[Example] A Hints for writing a GAP Package

Sections

The Example package is intended to be a prototype for a package. Here we describe just what features one should emulate when writing one's own GAP package for popular consumption, and a few pointers as to where to go for more information. Much of what is written here is amplified in the section Writing a GAP Package in the Extending GAP Manual.

A.1 Structure of a GAP Package

This section is intended to amplify the recommendations made in Section The Files of a GAP Package of the Extending GAP Manual.

A GAP package should have an alphanumeric name (package-name, say); mixed case is fine, but there should be no whitespace. The directory package-dir containing the files of package package-name should be just package-name converted to lowercase (the restriction that package-dir must contain only lowercase characters may change in the future).

The directory package-dir should be a subdirectory of pkg and preferably should have the following structure (below, a trailing / distinguishes directories from ordinary files):

package-dir/
README
configure
Makefile.in
PackageInfo.g
init.g
read.g
doc/
lib/
src/

We now describe the above files and directories:

README: This should contain ``how to get it'' (from the GAP ftp- and web-sites) instructions, as well as installation instructions and names of the package authors and their email addresses. The installation instructions and authors' names and addresses should be repeated in the package's documentation (which should be in the doc directory).
configure, Makefile.in: These files are only necessary if the package has 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. 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 a subdirectory of bin with name 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: Since GAP 4.4, a GAP package must have a PackageInfo.g file. The Example package's PackageInfo.g file is well-commented and should be used as a prototype.
init.g, read.g: A GAP package must have a file init.g (see Section The Files of a GAP Package in the Extending GAP Manual). As of GAP 4.4, the typical init.g and read.g files should normally consist entirely of ReadPackage (see ReadPackage in the GAP 4 Reference Manual) commands (and possibly also Read commands). 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. 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. Traditionally, a TeX-based system has been used for GAP documentation, which is thoroughly described in Section The gapmacro.tex Manual Format of the Extending GAP Manual. There is now an alternative XML-based system provided by the GAP package GAPDoc (see Chapter gapdoc:Introduction and Example of the GAPDoc Manual). Please spend some time reading the documentation for whichever system you decide to use for writing your package's documentation. The Example package's documentation was written using the traditional TeX-based system. If you plan on using this, please use the Example package's doc directory as a prototype, which you will observe contains the following files:
manual.tex # master file
chapi.tex # chapter file(s) ... 1 for each chapter
manual.mst # MakeIndex style file
make_doc # script that generates the manuals
: Generally, one should also provide a manual.bib BibTeX database file (or write one's own bibliography manual.bbl file). Generating the various formats of the manuals requires various software tools which are called directly or indirectly by make_doc and these are listed in Section Documentation Software Tools Needed. The file manual.mst is needed for generating a manual index; it should be a copy of the one provided in the Example package. The only adjustments that a package writer should need to make to make_doc is to replace occurrences of the word Example with package-name.
lib: This is the preferred place for the GAP code, i.e. the .g, .gd and .gi files (other than PackageInfo.g, init.g and read.g). For some packages (the Example package included), the directory gap has been used instead of lib; lib has the slight advantage that it is the default subdirectory of a package directory searched for by the DirectoriesPackageLibrary command (see DirectoriesPackageLibrary in the GAP 4 Reference Manual).
src: If the package has 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.

A.2 Documentation Software Tools Needed

Whether you use the traditional gapmacro.tex TeX-based system or GAPDoc you will need to have the various following TeX tools installed:

tex (or latex for GAPDoc), bibtex and makeindex: for generating .dvi;
dvips: for generating .ps; and
pdftex or gs and ps2pdf (or pdflatex for GAPDoc): for generating .pdf;

Note that using gs and ps2pdf in lieu of pdftex or pdflatex is a poor substitute unless your gs is at least version 6.xx for some xx.

The rest of this section describes the various additional tools needed for the gapmacro.tex documentation system.

