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 [Top of Book]  [Contents]   [Previous Chapter]   [Next Chapter] 

76 Using and Developing GAP Packages
 76.1 Installing a GAP Package
 76.2 Loading a GAP Package
 76.3 Functions for GAP Packages
 76.4 Guidelines for Writing a GAP Package
 76.5 Structure of a GAP Package
 76.6 Writing Documentation and Tools Needed
 76.7 An Example of a GAP Package
 76.8 File Structure
 76.9 Creating the PackageInfo.g File
 76.10 Functions and Variables and Choices of Their Names
 76.11 Package Dependencies (Requesting one GAP Package from within Another)
 76.12 Declaration and Implementation Part of a Package
 76.13 Autoreadable Variables
 76.14 Standalone Programs in a GAP Package
 76.15 Having an InfoClass
 76.16 The Banner
 76.17 Version Numbers
 76.18 Testing a GAP package
 76.19 Access to the GAP Development Version
 76.20 Version control and continuous integration for GAP packages
 76.21 Selecting a license for a GAP Package
 76.22 Releasing a GAP Package
 76.23 The homepage of a Package
 76.24 Some things to keep in mind
 76.25 Package release checklists

76 Using and Developing GAP Packages

The functionality of GAP can be extended by loading GAP packages. The GAP distribution already contains all currently redistributed GAP packages in the gap-4.9.2/pkg directory.

GAP packages are written by (groups of) GAP users who may not necessarily be members of the GAP developer team. The responsibility and copyright of a GAP package remains with the original author(s).

GAP packages have their own documentation which is smoothly integrated into the GAP help system. (When GAP is started, LoadPackageDocumentation is called for all packages.)

All GAP users who develop new code are invited to share the results of their efforts with other GAP users by making the code and its documentation available in form of a package. Guidance on how to do this is available from the GAP website (https://www.gap-system.org) and in the GAP package Example (see https://www.gap-system.org/Packages/example.html).

The GAP development team will assist in making any new package suitable for distribution with GAP. It is also possible to submit a package to a formal refereeing process.

In this chapter we first describe how to use existing packages, and then provide guidelines for writing a GAP package.

76.1 Installing a GAP Package

Before a package can be used it must be installed. A standard distribution of GAP already contains all the packages currently redistributed with GAP. This set of packages has been checked for compatibility with the system and with each other during release preparation. Most of the packages can be used immediately, but some of them may require further installation steps (see below).

Also, since GAP packages are released independently of the main GAP system, it may sometimes be useful to upgrade or install new packages between upgrades of your GAP installation, e.g. if a new version of a package adds new capabilities or bug fixes that you need.

A package consists of a collection of files within a single directory that must be a subdirectory of the pkg directory in one of the GAP root directories (see 9.2). If you don't have access to the pkg directory in your main GAP installation you can add private root directories as explained in section 9.2.

Whenever you download or clone an archive of a GAP package, it will contain a README file (or README.md etc.) that explains how it should be installed. Some packages just consist of GAP code and the installation is done by unpacking the archive in one of the places described above. There are also packages that need further installation steps, such as compilation or installing additional software to satisfy their dependencies. If there are some external programs which have to be compiled, this is often done by executing ./configure; make inside the unpacked package directory (but check the individual README files).

Most of the packages that require compilation can be compiled in a single step by changing to the pkg directory of your GAP installation and calling the ../bin/BuildPackages.sh script.

Note that if you use Windows you may not be able to use some or all external binaries.

76.2 Loading a GAP Package

If a package is not already loaded, it may be loaded using the function LoadPackage (76.2-1).

Some GAP packages are prepared for automatic loading, that is they will be loaded automatically when GAP starts (see 76.2-2).

76.2-1 LoadPackage
‣ LoadPackage( name[, version][, banner] )( function )

loads the GAP package with name name.

As an example, the following loads the GAP package SONATA (case insensitive) which provides methods for the construction and analysis of finite nearrings:

gap> LoadPackage("sonata");
... some more lines with package banner(s) ...
true

The package name is case insensitive and may be appropriately abbreviated. At the time of writing, for example, LoadPackage("semi"); will load the Semigroups package, and LoadPackage("j"); will load the json package. If the abbreviation cannot be uniquely completed, a list of available completions will be offered, and LoadPackage returns fail. Thus the names of all installed packages can be shown by calling LoadPackage("");.

When the optional argument string version is present, the package will only be loaded in a version number equal to or greater than version (see CompareVersionNumbers (76.3-7)). If the first character of version is = then only that version will be loaded.

LoadPackage will return true if the package has been successfully loaded, and will return fail if the package could not be loaded. The latter may be the case if the package is not installed, if necessary binaries have not been compiled, or if the version number of the available version is too small. If the package cannot be loaded, TestPackageAvailability (76.3-2) can be used to find the reasons. Also, DisplayPackageLoadingLog (76.2-5) can be used to find out more about the failure. To see the problems directly, one can change the verbosity using the user preference InfoPackageLoadingLevel, see InfoPackageLoading (76.2-5) for details.

If the package name has already been loaded in a version number equal to or greater than version, LoadPackage returns true without doing anything else.

If the optional argument banner is present then it must be either true or false; in the latter case, the effect is that no package banner is printed.

After a package has been loaded, all its code becomes available to use with the rest of the GAP library.

76.2-2 Automatic loading of GAP packages

When GAP is started some packages are loaded automatically, and these belong to two categories. The first are those packages which are needed to start GAP (at the present time, the only such package is GAPDoc). Their list is contained in GAPInfo.Dependencies.NeededOtherPackages. The second are packages which are loaded during GAP startup by default. The latter list may be obtained by calling UserPreference("PackagesToLoad") and is customisable as described in Section Reference: Configuring User preferences.

While GAP will not start if any of the packages from the former group is missing, loading of the packages from the latter group may be suppressed by using the -A command line option (see 3.1).

If for some reason you don't want certain packages to be automatically loaded, GAP provides three levels for disabling autoloading.

The autoloading of specific packages can be overwritten for the whole GAP installation by putting a file NOAUTO into a pkg directory that contains lines with the names of packages which should not be automatically loaded.

Furthermore, individual users can disable the autoloading of specific packages by putting the names of these packages into the list that is assigned to the user preference "ExcludeFromAutoload", for example in the user's gap.ini file (see 3.2-1).

Using the -A command line option when starting GAP (see 3.1), automatic loading of packages is switched off for this GAP session.

In any of the above three cases, the packages listed in GAPInfo.Dependencies.NeededOtherPackages are still loaded automatically, and an error is signalled if any of these packages is unavailable.

See SetPackagePath (76.2-3) for a way to force the loading of a prescribed package version. See also ExtendRootDirectories (76.2-4) for a method of adding directories containing packages after GAP has been started.

76.2-3 SetPackagePath
‣ SetPackagePath( pkgname, pkgpath )( function )

This function can be used to force GAP to load a particular version of a package, even though newer versions of the package are available.

Let pkgname and pkgpath be strings denoting the name of a GAP package and the path to a directory where a version of this package can be found (i. e., calling Directory (9.3-2) with the argument pkgpath will yield a directory that contains the file PackageInfo.g of the package).

If the package pkgname is already loaded with an installation path different from pkgpath then SetPackagePath signals an error. If the package pkgname is not yet loaded then SetPackagePath erases the information about available versions of the package pkgname, and stores the record that is contained in the PackageInfo.g file at pkgpath instead, such that only the version installed at pkgpath can be loaded with LoadPackage (76.2-1).

76.2-4 ExtendRootDirectories
‣ ExtendRootDirectories( paths )( function )

Let paths be a list of strings that denote paths to intended GAP root directories (see 9.2). The function ExtendRootDirectories adds these paths to the global list GAPInfo.RootPaths and calls the initialization of available GAP packages, such that later calls to LoadPackage (76.2-1) will find the GAP packages that are contained in pkg subdirectories of the directories given by paths.

Note that the purpose of this function is to make GAP packages in the given directories available. It cannot be used to influence the start of GAP, because the GAP library is loaded before ExtendRootDirectories can be called (and because GAPInfo.RootPaths is not used for reading the GAP library).

76.2-5 DisplayPackageLoadingLog
‣ DisplayPackageLoadingLog( [severity] )( function )
‣ InfoPackageLoading( info class )
‣ PACKAGE_ERROR( global variable )
‣ PACKAGE_WARNING( global variable )
‣ PACKAGE_INFO( global variable )
‣ PACKAGE_DEBUG( global variable )
‣ LogPackageLoadingMessage( severity, message[, name] )( function )

Whenever GAP considers loading a package, log messages are collected in a global list. The messages for the current GAP session can be displayed with DisplayPackageLoadingLog. To each message, a "severity" is assigned, which is one of PACKAGE_ERROR, PACKAGE_WARNING, PACKAGE_INFO, PACKAGE_DEBUG, in increasing order. The function DisplayPackageLoadingLog shows only the messages whose severity is at most severity, the default for severity is PACKAGE_WARNING.

The intended meaning of the severity levels is as follows.

PACKAGE_ERROR

should be used whenever GAP will run into an error during package loading, where the reason of the error shall be documented in the global list.

PACKAGE_WARNING

should be used whenever GAP has detected a reason why a package cannot be loaded, and where the message describes how to solve this problem, for example if a package binary is missing.

PACKAGE_INFO

should be used whenever GAP has detected a reason why a package cannot be loaded, and where it is not clear how to solve this problem, for example if the package is not compatible with other installed packages.

