Modules

We now have an idea of the overall structure of our application and can now turn this into the top-level code structure. We use the module concept of ECLiPSe to clearly separate the components and to define fixed interfaces between them. Each component of our top-level structure should define a module in one file. We place a module directive at the beginning of the file to indicate the module name.

The following examples are taken from the file flow_prepare_data.ecl.

:-module(flow_prepare_data).

Making predicates available

The export directive makes some predicate definition available to users outside the module. All other predicates can be only used inside the module, but cannot be used from outside. A module effectively hides all non exported predicates in a separate name space, so that we do not have to worry about using the same name in two different modules.

In our example, the module exports a single predicate prepare_data/12.

:-export(prepare_data/12).

Using packaged functionality

If we want to use some function which has been exported from some module in another module, we have to use the use_module directive. It says that all exported predicates of a module should be available in the current module, and ensures that the module is loaded into the system. We can use the same module in more than one place, but the directive makes sure it is loaded only once. The use_module directive assumes that the module is defined in a file in the current directory, which has the same name as the module.

The flow_prepare_data module uses predicates from a variety of sources.

:-use_module(data_topology).
:-use_module(data_peering_policy).
:-use_module(data_traffic).
:-use_module(data_routing).
:-use_module(data_group).
:-use_module(data_general).

:-use_module(flow_statistics).
:-use_module(flow_structures).
:-use_module(flow_utility).
:-use_module(report).
:-use_module(report_utility).
:-use_module(utility).

If we want to use predicates defined in some library, we have to use the lib directive. It loads the library from the library directory in the ECLiPSe installation, unless we have changed the library path setting to include other directories in the search path. Libraries in ECLiPSe are modules like all others, they are just stored in a different place in the file system.

In our example, a single library called ``hash'' is loaded.

:-lib(hash).