14.4 The angle library

The angle library is the second client of the sixty-unit substrate. The angle library extends the say protocol to handle objects of the classes that it defines, such as <latitude>, <longitude>, and <absolute-position>. For the time being, we have included positions with angles, as we do not foresee any benefit to breaking them out into yet another library, at least for the current application. Nevertheless, we have defined separate interface and implementation modules for positions, and we have broken out the position source records into a separate interchange file.

Like with the time library, the angle library file does not have to specify the use of the format-out library. It will be transitively included because it is exported by the say library. Similarly, clients of the angle library do not need to know anything about the say and sixty-unit libraries, since those libraries are imported and re-exported to clients of angle.

Note that the position-implementation module uses the angle module — it is an internal client of the angle module. This structure means that we can easily break out positions as a separate library, should the need arise.

Also note that we have used the angle interface module to enforce access control on the internal-direction slot. It should be accessed only through the direction and direction-setter methods, which ensure that valid values are used for our <latitude> and <longitude> classes. Because only the approved generic functions are created in the interface module, only they will be accessible to clients of the angle library. The internal-direction slot is truly internal to the angle library — no client library can even determine its existence.