PACKAGE_DEBUG

should be used for other messages reporting what GAP does when it loads packages (checking dependencies, reading files, etc.). One purpose is to record in which order packages have been considered for loading or have actually been loaded.

The log messages are created either by the functions of GAP's package loading mechanism or in the code of your package, for example in the AvailabilityTest function of the package's PackageInfo.g file (see 76.3-12), using LogPackageLoadingMessage. The arguments of this function are severity (which must be one of the above severity levels), message (which must be either a string or a list of strings), and optionally name (which must be the name of the package to which the message belongs). The argument name is not needed if the function is called from a call of a package's AvailabilityTest function (see 76.3-12) or is called from a package file that is read from init.g or read.g; in these cases, the name of the current package (stored in the record GAPInfo.PackageCurrent) is taken. According to the above list, the severity argument of LogPackageLoadingMessage calls in a package's AvailabilityTest function is either PACKAGE_WARNING or PACKAGE_INFO.

If you want to see the log messages already during the package loading process, you can set the level of the info class InfoPackageLoading to one of the severity values listed above; afterwards the messages with at most this severity are shown immediately when they arise. In order to make this work already for autoloaded packages, you can call SetUserPreference("InfoPackageLoadingLevel", lev); to set the desired severity level lev. This can for example be done in your gap.ini file, see Section 3.2-1.

76.3 Functions for GAP Packages

The following functions are mainly used in files contained in a package and not by users of a package. They are needed to organise reading package files into GAP in the right order, performing maintenance tasks like building documentation and running package tests, checking package dependencies, etc. You will find further information about their use in Section 76.4 and subsequent sections.

76.3-1 ReadPackage
‣ ReadPackage( [name, ]file )( function )
‣ RereadPackage( [name, ]file )( function )

Called with two strings name and file, ReadPackage reads the file file of the GAP package name, where file is given as a path relative to the home directory of name. Note that file is read in the namespace of the package, see Section 4.10 for details.

If only one argument file is given, this should be the path of a file relative to the pkg subdirectory of GAP root paths (see 9.2). Note that in this case, the package name is assumed to be equal to the first part of file, so the one argument form is not recommended.

The absolute path is determined as follows. If the package in question has already been loaded then the file in the directory of the loaded version is read. If the package is available but not yet loaded then the directory given by TestPackageAvailability (76.3-2) is used, without prescribed version number. (Note that the ReadPackage call does not force the package to be loaded.)

If the file is readable then true is returned, otherwise false.

Each of name and file should be a string. The name argument is case insensitive.

RereadPackage does the same as ReadPackage, except that also read-only global variables are overwritten (cf. Reread (9.7-10)).

76.3-2 TestPackageAvailability
‣ TestPackageAvailability( name[, version][, checkall] )( function )

For strings name and version, this function tests whether the GAP package name is available for loading in a version that is at least version, or equal to version if the first character of version is = (see CompareVersionNumbers (76.3-7) for further details about version numbers).

The result is true if the package is already loaded, fail if it is not available, and the string denoting the GAP root path where the package resides if it is available, but not yet loaded. So the package name is available if the result of TestPackageAvailability is not equal to fail.

If the optional argument checkall is true then all dependencies are checked, even if some have turned out to be not satisfied. This is useful when one is interested in the reasons why the package name cannot be loaded. In this situation, calling first TestPackageAvailability and then DisplayPackageLoadingLog (76.2-5) with argument PACKAGE_INFO (76.2-5) will give an overview of these reasons.

You should not call TestPackageAvailability in the test function of a package (the value of the component AvailabilityTest in the PackageInfo.g file of the package, see 76.3-12), because TestPackageAvailability calls this test function.

The argument name is case insensitive.

76.3-3 TestPackage
‣ TestPackage( pkgname )( function )

It is recommended that a GAP package specifies a standard test in its PackageInfo.g file. If pkgname is a string with the name of a GAP package, then TestPackage(pkgname) will check if this package is loadable and has the standard test, and will run this test in the current GAP session.

The output of the test depends on the particular package, and it also may depend on the current GAP session (loaded packages, state of the random sources, defined global variables etc.). If you would like to run the test for the same package in the same setting that is used for the testing of GAP releases, you have to call

make testpackage PKGNAME=pkgname

in the UNIX shell (without quotes around pkgname). This will run the standard test for the package pkgname three times in different settings, and will write test output to three files in the dev/log directory. These output files will be named in the format testpackageX_timestamp.pkgname, where X=A for the test with packages loaded by default, X=1 for the test without other packages (i.e. when GAP is started with -A command line option), and X=2 when the test is run with all packages loaded.

76.3-4 InstalledPackageVersion
‣ InstalledPackageVersion( name )( function )

If the GAP package with name name has already been loaded then InstalledPackageVersion returns the string denoting the version number of this version of the package. If the package is available but has not yet been loaded then the version number string for that version of the package that currently would be loaded. (Note that loading another package might force loading another version of the package name, so the result of InstalledPackageVersion will be different afterwards.) If the package is not available then fail is returned.

The argument name is case insensitive.

76.3-5 DirectoriesPackageLibrary
‣ DirectoriesPackageLibrary( name[, path] )( function )

takes the string name, a name of a GAP package, and returns a list of directory objects for those sub-directory/ies containing the library functions of this GAP package, for the version that is already loaded or is currently going to be loaded or would be the first version GAP would try to load if no other version is explicitly prescribed. (If the package name is not yet loaded then we cannot guarantee that the returned directories belong to a version that really can be loaded.)

The default is that the library functions are in the subdirectory lib of the GAP package's home directory. If this is not the case, then the second argument path needs to be present and must be a string that is a path name relative to the home directory of the GAP package with name name.

Note that DirectoriesPackageLibrary is likely to be called in the AvailabilityTest function in the package's PackageInfo.g file (see 76.3-12).

As an example, the following returns a directory object for the library functions of the GAP package Example:

gap> DirectoriesPackageLibrary( "Example", "gap" );
[ dir("/home/werner/gap/4.0/pkg/example/gap/") ]

Observe that we needed the second argument "gap" here, since Example's library functions are in the subdirectory gap rather than lib.

In order to find a subdirectory deeper than one level in a package directory, the second argument is again necessary whether or not the desired subdirectory relative to the package's directory begins with lib. The directories in path should be separated by / (even on systems, like Windows, which use \ as the directory separator). For example, suppose there is a package somepackage with a subdirectory m11 in the directory data, then we might expect the following:

gap> DirectoriesPackageLibrary( "somepackage", "data/m11" );
[ dir("/home/werner/gap/4.0/pkg/somepackage/data/m11") ]

76.3-6 DirectoriesPackagePrograms
‣ DirectoriesPackagePrograms( name )( function )

returns a list of the bin/architecture subdirectories of all packages name where architecture is the architecture on which GAP has been compiled (this can be accessed as GAPInfo.Architecture, see GAPInfo (3.5-1)) and the version of the installed package coincides with the version of the package name that is already loaded or is currently going to be loaded or would be the first version GAP would try to load if no other version is explicitly prescribed. (If the package name is not yet loaded then we cannot guarantee that the returned directories belong to a version that really can be loaded.)

Note that DirectoriesPackagePrograms is likely to be called in the AvailabilityTest function in the package's PackageInfo.g file (see 76.3-12).

The directories returned by DirectoriesPackagePrograms are the place where external binaries of the GAP package name for the current package version and the current architecture should be located.

gap> DirectoriesPackagePrograms( "nq" );
[ dir("/home/gap/4.0/pkg/nq/bin/x86_64-unknown-linux-gnu-gcc/64-bit/"),
  dir("/home/gap/4.0/pkg/nq/bin/x86_64-unknown-linux-gnu-gcc/") ]

76.3-7 CompareVersionNumbers
‣ CompareVersionNumbers( supplied, required[, "equal"] )( function )

A version number is a string which contains nonnegative integers separated by non-numeric characters. Examples of valid version numbers are for example:

"1.0"   "3.141.59"  "2-7-8.3" "5 release 2 patchlevel 666"

CompareVersionNumbers compares two version numbers, given as strings. They are split at non-digit characters, the resulting integer lists are compared lexicographically. The routine tests whether supplied is at least as large as required, and returns true or false accordingly. A version number ending in dev is considered to be infinite.

76.3-8 IsPackageMarkedForLoading
‣ IsPackageMarkedForLoading( name, version )( function )

This function can be used in the code of a package \(A\), say, for testing whether the package name in version version will be loaded after the LoadPackage (76.2-1) call for the package \(A\) has been executed. This means that the package name had been loaded before, or has been (directly or indirectly) requested as a needed or suggested package of the package \(A\) or of a package whose loading requested that \(A\) was loaded.

76.3-9 DeclareAutoreadableVariables
‣ DeclareAutoreadableVariables( pkgname, filename, varlist )( function )

Let pkgname be the name of a package, let filename be the name of a file relative to the home directory of this package, and let varlist be a list of strings that are the names of global variables which get bound when the file is read. DeclareAutoreadableVariables notifies the names in varlist such that the first attempt to access one of the variables causes the file to be read.

76.3-10 Kernel modules in GAP packages

If the package has a kernel module, then it can be compiled using the gac script. A kernel module is implemented in C and follows certain conventions to comply with the GAP kernel interface, which we plan to document later. In the meantime, we advice to get in touch with GAP developers if you plan to develop such a package.

