B.5. Global Object Symbols

The grid.h header file above also contains the declaration:

id <Symbol>  North, East, South, West;

Lines that declare global id variables of type Symbol, EventType, Warning, or Error (using the angle bracket syntax of id variables conforming to a protocol) are processed somewhat like deftype declarations in that they also produce global id variables initialized to support particular messages defined by defobj. These global variables, however, are not used to define or implement other message interfaces, but only to define certain fixed capabilities referenced through their global object names.

If declared as a Symbol, as in the case here, the generated objects have no particular behavior of their own (other than the character string of their name), but only serve to define unique global id constants which may be used as distinct named values in messages. In this example, the current direction of a GridTurtle object is represented by one of the symbol names North, East, South, or West. These values are like the enum constants of the C language, except that they are defined as full Objective C objects, and may be used with further restrictions. An EventType, or a Warning or Error, defines a further subtype of a Symbol constant with further specialized messages documented in defobj.