4.2.9 Executing Statements at a Later Time

It is sometimes useful to have some sequence of statements execute at a later time, without human intervention. For example, one might implement an object that, when thrown into the air, eventually falls back to the ground; the `throw' verb on that object should arrange to print a message about the object landing on the ground, but the message shouldn't be printed until some number of seconds have passed.

The `fork' statement is intended for just such situations and has the following syntax:

fork (expression) statements endfork

The `fork' statement first executes the expression, which must return a integer; call that integer n. It then creates a new MOO task that will, after at least n seconds, execute the statements. When the new task begins, all variables will have the values they had at the time the `fork' statement was executed. The task executing the `fork' statement immediately continues execution. The concept of tasks is discussed in detail in the next section.

By default, there is no limit to the number of tasks any player may fork, but such a limit can be imposed from within the database. See the chapter on server assumptions about the database for details.

Occasionally, one would like to be able to kill a forked task before it even starts; for example, some player might have caught the object that was thrown into the air, so no message should be printed about it hitting the ground. If a variable name is given after the `fork' keyword, like this:

fork name (expression) statements endfork

then that variable is assigned the task ID of the newly-created task. The value of this variable is visible both to the task executing the fork statement and to the statements in the newly-created task. This ID can be passed to the kill_task() function to keep the task from running and will be the value of task_id() once the task begins execution.