0.0.0.1 1.4.1 Interfaces.

One of Modula-2's most successful features is the provision for explicit interfaces between modules. Interfaces are retained with essentially no changes in Modula-3. An interface to a module is a collection of declarations that reveal the public parts of a module; things in the module that are not declared in the interface are private. A module imports the interfaces it depends on and exports the interface (or, in Modula-3, the interfaces) that it implements.

Interfaces make separate compilation type-safe; but it does them an injustice to look at them in such a limited way. Interfaces make it possible to think about large systems without holding the whole system in your head at once.

Programmers who have never used Modula-style interfaces tend to underestimate them, observing, for example, that anything that can be done with interfaces can also be done with C-style include files. This misses the point: many things can be done with include files that cannot be done with interfaces. For example, the meaning of an include file can be changed by defining macros in the environment into which it is included. Include files tempt programmers into shortcuts across abstraction boundaries. To keep large programs well structured, you either need super-human will power, or proper language support for interfaces.