To use the gac script to produce dynamically loadable modules, call it with the -d option, for example:

$ gap4/bin/i386-ibm-linux-gcc2/gac -d test.c

This will produce a file test.so, which then can be loaded into GAP with LoadDynamicModule (76.3-11).

76.3-11 LoadDynamicModule
‣ LoadDynamicModule( filename[, crc] )( function )

To load a compiled file, the command LoadDynamicModule is used. This command loads filename as module. If given, the CRC checksum crc must match the value of the module (see 9.7-7).

gap> LoadDynamicModule("./test.so");
gap> CrcFile("test.so");
2906458206
gap> LoadDynamicModule("./test.so",1);
Error, <crc> mismatch (or no support for dynamic loading) called from
<function>( <arguments> ) called from read-eval-loop
Entering break read-eval-print loop ...
you can 'quit;' to quit to outer loop, or
you can 'return;' to continue
brk> quit;
gap> LoadDynamicModule("./test.so",2906458206);

On some operating systems, once you have loaded a dynamic module with a certain filename, loading another with the same filename will have no effect, even if the file on disk has changed.

76.3-12 The PackageInfo.g File

Each package has the file PackageInfo.g which contains meta-information about the package (package name, version, author(s), relations to other packages, homepage, download archives, etc.). This file is used by the package loading mechanism and also for the redistribution of a package with GAP

76.3-13 ValidatePackageInfo
‣ ValidatePackageInfo( info )( function )

This function is intended to support package authors who create or modify PackageInfo.g files. (It is not called when these files are read during the startup of GAP or when packages are actually loaded.)

The argument info must be either a record as is contained in a PackageInfo.g file or a a string which describes the path to such a file. The result is true if the record or the contents of the file, respectively, has correct format, and false otherwise; in the latter case information about the incorrect components is printed.

Note that the components used for package loading are checked as well as the components that are needed for composing the package overview web page or for updating the package archives.

If info is a string then ValidatePackageInfo checks additionally whether those package files exist that are mentioned in the file info, for example the manual.six file of the package documentation.

76.3-14 ShowPackageVariables
‣ ShowPackageVariables( pkgname[, version][, arec] )( function )
‣ PackageVariablesInfo( pkgname, version )( function )

Let pkgname be the name of a GAP package. If the package pkgname is available but not yet loaded then ShowPackageVariables prints a list of global variables that become bound and of methods that become installed when the package is loaded. (For that, GAP actually loads the package.)

If a version number version is given (see Section Reference: Version Numbers) then this version of the package is considered.

An error message is printed if (the given version of) the package is not available or already loaded.

Information is printed about new and redeclared global variables, and about names of global variables introduced in the package that differ from existing globals only by case; note that the GAP help system is case insensitive, so it is difficult to document identifiers that differ only by case.

Info lines for undocumented variables are marked with an asterisk *.

The following entries are omitted from the list: default setter methods for attributes and properties that are declared in the package, and Setattr and Hasattr type variables where attr is an attribute or property.

The output can be customized using the optional record arec, the following components of this record are supported.

show

a list of strings describing those kinds of variables which shall be shown, such as "new global functions"; the default are all kinds that appear in the package,

showDocumented

true (the default) if documented variables shall be shown, and false otherwise,

showUndocumented

true (the default) if undocumented variables shall be shown, and false otherwise,

showPrivate

true (the default) if variables from the package's name space (see Section 4.10) shall be shown, and false otherwise,

Display

a function that takes a string and shows it on the screen; the default is Print (6.3-4), another useful value is Pager (2.4-1).

An interactive variant of ShowPackageVariables is the function BrowsePackageVariables (Browse: BrowsePackageVariables) that is provided by the GAP package Browse. For this function, it is not sensible to assume that the package pkgname is not yet loaded before the function call, because one might be interested in packages that must be loaded before Browse itself can be loaded. The solution is that BrowsePackageVariables (Browse: BrowsePackageVariables) takes the output of PackageVariablesInfo as its second argument. The function PackageVariablesInfo is used by both ShowPackageVariables and BrowsePackageVariables (Browse: BrowsePackageVariables) for collecting the information about the package in question, and can be called before the package Browse is loaded.

76.3-15 BibEntry
‣ BibEntry( pkgname[, key] )( function )

Returns: a string in BibXMLext format (see GAPDoc: The BibXMLext Format) that can be used for referencing the GAP system or a GAP package.

If the argument pkgname is the string "GAP", the function returns an entry for the current version of GAP.

Otherwise, if a string pkgname is given, which is the name of a GAP package, an entry for this package is returned; this entry is computed from the PackageInfo.g file of the current version of the package, see InstalledPackageVersion (76.3-4). If no package with name pkgname is installed then the empty string is returned.

A string for a different version of GAP or a package can be computed by entering, as the argument pkgname, the desired record from the PackageInfo.g file. (One can access these records using the function PackageInfo.)

In each of the above cases, an optional argument key can be given, a string which is then used as the key of the BibTeX entry instead of the default key that is generated from the system/package name and the version number.

BibEntry requires the functions FormatParagraph (GAPDoc: FormatParagraph) and NormalizedNameAndKey (GAPDoc: NormalizedNameAndKey) from the GAP package GAPDoc.

The functions ParseBibXMLextString (GAPDoc: ParseBibXMLextString) and StringBibXMLEntry (GAPDoc: StringBibXMLEntry) can be used to create for example a BibTeX entry from the return value, as follows.

gap> bib:= BibEntry( "GAP", "GAP4.5" );;
gap> Print( bib, "\n" );
<entry id="GAP4.5"><misc>
  <title><C>GAP</C> &ndash; <C>G</C>roups, <C>A</C>lgorithms,
         and <C>P</C>rogramming, <C>V</C>ersion 4.5.1</title>
  <howpublished><URL>https://www.gap-system.org</URL></howpublished>
  <key>GAP</key>
  <keywords>groups; *; gap; manual</keywords>
  <other type="organization">The GAP <C>G</C>roup</other>
</misc></entry>
gap> parse:= ParseBibXMLextString( bib );;
gap> Print( StringBibXMLEntry( parse.entries[1], "BibTeX" ) );
@misc{ GAP4.5,
  title =            {{GAP}   {\textendash}   {G}roups,   {A}lgorithms,  and
                      {P}rogramming, {V}ersion 4.5.1},
  organization =     {The GAP {G}roup},
  howpublished =     {\href                      {https://www.gap-system.org}
                      {\texttt{https://www.gap-system.org}}},
  key =              {GAP},
  keywords =         {groups; *; gap; manual}
}

76.3-16 Cite
‣ Cite( [pkgname[, key]] )( function )

Used with no arguments or with argument "GAP" (case-insensitive), Cite displays instructions on citing the version of GAP that is being used. Suggestions are given in plain text, HTML, BibXML and BibTeX formats. The same instructions are also contained in the CITATION file in the GAP root directory.

If pkgname is the name of a GAP package, instructions on citing this package will be displayed. They will be produced from the PackageInfo.g file of the working version of this package that must be available in the GAP installation being used. Otherwise, one will get a warning that no working version of the package is available.

The optional 2nd argument key has the same meaning as in BibEntry (76.3-15).

76.4 Guidelines for Writing a GAP Package

The remaining part of this chapter explains the basics of how to write a GAP package so that it integrates properly into GAP.

There are two basic aspects of creating a GAP package.

First, it is a convenient possibility to load additional functionality into GAP including a smooth integration of the package documentation. Second, a package is a way to make your code available to other GAP users.

Moreover, the GAP Group may provide some help with redistributing your package via the GAP website after checking if the package provides some new or improved functionality which looks interesting for other users, if it contains reasonable documentation, and if it seems to work smoothly with the GAP library and other distributed packages. In this case the package can take part in the GAP distribution update mechanism and becomes a deposited package.

Furthermore, package authors are encouraged to check if the package would be appropriate for the refereeing process and submit it. If the refereeing has been successful, the package becomes an accepted package. Check out https://www.gap-system.org/Packages/Authors/authors.html on the GAP website for more details.

Below we start with a description how the directory structure of a GAP package should be constructed and then add remarks on certain aspects of creating a package, some of these only apply to some packages. Finally, we provide guidelines for the release preparation and its distribution.

76.5 Structure of a GAP Package

A GAP package should have an alphanumeric name (packagename, say); mixed case is fine, but there should be no whitespace characters. All files of a GAP package packagename must be collected in a single directory packagedir, where packagedir should be just packagename optionally converted to lowercase and optionally followed by the package version (with or without hyphen to separate the version from packagename). Let us call this directory the home directory of the package.

To use the package with GAP, the directory packagedir must be a subdirectory of a pkg directory in (one of) the GAP root directories (see 9.2). For example, if GAP is installed in /usr/local/gap4 then the files of the package MyPack may be placed in the directory /usr/local/gap4/pkg/mypack. The directory packagedir preferably should have the following structure (below, a trailing / distinguishes directories from ordinary files):

packagedir/
  doc/
  lib/
  tst/
  CHANGES
  LICENSE
  README
  PackageInfo.g
  init.g
  read.g

This layout of directories and files may be created manually, or automatically using the tool called PackageMaker, available at https://github.com/gap-system/PackageMaker. The PackageMaker asks several questions about the intended package and then creates a new directory for it and populates it with all the files needed for a basic package.

Packages that contain some code that requires compilation will usually have it in the src subdirectory. They may also have extra files such as configure, Makefile.in etc. that automate the build procedure. There are three file names with a special meaning in the home directory of a package: PackageInfo.g and init.g which must be present, and read.g which is optional.

