Chapter 16

Library Collections and MzLib

A library is module declaration for use by multiple programs. MzScheme provides a mechanism for grouping libraries into collections that can be easily distributed and easily added to a local MzScheme installation. A collection is normally installed into a directory named collects that is in the same directory as the MzScheme executable.⁵⁴ Each installed collection is represented as a subdirectory within the collects directory.

Client programs incorporate a library by using a module path of the form (lib library-file-path collection ···). For example, the following module uses the match.ss library module from the default mzlib collection, the getinfo.ss library module from the setup collection, and the cards.ss library module from the games collection's cards subcollection:

(module my-game mzscheme
  (require (lib "match.ss")
           (lib "getinfo.ss" "setup")
           (lib "cards.ss" "games" "cards"))
  ....)

In general,(lib library-file-path collection ···) accesses the module in the file library-file-path in the collection named by the first collection, where both library-file-path and collection are literal strings that will be used as elements in a path. If additional collection strings are provided, they are used to form a path into a subcollection. If the collection arguments are omitted, the library is accessed in the mzlib collection.

The info.ss library in a collection is special by convention. This library is used to provide information about the collection to mzc (the MzScheme compiler) or MrEd. For more information see PLT mzc: MzScheme Compiler Manual and PLT MrEd: Graphical Toolbox Manual.

There is usually one standard collects directory, but MzScheme supports any number of directories containing collections. The search path for collections is determined by the current-library-collection-paths parameter (see section 7.9.1.6). The list of paths in current-library-collection-paths is searched from first to last to locate a collection. To find a sub-collection, the enclosing collection is first found; if the sub-collection is not present in the found enclosing collection, then the search continues by looking for another instance of the enclosing collection, and so on. In other words, the directory tree for each element in the search path is spliced together with the directory trees of other path elements. (The ``splicing'' of tress applies only to directories; a file within a collection is found only within the first instance of the collection.)

The value of the current-library-collection-paths parameter is initialized by the stand-alone version of MzScheme to the result of (find-library-collection-paths).⁵⁵ The find-library-collection-paths procedure produces a list of paths as follows:

• 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.

• If the executable embeds a list of search paths, they are included (in order) after the first element in the default collection path list. Embedded relative paths are included only when the corresponding directory exists 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.

• If the PLTCOLLECTS environment variable is defined, it is combined with the default list using path-list-string->path-list (see section 11.3.2). If it is not defined, the default collection path list (as constructed by the first three bullets above) is used directly.

The path produced by (find-system-path 'collects-dir) is normally embedded in an executable; in stand-alone MzScheme (or MrEd), it can be overridden via a --collects or -X command-line flag.

(collection-path collection ···¹) returns the path containing the libraries of collection; if the collection is not found, the exn:fail:filesystem exception is raised.

MzScheme is distributed with a standard collection of utility libraries with MzLib as the representative library. The name of this collection is mzlib, so the libraries are distributed in a mzlib subdirectory of the collects library collection directory. MzLib is described in PLT MzLib: Libraries Manual.