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Use of Global Variable Names

As the number of packages and library modules in GAP grows, it is important to try to avoid clashes where two independently developed pieces of code use the same global variable names in inconsistent ways. Note that "global variable names" include all names in the global namespace, not just those created with DeclareGlobalVariable. We therefore recommend the following guidelines for authors:

Firstly, new global variables should be introduced with one of the Declare... functions such as DeclareAttribute or DeclareGlobalVariable, or, if this not suitable for some reason, set using BindGlobal rather than simple assignment. (See section 79.18 of the GAP Reference Manual for more information on these function)
This will help ensure that any clashes cause error messages rather than wrong results.

After this, it is important to distinguish between documented names and undocumented names. Documented names are those that appear in the reference manual or the manual of the package that defines them, or which are, perhaps, intended to do so in the future. Undocumented names are all other global variables created by a module or package.

Concerning undocumented names, the first rule is not to create them unless you need them. For instance the common pattern

workerFunction := function(a,b,c,d)
        ......
end;

InstallMethod( DocumentedFunction, ......,
    function( ....)


    # supply missing argyments, handle special cases
    # do setup

    workerFunction(x,y,z,q);

    # clean up
    #
end);

is much better implemented as

InstallMethod( DocumentedFunction, ......,
    function( ....)
    local workerFunction,....;
    workerFunction := function(a,b,c,d)
       ......
    end;


    # supply missing argyments, handle special cases
    # do setup

    workerFunction(x,y,z,q);

    # clean up
    #
end);

unless the same workerFunction is used in two or more places.

Next, all undocumented names from a module or package should be chosen so that they are not reasonably going to occur in any other module or package. A simple way to achieve this is to prepend the module or package name. So:

BindGlobal("MyPackageWorkerFunctionUsedInTwoPlaces", ....);

or

MY_PACKAGE := rec();

MY_PACKAGE.WorkerFunctionUsedInTwoPlaces := .....;

As an example of this latter approach see the new file ffeconway.gi in the library, which store many internal functions and some data in a record called FFECONWAY.

Moving on to the documented names, the main rule here is that functions with "short" or "common" names, such as "Tail" or "NormalForm" should usually be Operations (or Attributes or Properties if appropriate). See chapter 8 of the GAP tutorial and chapter 79 of the GAP Reference Manual for more detail of these concepts. Even if you only plan one method, or you only plan to apply them to non-attribute storing objects, declaring them as Properties, Attributes or Operations will allow them to be used for unrelated purposes in other modules or packages, since GAP allows multiple declarations quite flexibly. Although see section 78.8 of the GAP Reference Manual for a discussion of the risks of overusing names.

There are only two real reasons for declaring something as a GlobalFunction rather than an Operation. One is that you plan to have a single method which takes a variable number of arguments (for instance Print and Concatenation cannot be Operations). The other is that your package represents mathematical constructs by simple GAP objects such as plain lists or records, so that method selection could not sensibly recognise your objects. In these cases you should use a name that is not likely to clash with other packages or library developments.

If you have concerns about what is, or isn't appropriate practice in a package you are developing, feel free to contact GAP support at any stage for help and advice.