On the other hand, the names of CHANGES, LICENSE and README files are not strictly fixed. They may have extensions .txt or .md, and instead of LICENSE one could use e.g. COPYING or GPL for packages distributed under the GNU General Public License, or use HISTORY instead of CHANGES.

We now describe the above files and directories in more details:

README

The filename may optionally have an extension, e.g. .txt or .md.

This should contain "how to get it" instructions (covering the way of getting it with the GAP distribution and from the GAP website, if applicable), as well as installation instructions and names of the package authors and their email addresses. These installation instructions should be repeated or referenced from the package's documentation, which should be in the doc directory (see 76.6). Authors' names and addresses should be repeated both in the package's documentation and in the PackageInfo.g (see below).

CHANGES

For further versions of the package, it will be also useful to have a CHANGES file that records the main changes between versions of the package.

The filename may optionally have an extension, e.g. .txt or .md.

LICENSE

The file which explains conditions on which the package is distributed.

We advise all package authors to make clear in the documentation of their package the basis on which it is being distributed to users. Technically, this is the terms of the license which you give the users to copy, modify and redistribute your software (of which you presumably own the copyright) for their purposes.

GAP itself is distributed under the GNU General Public License version 2, a popular "free software" license which allows users to redistribute it freely under the same terms, and requires that any software which incorporates GAP (technically, any "derived work") also be distributed under those terms. We would encourage you to consider the GPL for your packages, but you might wish to be more restrictive (for instance forbidding redistribution for profit) or less restrictive (allowing your software to be incorporated into commercial software).

The filename may optionally have an extension, e.g. .txt or .md. Some packages also use different filenames, like COPYING.

configure, Makefile.in

These files are typically only used by packages which have a non-GAP component, e.g. some C code (the files of which should be in the src directory). The configure and Makefile.in files of the Example package provide prototypes (or they may be created using the PackageMaker mentioned above). The configure file typically takes a path path to the GAP root directory as argument and uses the value assigned to GAParch in the file sysinfo.gap, created when GAP was compiled to determine the compilation architecture, inserts this in place of the string @GAPARCH@ in Makefile.in and creates a file Makefile. When make is run (which, of course, reads the constructed Makefile), a directory bin (if necessary) and subdirectories of bin with the path equal to the string assigned to GAParch in the file sysinfo.gap should be created; any binaries constructed by compiling the code in src should end up in this subdirectory of bin.

PackageInfo.g

Every GAP package must have a PackageInfo.g file which contains meta-information about the package (package name, version, author(s), relations to other packages, homepage, download archives, etc.). This information is used by the package loading mechanism and also for the redistribution of a package with GAP. The Example package's PackageInfo.g file is well-commented and can be used as a prototype (see also 76.3-12 for further details). It may also be created using the PackageMaker mentioned above.

init.g, read.g

A GAP package must have a file init.g. Typical init.g and read.g files should normally consist entirely of ReadPackage (76.3-1) commands (and possibly also Read (9.7-1) commands) for reading further files of the package. If the "declaration" and "implementation" parts of the package are separated (and this is recommended), there should be a read.g file. The "declaration" part of a package consists of function and variable name declarations and these go in files with .gd extensions; these files are read in via ReadPackage commands in the init.g file. The "implementation" part of a package consists of the actual definitions of the functions and variables whose names were declared in the "declaration" part, and these go in files with .gi extensions; these files are read in via ReadPackage commands in the read.g file. The reason for following the above dichotomy is that the read.g file is read after the init.g file, thus enabling the possibility of a function's implementation to refer to another function whose name is known but is not actually defined yet (see 76.12 below for more details).

The GAP code (whether or not it is split into "declaration" and "implementation" parts) should go in the package's lib directory (see below).

doc

This directory should contain the package's documentation, written in an XML-based documentation format supported by the GAP package GAPDoc (see GAPDoc: Introduction and Example) which is used for the GAP documentation itself.

The Example package's documentation (see its doc directory) may be used as a prototype. It consists of the master file main.xml, further .xml files for manual chapters (included in the manual via Include directives in the master file) and the GAP input file ../makedocrel.g which generates the manuals. Generally, one should also provide a manual.bib BibTeX database file or an xml file in the BibXMLext format (see GAPDoc: The BibXMLext Format).

One could also use the AutoDoc which simplifies writing documentation by generating most of the GAPDoc code automatically.

lib

This is the preferred place for the GAP code of the package, i.e. the .g, .gd and .gi files (other than PackageInfo.g, init.g and read.g). For some packages, the directory gap has been used instead of lib; lib has the advantage that it is the default subdirectory of a package directory searched for by the DirectoriesPackageLibrary (76.3-5) command.

src

If the package contains non-GAP code, e.g. C code, then this source code should go in the src directory. If there are .h "include" files you may prefer to put these all together in a separate include directory. There is one further rule for the location of kernel library modules or external programs which is explained in 76.14-1 below.

tst

It is highly recommended that a package should have test files, which then should go in the tst directory. For a deposited package, a test file with a basic test of the package (for example, to check that it works as expected and/or that the manual examples are correct) may be specified in the PackageInfo.g to be included in the GAP standard test suite and run as a part of the GAP release preparation. More specific and time consuming tests are not supposed to be a part of the GAP standard test suite but may be placed in the tst directory with further instructions on how to run them. See Section 76.18 about the requirements to the test files formats and further recommendations.

All other files can be organised as you like. But we suggest that you have a look at existing packages and use a similar scheme, for example, put examples in the examples subdirectory, data libraries in extra subdirectories, and so on.

Sometimes there may be a need to include an empty directory in the package distribution (for example, as a place to store some data that may appear at runtime). In this case package authors are advised to put in this directory a short README file describing its purpose to ensure that such directory will be included in the redistribution.

Concerning the GAP code in packages, it is recommended to use only documented GAP functions, see 83.3. In particular if you want to make your package available to other GAP users it is advisable to avoid using "obsolete" variables (see 77). To test that the package does not use obsolete variables you can set the ReadObsolete component in your gap.ini file to false (see 3.2) or start GAP with -A -O command line options (note that this may also cause problems with loading other packages that use "obsolete" variables).

76.6 Writing Documentation and Tools Needed

If you intend to make your package available to other users it is essential to include documentation explaining how to install and use your programs.

Concerning the installation you should produce a README file which gives a short description of the purpose of the package and contains proper instructions how to install your package. Again, check out some existing packages to get an idea how this could look like.

Documentation for GAP package should be prepared in an XML-based documentation format that is defined in and can be used with the GAPDoc package (see GAPDoc: Introduction and Example).

There should be at least a text version of your documentation provided for use in the terminal running GAP and some nicely printable version in .pdf format. Many GAP users like to browse the documentation in HTML format via their Web browser. As a package author, you are not obliged to provide an HTML version of your package manual, but if you use the GAPDoc package you should have no trouble in producing one.

Moreover, using the GAPDoc package, it is also possible to produce HTML version of the documentation supporting MathJax (http://www.mathjax.org/) for the high quality rendering of mathematical symbols while viewing it online. For example, if you are viewing the HTML version of the manual, compare how this formula will look with MathJax turned on/off:

\[ [ \chi, \psi ] = \left( \sum_{{g \in G}} \chi(g) \psi(g^{{-1}}) \right) / |G|. \]

The manual of the Example package is written in the GAPDoc format, and commands needed to build it are contained in the file makedocrel.g (you don't need to re-build the manual since it is already included in the package). You will also need to have certain TeX tools installed: to produce manuals in the .pdf format, you need pdflatex.

In principle it is also possible to use alternative documentation systems. Historically, there is one such TeX-based system, which predates GAPDoc, and which is still in use by several packages. However, we do not recommend using it for new packages.

76.7 An Example of a GAP Package

We illustrate the creation of a GAP package by an example of a very basic package.

Create the following directories in your home directory: .gap, .gap/pkg and .gap/pkg/test. Then inside the directory .gap/pkg/test create an empty file init.g, and a file PackageInfo.g with the following contents:

SetPackageInfo( rec(
  PackageName := "test",
  Version := "1.0",
  PackageDoc := rec(
      BookName  := "test",
      SixFile   := "doc/manual.six",
      Autoload  := true ),
  Dependencies := rec(
      GAP       := "4.9",
      NeededOtherPackages := [ ["GAPDoc", "1.6"] ],
      SuggestedOtherPackages := [ ] ),
  AvailabilityTest := ReturnTrue ) );

This file declares the GAP package with name "test" in version 1.0. The package documentation consists of one autoloaded book; the SixFile component is needed by the GAP help system. Package dependencies (picked for the purposes of this example) require at least GAP 4.9 and GAPDoc package at version at least 1.6, and these conditions will be checked when the package will be loaded (see 76.17). Since there are no requirements that have to be tested, AvailabilityTest just uses ReturnTrue (5.4-1).

Now start GAP (without using the -r option) and the .gap directory will be added to the GAP root directory to allow GAP to find the packages installed there (see 9.2).

gap> LoadPackage("test");
true

This GAP package is too simple to be useful, but we have succeeded in loading it via LoadPackage (76.2-1), satisfying all specified dependencies.

