4.11.4 Module inheritance

The export-import mechanism doesn't work well in some cases, such as:

• You want to create a module that is mostly the same as the existing one, but adding or altering some definitions.
• You wrote a bunch of related modules that are often used together, and not want your users to repeat a bunch of 'use' forms every time they use your module.

You can use module inheritance in these cases.

Macro: extend module-name ...

Makes the current module inherit from named modules. The current inheritance information is altered by the inheritance information calculated from given modules.

A new module inherits from gauche module when created. If you put (extend scheme) in that module, for example, the module resets to inherit directly from scheme module that has only bindings defined in R5RS, hence, after the export form, you can't use 'import' or any other gauche-specific bindings in the module.

If a named module is not defined yet, extend tries to load it, using the same convention use macro does.

A module can inherit multiple modules, exactly the same way as a class can inherit from multiple classes. The resolution of order of inheritance needs to be explained a bit.

Each module has a module precedence list, which lists modules in the order of how they are searched. When the module inherits multiple modules, module precedence lists of inherited modules are merged into a single list, keeping the constraints that: (1) if a module A appears before module B in some module precedence list, A has to appear before B in the resulting module precedence list; and (2) if a module A appears before module B in extend form, A has to appear before B in the resulting module precedence list. If no precedence list can be constructed with these constraints, an error is signalled.

For example, suppose you wrote a library in modules mylib.base, mylib.util and mylib.system. You can bundle those modules into one module by creating a module mylib, as follows:

(define-module mylib (extend mylib.system mylib.util mylib.base))

The user of your module just says (use mylib) and all exported symbols from three submodules become available.