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# A Hints for writing a GAP Package

### Sections

1. Structure of a GAP Package
2. Documentation Software Tools Needed
3. Functions and Variables and Choices of Their Names
4. Having an InfoClass
5. Having a Banner
6. Packing up your GAP Package
7. CVS

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.

## A.1 Structure of a GAP Package

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 be contain only lowercase characters may change in the future); this is so that a user may load the package-name package by calling the `RequirePackage` command (which lowercases its first argument; see RequirePackage in the Reference Manual) with either package-name or package-dir (in doublequotes).

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`
`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`.

`init.g`, `read.g`
  A GAP package must have a file `init.g` (see Sections The Structure of a GAP Package and The Init File of a GAP Package in the Reference Manual; the latter section describes what `init.g` should contain). 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 `ReadPkg` 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 `ReadPkg` 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 GAP 4 Programming Reference Manual. There is now an alternative XML-based system provided by the GAP package GAPDoc (see gapdoc:Title page). 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 `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 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 describe 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 `tools` XXX`.zoo` for some version number XXX at the GAP `ftp`- or web-sites, or can be obtained by emailing gap-trouble@dcs.st-and.ac.uk).

`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. This is done by starting GAP (at least GAP 4.3) 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 `tools` XXX`.zoo` for some version number XXX at the GAP `ftp`- or web-sites, or can be obtained by emailing gap-trouble@dcs.st-and.ac.uk.

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 GAP 4 Programming Reference 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 `Set` XXX`` or `Has` XXX`` 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 `Has` XXX`` and setters of form `Set` XXX``).

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 `ReadPkg` statements in the package `init.g` file, and the `Install...` definitions should go in files with `.gi` extensions and be loaded by `ReadPkg` 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 `ReadPkg` 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 Having a Banner

Typically a banner for a package is produced by a sequence of `Print` statements in a file named `banner.g` which is loaded via a `ReadPkg` statement in the package `init.g` file. The execution of the printing of the banner should be nested inside `if not QUIET and BANNER then ... fi;` (see the Example package `init.g` file) so that the banner is suppressed during package autoloading and when GAP is started up with the `-q` or `-b` command line options.

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

Ideally, your GAP package should be packed via the `zoo` program (Section Wrapping Up a GAP Package in the ``Extending GAP'' Manual contains more information). The Example package file `make_zoo` provides a template packing-up script that uses `zoo`. The `etc` directory obtained from `tools` XXX`.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 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 gap-trouble@dcs.st-and.ac.uk.

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