76.8 File Structure

Package files may follow the style used for the GAP library. Every file in the GAP library starts with a header that lists the filename, copyright, a short description of the file contents and the original authors of this file, and ends with a comment line #E. Indentation in functions and the use of decorative spaces in the code are left to the decision of the authors of each file. Global (i.e. re-used elsewhere) comments usually are indented by two hash marks and two blanks, in particular, every declaration or method or function installation which is not only of local scope is separated by a header.

Facilities to distribute a document over several files to allow the documentation for parts of some code to be stored in the same file as the code itself are provided by the GAPDoc package (see GAPDoc: Distributing a Document into Several Files). The same approach is demonstrated by the Example package. E.g. example/doc/example.xml has the statement <#Include Label="ListDirectory"> and example/lib/files.gd contains

##  <#GAPDoc Label="ListDirectory">
##  <ManSection>
##  <Func Name="ListDirectory" Arg="[dir]"/>
##
##  <Description>
##  lists the files in directory <A>dir</A> (a string) 
##  or the current directory if called with no arguments.
##  </Description>
##  </ManSection>
##  <#/GAPDoc>
DeclareGlobalFunction( "ListDirectory" );

This is all put together in the file example/makedocrel.g which builds the package documentation, calling MakeGAPDocDoc (GAPDoc: MakeGAPDocDoc) with locations of library files containing parts of the documentation.

Alternatively, one could use the AutoDoc, which simplifies writing documentation by generating most of the GAPDoc code automatically. The equivalent of the fragment of the code above for AutoDoc would look like

#! @Arguments [dir]
#! @Description
#!  lists the files in directory <A>dir</A> (a string) 
#!  or the current directory if called with no arguments.
DeclareGlobalFunction( "ListDirectory" );

76.9 Creating the PackageInfo.g File

While the minimalistic PackageInfo.g file described in 76.7 is enough to let GAP load the package, and check all specified dependencies, it is actually missing many extra fields which become relevant if you want to distribute your package: they contain lists of authors and/or maintainers including contact information, URLs of the package archives and README files, status information, text for a package overview webpage, and so on. All these details are required for a package to be redistributed with GAP.

The command ValidatePackageInfo (76.3-13) can be used to get a quick idea about which fields are missing:

gap> ValidatePackageInfo("PackageInfo.g");
#E  component `Subtitle' must be bound to a string
#E  component `Date' must be bound to a string of the form `dd/mm/yyyy'
#E  component `ArchiveURL' must be bound to a string started with http://, https:// or ftp://
#E  component `ArchiveFormats' must be bound to a string
#E  component `Status' must be bound to one of "accepted", "deposited", "dev", "other"
#E  component `README_URL' must be bound to a string started with http://, https:// or ftp://
#E  component `PackageInfoURL' must be bound to a string started with http://, https:// or ftp://
#E  component `AbstractHTML' must be bound to a string
#E  component `PackageWWWHome' must be bound to a string started with http://, https:// or ftp://
#E  component `ArchiveURLSubset' must be bound to a list of strings denoting relative paths to readable files or directories
#E  component `HTMLStart' must be bound to a string denoting a relative path to a readable file
#E  component `PDFFile' must be bound to a string denoting a relative path to a readable file
#E  component `SixFile' must be bound to a string denoting a relative path to a readable file
#E  component `LongTitle' must be bound to a string
false

We suggest to create a PackageInfo.g file for your package by copying the one in the Example package, distributed with GAP, or using the PackageMaker (https://github.com/gap-system/PackageMaker), and then adjusting it for your package. Within GAP you can look at this template file for a list and explanation of all recognised entries by

Pager(StringFile(Filename(DirectoriesLibrary(), 
                          "../pkg/example/PackageInfo.g")));

Instead of populating the rest of the PackageInfo.g by hands, you can also create a basic GAP package with the help of the tool called PackageMaker, available at https://github.com/gap-system/PackageMaker. The PackageMaker asks several questions about the intended package and then creates a new directory for it and populates it with all the files needed for a basic package.

76.10 Functions and Variables and Choices of Their Names

In writing the GAP code for your package you need to be a little careful on just how you define your functions and variables.

Firstly, in general one should avoid defining functions and variables via assignment statements in the way you would interactively, e.g.

gap> Squared := x -> x^2;;
gap> Cubed := function(x) return x^3; end;;

The reason for this is that such functions and variables are easily overwritten and what's more you are not warned about it when it happens.

To protect a function or variable against overwriting there is the function BindGlobal (4.9-8), or alternatively (and equivalently) you may define a global function via a DeclareGlobalFunction (79.18-14) and InstallGlobalFunction (79.18-14) pair or a global variable via a DeclareGlobalVariable (79.18-15) and InstallValue (79.18-16) pair. There are also operations and their methods, and related objects like attributes and filters which also have Declare... and Install... pairs.

Secondly, it's a good idea to reduce the chance of accidental overwriting by choosing names for your functions and variables that begin with a string that identifies it with the package, e.g. some of the undocumented functions in the Example package begin with Eg. This is especially important in cases where you actually want the user to be able to change the value of a function or variable defined by your package, for which you have used direct assignments (for which the user will receive no warning if she accidentally overwrites them). It's also important for functions and variables defined via BindGlobal, DeclareGlobalFunction/InstallGlobalFunction and DeclareGlobalVariable/InstallValue, in order to avoid name clashes that may occur with (extensions of) the GAP library and other packages.

Additionally, since GAP 4.5 a package may place global variables into a local namespace as explained in 4.10 in order to avoid name clashes and preserve compatibility. This new feature allows you to define in your package global variables with the identifier ending with the @ symbol, e.g. xYz@. Such variables may be used in your package code safely, as they may be accessed from outside the package only by their full name, i.e. xYz@YourPackageName. This helps to prevent clashes between different packages or between a package and the GAP library because of the same variable names.

On the other hand, operations and their methods (defined via DeclareOperation (79.18-12), InstallMethod (78.2-1) etc. pairs) and their relatives do not need this consideration, as they avoid name clashes by allowing for more than one "method" for the same-named object.

To demonstrate the definition of a function via a DeclareOperation/InstallMethod pair, the method Recipe (Example: Recipe) was included in the Example package; Recipe( FruitCake ); gives a "method" for making a fruit cake (forgive the pun).

Thirdly, functions or variables with SetXXX or HasXXX names (even if they are defined as operations) should be avoided as these may clash with objects associated with attributes or properties (attributes and properties XXX declared via the DeclareAttribute and DeclareProperty commands have associated with them testers of form HasXXX and setters of form SetXXX).

Fourthly, it is a good idea to have some convention for internal functions and variables (i.e. the functions and variables you don't intend for the user to use). For example, they might be entirely CAPITALISED.

Additionally, there is a recommended naming convention that the GAP core system and GAP packages should not use global variables starting in the lowercase. This allows to reserve variables with names starting in lowercase to the GAP user so they will never clash with the system. It is extremely important to avoid using for package global variables very short names started in lowercase. For example, such names like cs, exp, ngens, pc, pow which are perfectly fine for local variables, should never be used for globals. Additionally, the package must not have writable global variables with very short names even if they are starting in uppercase, for example, C1 or ORB, since they also could be easily overwritten by the user.

It is a good practice to follow naming conventions used in GAP as explained in 5.6 and Tutorial: Changing the Structure, which might help users to memorize or even guess names of functions provided by the package.

Finally, note the advantage of using DeclareGlobalFunction/InstallGlobalFunction, DeclareGlobalVariable/InstallValue, etc. pairs (rather than BindGlobal) to define functions and variables, which allow the package author to organise her function- and variable- definitions in any order without worrying about any interdependence. The Declare... statements should go in files with .gd extensions and be loaded by ReadPackage statements in the package init.g file, and the Install... definitions should go in files with .gi extensions and be loaded by ReadPackage statements in the package read.g file; this ensures that the .gi files are read after the .gd files. All other package code should go in .g files (other than the init.g and read.g files themselves) and be loaded via ReadPackage statements in the init.g file.

In conclusion, here is some practical advice on how to check which variables are used by the package.

Firstly, there is a function ShowPackageVariables (76.3-14). If the package pkgname is available but not yet loaded then ShowPackageVariables( pkgname ) prints a list of global variables that become bound and of methods that become installed when the package is loaded (for that, the package will be actually loaded, so ShowPackageVariables can be called only once for the same package in the same GAP session.) The second optional argument version may specify a particular package version to be loaded. An error message will be printed if (the given version of) the package is not available or already loaded.

Info lines for undocumented variables will be marked with an asterisk *. Note that the GAP help system is case insensitive, so it is difficult to document identifiers that differ only by case.

The following entries are omitted from the list: default setter methods for attributes and properties that are declared in the package, and Setattr and Hasattr type variables where attr is an attribute or property.

For example, for the Example package it may produce the output looking like this:

gap> ShowPackageVariables("example");
----------------------------------------------------------------
Loading  Example 3.3 (Example/Template of a GAP Package)
by Werner Nickel (http://www.mathematik.tu-darmstadt.de/~nickel),
   Greg Gamble (http://www.math.rwth-aachen.de/~Greg.Gamble), and
   Alexander Konovalov (http://www.cs.st-andrews.ac.uk/~alexk/).
----------------------------------------------------------------
new global functions:
  EgSeparatedString( str, c )*
  FindFile( dir, file )
  HelloWorld(  )
  ListDirectory( arg )
  LoadedPackages(  )
  WhereIsPkgProgram( prg )
  Which( prg )

new global variables:
  FruitCake

new operations:
  Recipe( arg )

new methods:
  Recipe( cake )

Another trick is to start GAP with -r -A options, immediately load your package and then call NamesUserGVars (4.9-11) which returns a list of the global variable names created since the library was read, to which a value is currently bound. For example, for the Example it produces

gap> NamesUserGVars();
[ "EgSeparatedString", "FindFile", "FruitCake", "HelloWorld", "ListDirectory",
  "LoadedPackages", "Recipe", "WhereIsPkgProgram", "Which" ]

but for packages with dependencies it will also contain variables created by other packages. Nevertheless, it may be a useful check to search for unwanted variables appearing after package loading. A potentially dangerous situation which should be avoided is when the package uses some simply named temporary variables at the loading stage. Such "phantom" variables may then remain unnoticed and, as a result, there will be no warnings if the user occasionally uses the same name as a local variable name in a function. Even more dangerous is the case when the user variable with the same name already exists before the package is loaded so it will be silently overwritten.

