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<TITLE>LambdaMOO Programmer's Manual - Verbs</TITLE>
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<H3><A NAME="SEC7" HREF="ProgrammersManual_toc.html#TOC7">Verbs on Objects</A></H3>
<P>
The final kind of piece making up an object is <STRONG>verbs</STRONG>. 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 <VAR>object</VAR> in <VAR>container</VAR>'. 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.
</P>
<P>
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.
</P>
<P>
The permission bits on verbs are drawn from this set: <SAMP>`r'</SAMP> (read),
<SAMP>`w'</SAMP> (write), <SAMP>`x'</SAMP> (execute), and <SAMP>`d'</SAMP> (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.
</P>
<P>
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 <SAMP>`put'</SAMP> verb
on containers). If the <SAMP>`x'</SAMP> bit is not set, the verb cannot be called
from inside a program. The <SAMP>`x'</SAMP> bit is usually set.
</P>
<P>
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 <SAMP>`d'</SAMP> bit is set, then the server <STRONG>raises</STRONG> an error value; such
raised errors can be <STRONG>caught</STRONG> 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 <SAMP>`d'</SAMP> 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.
</P>
<BLOCKQUOTE>
<P>
<STRONG>Note:</STRONG> the <SAMP>`d'</SAMP> 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 <CODE>try</CODE>-<CODE>except</CODE> statement and the error-catching expression,
the <SAMP>`d'</SAMP> bit is no longer useful. All new verbs should have the <SAMP>`d'</SAMP>
bit set, using the newer facilities for error handling if desired. Over time,
old verbs written assuming the <SAMP>`d'</SAMP> bit would not be set should be changed
to use the new facilities instead.
</BLOCKQUOTE>
<P>
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: <SAMP>`this'</SAMP>, <SAMP>`any'</SAMP>, or <SAMP>`none'</SAMP>. The preposition specifier
is <SAMP>`none'</SAMP>, <SAMP>`any'</SAMP>, or one of the items in this list:
</P>
<PRE>
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
</PRE>
<P>
The argument specifiers are used in the process of parsing commands,
described in the next chapter.
</P>
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