2.2.3 Verbs on Objects

The final kind of piece making up an object is verbs. A verb is a named MOO program that is associated with a particular object. Most verbs implement commands that a player might type; for example, in the LambdaCore database, there is a verb on all objects representing containers that implements commands of the form `put object in container'. It is also possible for MOO programs to invoke the verbs defined on objects. Some verbs, in fact, are designed to be used only from inside MOO code; they do not correspond to any particular player command at all. Thus, verbs in MOO are like the `procedures' or `methods' found in some other programming languages.

As with properties, every verb has an owner and a set of permission bits. The owner of a verb can change its program, its permission bits, and its argument specifiers (discussed below). Only a wizard can change the owner of a verb. The owner of a verb also determines the permissions with which that verb runs; that is, the program in a verb can do whatever operations the owner of that verb is allowed to do and no others. Thus, for example, a verb owned by a wizard must be written very carefully, since wizards are allowed to do just about anything.

The permission bits on verbs are drawn from this set: `r' (read), `w' (write), `x' (execute), and `d' (debug). Read permission lets non-owners see the program for a verb and, symmetrically, write permission lets them change that program. The other two bits are not, properly speaking, permission bits at all; they have a universal effect, covering both the owner and non-owners.

The execute bit determines whether or not the verb can be invoked from within a MOO program (as opposed to from the command line, like the `put' verb on containers). If the `x' bit is not set, the verb cannot be called from inside a program. The `x' bit is usually set.

The setting of the debug bit determines what happens when the verb's program does something erroneous, like subtracting a number from a character string. If the `d' bit is set, then the server raises an error value; such raised errors can be caught by certain other pieces of MOO code. If the error is not caught, however, the server aborts execution of the command and, by default, prints an error message on the terminal of the player whose command is being executed. (See the chapter on server assumptions about the database for details on how uncaught errors are handled.) If the `d' bit is not set, then no error is raised, no message is printed, and the command is not aborted; instead the error value is returned as the result of the erroneous operation.

The `d' bit exists only for historical reasons; it used to be the only way for MOO code to catch and handle errors. With the introduction of the try-except statement and the error-catching expression, the `d' bit is no longer useful. All new verbs should have the `d' bit set, using the newer facilities for error handling if desired. Over time, old verbs written assuming the `d' bit would not be set should be changed to use the new facilities instead.

In addition to an owner and some permission bits, every verb has three `argument specifiers', one each for the direct object, the preposition, and the indirect object. The direct and indirect specifiers are each drawn from this set: `this', `any', or `none'. The preposition specifier is `none', `any', or one of the items in this list:

with/using at/to in front of in/inside/into on top of/on/onto/upon out of/from inside/from over through under/underneath/beneath behind beside for/about is as off/off of

The argument specifiers are used in the process of parsing commands, described in the next chapter.