76.11 Package Dependencies (Requesting one GAP Package from within Another)

It is possible for one GAP package A, say, to require another package B. For that, one simply adds the name and the (least) version number of the package B to the NeededOtherPackages component of the Dependencies component of the PackageInfo.g file of the package A. In this situation, loading the package A forces that also the package B is loaded, and that A cannot be loaded if B is not available.

If B is not essential for A but should be loaded if it is available (for example because B provides some improvements of the main system that are useful for A) then the name and the (least) version number of B should be added to the SuggestedOtherPackages component of the Dependencies component of the PackageInfo.g file of A. In this situation, loading A forces an attempt to load also B, but A is loaded even if B is not available.

Also the component Dependencies.OtherPackagesLoadedInAdvance in PackageInfo.g is supported, which describes needed packages that shall be loaded before the current package is loaded. See 76.12 for details about this and more generally about the order in which the files of the packages in question are read.

All package dependencies must be documented explicitly in the PackageInfo.g file. It is important to properly identify package dependencies and make the right decision whether the other package should be "needed" or "suggested". For example, declaring package as "needed" when "suggested" might be sufficient may prevent loading of packages under Windows for no good reason.

It is not appropriate to explicitly call LoadPackage (76.2-1) when the package is loaded, since this may distort the order of package loading and result in warning messages. It is recommended to turn such dependencies into needed or suggested packages. For example, a package can be designed in such a way that it can be loaded with restricted functionality if another package (or standalone program) is missing, and in this case the missing package (or binary) is suggested. Alternatively, if the package author decides that loading the package in this situation makes no sense, then the missing component is needed.

On the other hand, if LoadPackage (76.2-1) is called inside functions of the package then there is no such problem, provided that these functions are called only after the package has been loaded, so it is not necessary to specify the other package as suggested. The same applies to test files and manual examples, which may be simply extended by calls to LoadPackage (76.2-1).

It may happen that a package B that is listed as a suggested package of package A is actually needed by A. If no explicit LoadPackage (76.2-1) calls for B occur in A at loading time, this can now be detected using the new possibility to load a package without loading its suggested packages using the global option OnlyNeeded which can be used to (recursively) suppress loading the suggested packages of the package in question. Using this option, one can check whether errors or warnings appear when B is not available (note that this option should be used only for such checks to simulate the situation when package B is not available; it is not supposed to be used in an actual GAP session when package B will be loaded later, since this may cause problems). In case of any errors or warnings, their consequence can then be either turning B into a needed package or (since apparently B was not intended to become a needed package) changing the code accordingly. Only if package A calls LoadPackage (76.2-1) for B at loading time (see above) then package B needs to be deinstalled (i.e. removed) to test loading of A without B.

Finally, if the package manual is in the GAPDoc format, then GAPDoc should still be listed as needed for this package (not as suggested), even though GAPDoc is now a needed package for GAP itself.

76.12 Declaration and Implementation Part of a Package

When GAP packages require each other in a circular way, a "bootstrapping" problem arises of defining functions before they are called. The same problem occurs in the GAP library, and it is resolved there by separating declarations (which define global variables such as filters and operations) and implementations (which install global functions and methods) in different files. Any implementation file may use global variables defined in any declaration file. GAP initially reads all declaration files (in the library they have a .gd suffix) and afterwards reads all implementation files (which have a .gi suffix).

Something similar is possible for GAP packages: if a file read.g exists in the home directory of the package, this file is read only after all the init.g files of all (implicitly) required GAP packages are read. Thus one can separate declaration and implementation for a GAP package in the same way as is done for the GAP library, by creating a file read.g, restricting the ReadPackage (76.3-1) statements in init.g to only read those files of the package that provide declarations, and to read the implementation files from read.g.

Examples:

Suppose that there are two packages A and B, each with files init.g and read.g.

In general, when GAP is asked to load a package then first the dependencies between this packages and its needed and suggested packages are inspected (recursively), and a list of package sets is computed such that no cyclic dependencies occur between different package sets and such that no package in any of the package sets needs any package in later package sets. Then GAP runs through the package sets and reads for each set first all init.g files and then all read.g files of the packages in the set. (There is one exception from this rule: Whenever packages are autoloaded before the implementation part of the GAP library is read, only the init.g files of the packages are read; as soon as the GAP library has been read, the read.g files of these packages are also read, and afterwards the above rule holds.)

It can happen that some code of a package depends on the availability of suggested packages, i.e., different initialisations are performed depending on whether a suggested package will eventually be loaded or not. One can test this condition with the function IsPackageMarkedForLoading (76.3-8). In particular, one should not call (and use the value returned by this call) the function LoadPackage (76.2-1) inside package code that is read during package loading. Note that for debugging purposes loading suggested packages may have been deliberately disabled via the global option OnlyNeeded.

Note that the separation of the GAP code of packages into declaration part and implementation part does in general not allow one to actually call functions from a package when the implementation part is read. For example, in the case of a "cyclic dependency" as in the second example above, suppose that B provides a new function f or a new global record r, say, which are declared in the declaration part of B. Then the code in the implementation part of A may contain calls to the functions defined in the declaration part of B. However, the implementation part of A may be read before the implementation part of B. So one can in general not assume that during the loading of A, the function f can be called, or that one can access components of the record r.

If one wants to call the function f or to access components of the record r in the code of the package A then the problem is that it may be not possible to determine a cyclic dependency between A and B from the packages A and B alone. A safe solution is then to add the name of B to the component Dependencies.OtherPackagesLoadedInAdvance of the PackageInfo.g file of A. The effect is that package B is completely loaded before the file read.g of A is read, provided that there is no cyclic dependency between A and B, and that package A is regarded as not available in the case that such a cyclic dependency between A and B exists.

A special case where Dependencies.OtherPackagesLoadedInAdvance can be useful is that a package wants to force the complete GAP library to be read before the file read.g of the package A is read. In this situation, the "package name" "gap" should be added to this component in the PackageInfo.g file of A.

In the case of cyclic dependencies, one solution for the above problem might be to delay those computations (typically initialisations) in package A that require package B to be loaded until all required packages are completely loaded. This can be done by moving the declaration and implementation of the variables that are created in the initialisation into a separate file and to declare these variables in the init.g file of the package, via a call to DeclareAutoreadableVariables (76.3-9) (see also 76.13).

76.13 Autoreadable Variables

Package files containing method installations must be read when the package is loaded. For package files not containing method installations (this applies, for example, to many data files) another mechanism allows one to delay reading such files until the data are actually accessed. See 76.3-9 for further details.

76.14 Standalone Programs in a GAP Package

GAP packages that involve stand-alone programs are fundamentally different from GAP packages that consist entirely of GAP code.

This difference is threefold: A user who installs the GAP package must also compile (or install) the package's binaries, the package must check whether the binaries are indeed available, and finally the GAP code of the package has to start the external binary and to communicate with it. We will cover these three points in the following sections.

If the package does not solely consist of an interface to an external binary and if the external program called is not just special-purpose code, but a generally available program, chances are high that sooner or later other GAP packages might also require this program. We therefore strongly recommend the provision of a documented GAP function that will call the external binary. We also suggest to create actually two GAP packages; the first providing only the binary and the interface and the second (requiring the first, see 76.11) being the actual GAP package.

76.14-1 Installation of GAP Package Binaries

The scheme for the installation of package binaries which is described further on is intended to permit the installation on different architectures which share a common file system (and share the architecture independent file).

A GAP package which includes external binaries contains a bin subdirectory. This subdirectory in turn contains subdirectories for the different architectures on which the GAP package binaries are installed. The names of these directories must be the same as the names of the architecture dependent subdirectories of the main bin directory. Unless you use a tool like autoconf yourself, you must obtain the correct name of the binary directory from the main GAP branch. To help with this, the main GAP directory contains a file sysinfo.gap which assigns the shell variable GAParch to the proper name as determined by GAP's configure process. For example on a Linux system, the file sysinfo.gap may look like this:

GAParch=i586-unknown-linux2.0.31-gcc

We suggest that your GAP package contains a file configure which is called with the path of the GAP root directory as parameter. This file then will read sysinfo.gap and set up everything for compiling under the given architecture (for example creating a Makefile from Makefile.in). As initial templates, you may use installation scripts of the Example package or files generated with the help of PackageMaker.

76.14-2 Test for the Existence of GAP Package Binaries

If an external binary is essential for the workings of a GAP package, the function stored in the component AvailabilityTest of the PackageInfo.g file of the package should test whether the program has been compiled on the architecture (and inhibit package loading if this is not the case). This is especially important if the package is loaded automatically.

