is relative (to the executable) and it exists, then it is added to the end of the default collection path list.

• Extra directories provided in post-extras are included last in the default collection path list, converted to complete paths relative to the executable.

• If the PLTCOLLECTS environment variable is defined, it is combined with the default list using path-list-string->path-list. If it is not defined, the default collection path list (as constructed by the first three bullets above) is used directly.

  Note that on Unix and Mac OS X, paths are separated by :, and on Windows by ;. Also, path-list-string->path-list splices the default paths at an empty path, for example, with many Unix shells you can set PLTCOLLECTS to ":`pwd`", "`pwd`:", or "`pwd`" to specify search the current directory after, before, or instead of the default paths, respectively.

17.2.1 Collection Links

The collection links files are used by collection-file-path, collection-path, and the default module name resolver to locate collections before trying the (current-library-collection-paths) search path, but only if the use-collection-link-paths parameter is set to #t. Furthermore, a user-specific collection links file takes precedence over an installation-wide collection links file, but the user-specific collection links file is used only the use-user-specific-search-paths parameter is set to #t.

The path of the user-specific collection links file is by (find-system-path 'links-file), while an installation-wide collection links file is "links.rktd" in the "config" collection within the installation’s main collection directory. Each collection links file is cached by Racket, but the file is re-read if its timestamp changes.

Each collection links file is read with default reader parameter settings to obtain a list. Every element of the list must be a link specification with one of the forms (list string path), (list string path regexp), (list 'root path), or (list 'root regexp). A string names a top-level collection, in which case path is a path that can be used as the collection’s path (directly, as opposed to a subdirectory of path named by string). A 'root entry, in contrast, acts like an path in (current-library-collection-paths). If path is a relative path, it is relative to the directory containing the collection links file. If regexp is specified in a link, then the link is used only if (regexp-match? regexp (version)) produces a true result.

A single top-level collection can have multiple links in a collection links file, and any number of 'root entries can appear. The corresponding paths are effectively spliced together, since the paths are tried in order to locate a file or sub-collection.

The raco link command-link tool can display, install, and remove links in the collection links file. See raco link: Library Collection Links in raco: Racket Command-Line Tools for more information.