8.2  Getting Started

• Creating a Module
• Exporting
• Importing
• Definitions, Visibility and Accessibility

8.2.1  Creating a Module

You create a module simply by starting your program code with a module/1 directive. This should usually be placed at the beginning of the source file and looks like

:- module(mymodule).

As a rule, the module name should be chosen to be the same as the file’s base name (the file name without directory/folder and suffix/extension part). For example, the module mymodule might be contained in a file mymodule.ecl.

Anything you define in your module is by default local to that module.

8.2.2  Exporting

A definition is made available to the outside world by exporting it. All the exports of a module together form the module’s interface. Exporting is done with the export/1 directive, which can take different forms depending on the kind of the exported item.

Predicates are exported as follows:

:- export p/2.

p(X,Y) :-
        ...

Structures are exported by defining them with an export/1 instead of a local/1 directive, e.g.,

:- export struct(book(author,title,publisher)).

And the same holds for operators and other syntax settings:

:- export op(500, xfx, before).
:- export chtab(0'$, lower_case).
:- export syntax_option(no_array_subscripts).
:- export macro(pretty/1, tr_pretty/2, []).

All these declarations are valid locally in the module where they appear and in every module that imports them.

Initialization goals are exported as follows:

:- export initialization(writeln("I have been imported")).

Unlike the other declarations above, an exported initialization/1 directive is not executed locally in they module where it appears, but only in the context of the module where it gets imported.¹

8.2.3  Importing

In order to use a definition that has been exported elsewhere, it has to be imported. Often it is desirable to import another module’s interface as a whole, i.e., everything it exports. This is achieved by an import/1 directive of the form

:- import amodule.

If the module is in a file and has to be compiled first, then use_module/1 can be used, which is a combination of ensure_loaded/1 (see chapter 6) and import/1:

:- use_module("/home/util/amodule").

If the module is a library in one of ECLⁱPS^e’s library directories, then it can be loaded and imported by

:- use_module(library(hash)).

or simply using lib/1 as in

:- lib(hash).

It is also possible to import only a part of another module’s interface, using an import-from directive

:- import p/2 from amodule.

Note that this is the only form of import that can refer to a module that has not yet been loaded, and therefore allows a restricted form of circularity in the import structure.

8.2.4  Definitions, Visibility and Accessibility

For a given predicate name and arity the following rules hold:

• Every module can contain at most one definition:
  • this definition may be local or exported.
• In every module, at most one definition is visible:
  • if there is a definition in the module itself, this is also the visible one in the module;
  • otherwise, if there is an (unambiguous) import or reexport, this is the visible one;
  • otherwise no definition is visible.
• All exported definitions are accessible everywhere:
  • this might require explicit module qualification (see 8.3.2).

1

For local initialization use :- local initialization(...).