The easiest way to accomplish this is to use Filename (9.4-1) for checking for the actual binaries in the path given by DirectoriesPackagePrograms (76.3-6) for the respective package. For example the example GAP package could use the following function to test whether the binary hello has been compiled; it will issue a warning if not, and will only load the package if the binary is indeed available:

...
AvailabilityTest := function()
  local path,file;
    # test for existence of the compiled binary
    path:= DirectoriesPackagePrograms( "example" );
    file:= Filename( path, "hello" );
    if file = fail then
      LogPackageLoadingMessage( PACKAGE_WARNING,
          [ "The program `hello' is not compiled,",
            "`HelloWorld()' is thus unavailable.",
            "See the installation instructions;",
            "type: ?Installing the Example package" ] );
    fi;
    return file <> fail;
  end,
...

However, if you look at the actual PackageInfo.g file of the example package, you will see that its AvailabilityTest function always returns true, and just logs the warning if the binary is not available (which may be later viewed with DisplayPackageLoadingLog (76.2-5)). This means that the binary is not regarded as essential for this package.

You might also have to cope with the situation that external binaries will only run under UNIX (and not, say, under Windows), or may not compile with some compilers or default compiler options. See 3.4 for information on how to test for the architecture.

Last but not least: do not print anything in the AvailabilityTest function of the package via Print or Info. Instead one should call LogPackageLoadingMessage (76.2-5) to store a message which may be viewed later with DisplayPackageLoadingLog (76.2-5) (the latter two functions have been introduced in GAP 4.5)

76.14-3 Calling of and Communication with External Binaries

There are two reasons for this: the input data has to be passed on to the stand-alone program and the stand-alone program has to be started from within GAP.

There are two principal ways of doing this.

The first possibility is to write all the data for the stand-alone to one or several files, then start the stand-alone with Process (11.1-1) or Exec (11.1-2) which then writes the output data to file, and finally read in the standalone's output file.

The second way is interfacing via input-output streams, see Section 10.8.

Some GAP packages use kernel modules (see 76.3-10) instead of external binaries. A kernel module is implemented in C and follows certain conventions to comply with the GAP kernel interface, which we plan to document later. In the meantime, we advise you to look at existing examples of such packages and get in touch with GAP developers if you plan to develop such a package.

76.15 Having an InfoClass

It is a good idea to declare an InfoClass for your package. This gives the package user the opportunity to control the verbosity of output and/or the possibility of receiving debugging information (see 7.4). Below, we give a quick overview of its utility.

An InfoClass is defined with a DeclareInfoClass( InfoPkgname ); statement and may be set to have an initial InfoLevel other than the zero default (which means no Info statement is to output information) via a SetInfoLevel( InfoPkgname, level ); statement. An initial InfoLevel of 1 is typical.

Info statements have the form: Info( InfoPkgname, level, expr1, expr2, ...); where the expression list expr1, expr2, ... appears just like it would in a Print statement. The only difference is that the expression list is only printed (or even executed) if the InfoLevel of InfoPkgname is at least level.

76.16 The Banner

When the package is loaded, GAP will display a default package banner, constructed from the package metadata provided in the PackageInfo.g file.

Alternatively, the package may establish its own banner by assigning a string to the BannerString field of the record argument of SetPackageInfo in the PackageInfo.g file.

If you will be designing a banner for your package, it is a good idea to suggest there how to access package documentation. For example, the banner of the Example package says:

For help, type: ?Example package

In order for this to display the introduction of the Example package the index-entry <Index>Example package</Index> was added just before the first paragraph of the introductory section in the file doc/example.xml of the Example package.

76.17 Version Numbers

Version numbers are strings containing nonnegative integers separated by non-numeric characters. They are compared by CompareVersionNumbers (76.3-7) which first splits them at non-digit characters and then lexicographically compares the resulting integer lists. Thus version "2-3" is larger than version "2-2-5" but smaller than "4r2p3" or "11.0".

It is possible for code to require GAP packages in certain versions. In this case, all versions, whose number is equal or larger than the requested number are acceptable. It is the task of the package author to provide upwards compatibility.

Loading a specific version of a package (that is, not one with a larger version number) can be achieved by prepending = to the desired version number. For example, LoadPackage( "example", "=1.0" ) will load version "1.0" of the package "example", even if version "1.1" is available. As a consequence, version numbers must not start with =, so "=1.0" is not a valid version number.

Package authors should choose a version numbering scheme that admits a new version number even after tiny changes to the package, and ensure that version numbers of successive package versions increase. The automatic update of package archives in the GAP distribution will only work if a package has a new version number.

It is a well-established custom to name package archives like name-version.tar.gz, name-version.tar.bz2 etc., where name is the lower case name, and version is the version (another custom is that the archive then should extract to a directory that has exactly the name name-version).

It is very important that there should not ever be, for a given GAP package, two different archives with the same package version number. If you make changes to your package and place a new archive of the package onto the public server, please ensure that a new archive has a new version number. This should be done even for very minor changes.

For most of the packages it will be inappropriate to re-use the date of the release as a version number. It's much more obvious how big are the changes between versions "4.4.12", "4.5.1" and "4.5.2" than between versions "2008.12.17", "2011.04.15" and "2011.09.14". The concept of using version numbers to convey the meaning of the status of the code and the way it has been modified is known as "Semantic Versioning", see http://semver.org/ for further recommendations on its use.

Since version information is duplicated in several places throughout the package documentation, for GAPDoc-based manuals you may define the version and the release manual in the comments in PackageInfo.g file close to the place where you specified its Version and Date components, for example

##  <#GAPDoc Label="PKGVERSIONDATA">
##  <!ENTITY VERSION "3.3">
##  <!ENTITY RELEASEDATE "12/09/2017">
##  <!ENTITY RELEASEYEAR "2017">
##  <#/GAPDoc>

notify MakeGAPDocDoc (GAPDoc: MakeGAPDocDoc) that a part of the document is stored in PackageInfo.g (see example/makedocrel.g), read this data into the header of the main document via <#Include Label="PKGVERSIONDATA"> directive and then use them via &VERSION; and &RELEASEDATE; entities almost everywhere where you need to refer to them (most commonly, in the title page and installation instructions).

76.18 Testing a GAP package

There are several aspects of testing a GAP package.

First, one should ensure that the package functionality works as expected. Below we give an advice on creating test files for automated tests that may be run by package authors, by GAP developers as part of the release preparation, and by package users interested in checking that the package works. Such tests should be included in the package distribution, and the responsibility for ensuring that they pass stays with package authors.

Second, the package should cleanly integrate into the GAP system and other packages, and should not break their functionality. In particular, all tests from the standard GAP testing suite should pass if the package is loaded. This is more comprehensive and time consuming test, which GAP developers regularly run using special tools. They will report to you any detected issues. Below we explain how to do several simple and less time consuming checks which package authors are recommended to perform themselves.

76.18-1 Tests files for a GAP package

The (optional) tst directory of your package may contain as many tests of the package functionality as appears appropriate. These tests should be organised into test files similarly to those in the tst directory of the GAP distribution as documented in 7.10.

For a deposited package, a test file with a basic test of the package (for example, to check that it works as expected and/or that the manual examples are correct) may be specified in the component TestFile in the PackageInfo.g to be included in the GAP standard test suite. This file can either consist of calls of TestDirectory (7.10-3) or Test (7.10-2) (in this case, it is common to call it testall.g) or be itself a test file having an extension .tst and supposed to be read via Test (7.10-2). It is assumed that the latter case occurs if and only if the file contains the substring

 "gap> START_TEST("

(with exactly one space after the GAP prompt).

For deposited packages, these tests are run by the GAP Group regularly, as a part of the standard GAP test suite. For the efficient testing it is important that the test specified in the PackageInfo.g file does not display any output (e.g. no test progress indicators) except reporting discrepancies if such occur and the completion report as in the example below:

gap> Test("tst/testall.tst");
Example package: testall.tst
GAP4stones: 3333
true

Tests which produce extended output and/or require substantial runtime are not supposed to be a part of the GAP standard test suite but may be placed in the tst directory of the packages with further instructions on how to run them elsewhere.

Because of different approaches to testing, used by different packages, it is not always easy to identify whether an automated test passed or failed. Presently, automated detection works fine if a package uses a single .tst file or uses TestDirectory (7.10-3) to run a collection of tests. Otherwise, one should follow the convention to output a string of a fixed format to indicate the result of the test in the test script along the following lines:

if testresult then
  Print("#I  No errors detected while testing"\n");
else
  Print("#I  Errors detected while testing\n");
fi;

76.18-2 Testing GAP package loading

To test that your package may be loaded into GAP without any problems and conflicts with other packages, test that it may be loaded in various configurations:

The test of loading all packages is the most subtle one. Quite often it reveals problems which do not occur in the default configuration but may cause difficulties to the users of specialised packages.

For your convenience, make testpackagesload called in the GAP root directory will run all package loading tests listed in this subsection and write their output in its dev/log directory.

It will produce four files with test logs, corresponding to the four cases above (the letter N in the filename stands for "needed", A stands for "autoloaded"):

Each file contains small sections for loading individual packages: among those, you need to find the section related to your package and may ignore all other sections. For example, the section for the Example package looks like

%%% Loading example 3.3.3
[  ]
### Loaded example 3.3.3

so it has clearly passed the test. If there are any error messages displayed between Loading ... and Loaded ... lines, they will signal on errors during loading of your package.