To produce the .dvi, .ps and .pdf manual formats, the following GAP tools (usually located in GAP's main doc directory) are needed (provided by toolsXXX.zoo for some version number XXX at the GAP ftp- or web-sites, or can be obtained by emailing support@gap-system.org).

gapmacro.tex

The macros file that dictates the style and mark-up for the traditional TeX-based system of GAP documentation.

manualindex

This is an awk script that adjusts the TeX-produced index entries and calls makeindex to process them.

mrabbrev.bib

This is usually supplied with your TeX tools but nevertheless a copy of mrabbrev.bib should be located in GAP's main doc directory. To find it on your system, try:

kpsewhich mrabbrev.bib

or if that doesn't work and you can't otherwise find it check out a CTAN site, e.g. search for it at: http://www.dante.de/cgi-bin/ctan-index

If your manual cross-refers to other gapmacro.tex-produced manuals (and so has \UseReferences commands in its manual.tex), then a manual.lab file (generated by running tex manual) for each such other manual is needed (this includes the ``main'' manuals, e.g. those in the doc/ref, doc/ext etc. directories).

If your manual cross-refers to GAPDoc-produced manuals (and so has \UseGapDocReferences commands in its manual.tex), then manual.lab files need to be generated for these too. Since GAP 4.3, this is done by starting GAP and running:

gap> GapDocManualLab( "package" );

for each package whose manual is cross-referred to.

To produce an HTML version of the manual one needs the Perl 5 program convert.pl which is usually located in the subdirectory etc of the GAP root directory. The etc directory is not part of the usual GAP distribution. The etc directory files are obtained from toolsXXX.zoo for some version number XXX at the GAP ftp- or web-sites, or can be obtained by emailing support@gap-system.org.

Finally, to ensure the mathematics formulae are rendered as well as they can be in the HTML version, one should also have the program tth (TeX to HTML converter); convert.pl calls tth to translate mathmode formulae to HTML (if it's available). The tth program is easy to compile and can be obtained from http://hutchinson.belmont.ma.us/tth/tth-noncom/download.html

As a package author, you are not obliged to provide an HTML version of your package manual, but if you have kept to the guidelines in Section The gapmacro.tex Manual Format of the Extending GAP Manual, you should have no trouble in producing one. A prototype of the command to execute is in the file make_doc; note that the HTML manual is produced in files with .htm extensions in a directory htm outside the doc directory. The beginning of the file convert.pl contains instructions on its usage should you need them.

A.3 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> 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 command BindGlobal (see BindGlobal in the GAP Reference Manual), or alternatively (and equivalently) you may define a global function via a DeclareGlobalFunction and InstallGlobalFunction pair or a global variable via a DeclareGlobalVariable and InstallValue 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 haved 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. On the other hand, operations and their methods (defined via DeclareOperation, InstallMethod 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.

The method Recipe was included in the Example package to demonstrate the definition of a function via a DeclareOperation/InstallMethod pair; 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.

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.

A.4 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 Info functions in the GAP Reference Manual). 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.

A.5 The Banner

Since GAP 4.4, the package banner, if one is desired, should be provided by assigning a string to the BannerString field of the record argument of SetPackageInfo in the PackageInfo.g file.

It is a good idea to have a hook into your package documentation from your banner. The Example package suggests to the GAP user:

For help, type: ?Example package

In order for this to display the introduction of the Example package an \atindex equivalent of the following index-entry:

\index{Example package}

was added just before the first paragraph of the introductory section in the file example.tex. The Example package uses the gapmacro.tex system (see Section Documentation Software Tools Needed) for documentation (you will need some different scheme to achieve this using GAPDoc).

A.6 Packing up your GAP Package

In the past, it was recommended that your GAP package should be packed via the zoo program, but now any of four different archive formats are accepted (see Section Wrapping Up a GAP Package in the Extending GAP Manual for the details). The Example package file make_zoo provides a template packing-up script that uses zoo. The etc directory obtained from toolsXXX.zoo for some version number XXX (this is described above in Section Documentation Software Tools Needed) contains a file packpack which provides a more versatile packing-up script.

A.7 New versions of your GAP Package

You will notice that there is a file VERSION which contains the current version of the Example package. Such a file is entirely optional. Note that this file is not read at all when GAP loads the package. GAP establishes the package version by reading the PackageInfo.g file. The current maintainer of the Example package finds it convenient to have a file VERSION that is read both by doc/manual.tex and make_zoo. It is however important that each new version of a package has a new number and that version numbers of successive package versions increase (see Version Numbers in the Extending GAP Manual for the details).

It's also useful to have a CHANGES file that records the main changes between versions of your package.

A.8 CVS

When your package is ready to be refereed and/or made available as an ``accepted'' GAP package, it may be of benefit to obtain CVS access to GAP; as a first step towards this you should make a request to the GAP team via an email to support@gap-system.org.

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Example manual
Januar 2006