Additionally, this test collects information about variables created since the library was read (obtained using NamesUserGVars (4.9-11)) with either short names (no more than three characters) or names breaking a recommended naming convention that the GAP core system and GAP packages should not use global variables starting in the lowercase (see Section 76.10). Their list will be displayed in the test log (in the example above, Example packages does not create any such variables, so an empty list is displayed). It may be hard to attribute a particular identifier to a package, since it may be created by another package loaded because of dependencies, so when a more detailed and precise report on package variables is needed, it may be obtained using ShowPackageVariables (76.3-14) (also, make testpackagesvars called in the GAP root directory produces such reports for each package and writes them to a file dev/log/testpackagesvars_...).

Finally, each log file finishes with two large sections for loading all packages in the alphabetical and reverse alphabetical order (to check more combinations of loading one package after another). We are aiming at releasing only collections of package which do not break LoadAllPackages (76.18-3) in any of the four configurations, so if it is broken when you plug in the development version of your package into the released version of GAP, it is likely that your package triggers this error. If you observe that LoadAllPackages (76.18-3) is broken and suspect that this is not the fault of your package, please contact the GAP Support.

76.18-3 LoadAllPackages
‣ LoadAllPackages( : reversed )( function )

loads all GAP packages from their list sorted in alphabetical order (needed and suggested packages will be loaded when required). This is a technical function to check packages compatibility, so it should NOT be used to run anything except tests; it is known that GAP performance is slower if all packages are loaded. To introduce some variations of the order in which packages will be loaded for testing purposes, LoadAllPackages accepts option reversed to load packages from their list sorted in the reverse alphabetical order.

76.18-4 Testing a GAP package with the GAP standard test suite

The tst directory of the GAP installation contains a selection of test files and scripts such as testinstall.g and teststandard.g which are a part of the GAP standard test suite.

It is important to check that your package does not break GAP standard tests. To perform a clean test and avoid interfering with other packages, first you must start a new GAP session and then read either testinstall.g or teststandard.g as demonstrated below.

The quicker test, testinstall.g, requires about 1 GB of memory and runs for several minutes. It may be started with the command

gap> Read( Filename( DirectoriesLibrary( "tst" ), "testinstall.g" ) );

You will get a large number of lines with output about the progress of the tests, for example:

You should start GAP4 using `gap -A -x 80 -r -m 100m -o 1g -K 2g'.

Architecture: x86_64-apple-darwin15.6.0-gcc-6-default64

testing: /Users/alexk/GITREPS/gap/tst/testinstall/alghom.tst
     140 msec for alghom.tst        
testing: /Users/alexk/GITREPS/gap/tst/testinstall/algmat.tst
    1309 msec for algmat.tst        
testing: /Users/alexk/GITREPS/gap/tst/testinstall/algsc.tst
     500 msec for algsc.tst         
... further lines deleted ...

(optionally, you may start GAP with the command line options which you will see in the test output, to run it in a more conservative settings).

The more thorough test is teststandard.g which exercises more of GAP's capabilities, also including all test files from teststandard.g. It requires about 1 GB of memory, runs for about one hour, and produces an output similar to the testinstall.g test. To run it, also start a new GAP session and then call

gap> Read( Filename( DirectoriesLibrary( "tst" ), "testall.g" ) );

You may repeat the same check loading your package with OnlyNeeded option. Remember to perform each subsequent test in a new GAP session.

Also you may perform individual tests from the tst directory of the GAP installation loading them with Test (7.10-2).

76.19 Access to the GAP Development Version

We are aiming at providing a stable platform for package development and testing with official GAP releases. We also invite everyone to contribute by submitting patches, pull requests, and bug reports. We would like to make the contributing process as easy as possible.

The main GAP development repository is hosted on GitHub at https://github.com/gap-system/gap. Many GAP packages also have public repositories and issue trackers, and we are keeping a list of such packages at https://gap-packages.github.io/.

For further information about contributing to the GAP development, please see https://github.com/gap-system/gap/blob/master/CONTRIBUTING.md.

76.20 Version control and continuous integration for GAP packages

As we have mentioned above, many GAP packages have public repositories and issue trackers on GitHub, and we are keeping a list of such packages at https://gap-packages.github.io/. We welcome establishing public repositories for new packages and migrating existing package repositories there as well. Such repositories may be hosted under their authors' accounts or under the gap-packages organisation (https://github.com/gap-packages/). The latter has the benefit that while the authors will preserve their deciding role on all aspects of the package development, the package will become more visible for potential collaborators and GAP developers may help to set up continuous integration for your package so that every commit to the repository will trigger automated running of package tests and reporting any failures to package maintainers.

76.21 Selecting a license for a GAP Package

As it was mentioned in the description of the LICENSE file in Section 76.5, it is advised to make clear in the documentation of the package the basis on which it is being distributed to users. GAP itself is distributed under the GNU Public License version 2 (version 2 or later). We would encourage you to consider the GPL license for your packages, but you might wish to be more restrictive (for instance forbidding redistribution for profit) or less restrictive (allowing your software to be incorporated into commercial software). See "Choosing a License for the Distribution of Your Package" from http://www.gap-system.org/Packages/Authors/authors.html and also https://choosealicense.com/ for further details.

In the past many GAP packages used the text "We adopt the copyright regulations of GAP as detailed in the copyright notice in the GAP manual" or a similar statement. We now advise to be more explicit by making the exact reference to the GPL license, for example:

packagename is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. and also including a copy of the full text of the license.

76.22 Releasing a GAP Package

Currently, the GAP distribution provides archives in four different formats.

.tar.gz

a standard UNIX tar archive, compressed with gzip

.tar.bz2

a standard UNIX tar archive, compressed with bzip2

.zip

an archive in zip format, where text files should have UNIX style line breaks

-win.zip

an archive in zip format, where text files should have Windows style line breaks

For convenience of possible users it is sensible that you provide an archive of your package in at least one of these formats.

For example, if you wish to supply a .tar.gz archive, you may create it with the command

 tar -cvzf packagename-version.tar.gz packagename

Because the release of the GAP package is independent of the version of GAP, a GAP package should be wrapped up in separate file that can be installed onto any version of GAP. In this way, a package can be upgraded any time without the need to wait for new GAP releases. To ensure this, the package should be archived from the GAP pkg directory, that is all files are archived with the path starting at the package's name.

The archive of a GAP package should contain all files necessary for the package to work. In particular there should be a compiled documentation, which includes the manual.six, manual.toc and manual.lab file in the documentation subdirectory which are created by GAPDoc while TeXing the documentation. (The first two files are needed by the GAP help system, and the manual.lab file is needed if the main manuals or another package is referring to your package. Use the command GAPDocManualLab( packagename ); to create this file for your help books if you use GAPDoc.)

For packages which are redistributed via the GAP website, we offer an automatic conversion of any of the formats listed above to all the others (note that this, as well as wrapping the GAP distribution as a single archive containing the core system and all currently redistributed packages, will change file timestamps, so one should not rely on them anywhere in the package).

To use the conversion and repackaging service, you can provide any of the four archive formats or even more than one, however you should adhere to the following rule: text files in .tar.gz and .tar.bz2 archives must have UNIX style line breaks, while text files in -win.zip archives must have DOS/Windows line breaks.

The package wrapping tools for the GAP distribution and webpages then will use a sensible list of file extensions to decide if a file is a text file (being conservative, it may miss a few text files). These rules may be prepended by the application of rules from the PackageInfo.g file:

Utility functions from the lib/lbutil.g file in GAP, namely DosUnixLinebreaks, UnixDosLinebreaks, MacUnixLinebreaks may be helpful. They are described in the comments to their source code.

For packages hosted on GitHub publishing package release and establishing its website can be very efficiently automated using two tools: ReleaseTools (https://github.com/gap-system/ReleaseTools) and GitHubPagesForGAP (https://github.com/gap-system/GitHubPagesForGAP).

76.23 The homepage of a Package

If you want to distribute your package you should create its homepage containing some basic information, archives for download, the README file with installation instructions, and a copy of the package's PackageInfo.g file.

The responsibility to maintain this homepage is with the package authors/maintainers.

If you tell the GAP Group about your package (say, by mail to support@gap-system.org) we may consider either

In the latter case we can also provide some services for producing several archive formats from the archive you provide (e.g., you produce a .tar.gz version of your archive and we produce also a .tar.bz2, .zip and a -win.zip version from it).

Please also consider submitting your package to the GAP package refereeing process (see http://www.gap-system.org/Contacts/submit.html for further information).

For packages hosted on GitHub publishing package release and establishing its website can be very efficiently automated using two tools: GitHubPagesForGAP (https://github.com/gap-system/GitHubPagesForGAP) and ReleaseTools (https://github.com/gap-system/ReleaseTools).

76.24 Some things to keep in mind

76.25 Package release checklists

The following checklists should be useful to package authors and maintainers, as well as to everyone involved in the depositing and refereeing of GAP packages.

76.25-1 Checklist for releasing a new package

76.25-2 Checklist for upgrading the package for the next major release of GAP

GAP ecosystem is not static: both the core GAP system and packages redistributed with GAP are in constant development. GAP has a policy that changes that may have a disruptive effect on packages redistributed with GAP should only be introduced in major GAP releases. When the next GAP major release is prepared, a beta version for package authors will be made available in order to give them an opportunity to check and update, if necessary, their packages for the public release of the next major version of GAP.

The following checklist will help you to check how well your package is ready to work with the next major release of GAP

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