OneTimePromptDaemon: PromptDaemon
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A OneTimePromptDaemon is a special kind of PromptDaemon that automatically deactivates itself after its first invocation, thereby ensuring that it is executed only once. A one-time-only prompt daemon is a regular command prompt daemon, except that it fires only once. After it fires once, the daemon automatically deactivates itself, so that it won't fire again.

Prompt daemons are occasionally useful for non-recurring processing, when you want to defer some bit of code until a "safe" time between turns. In these cases, the regular PromptDaemon is inconvenient to use because it automatically recurs. This subclass is handy for these cases, since it lets you schedule some bit of processing for a single deferred execution.

One special situation where one-time prompt daemons can be handy is in triggering conversational events - such as initiating a conversation - at the very beginning of the game. Initiating a conversation can only be done from within an action context, but no action context is in effect during the game's initialization. An easy way to deal with this is to create a one-time prompt daemon during initialization, and then trigger the event from the daemon's callback method. The prompt daemon will set up a daemon action environment just before the first command prompt is displayed, at which point the callback will be able to trigger the event as though it were in ordinary action handler code. We can't use a regular Fuse or Daemon for this, since a regular Fuse or Daemon will only fire at the end of the player's turn, and in the case just described, we need something that fires just before the first command prompt appears.

So, for example, to have an NPC (let's call her Sarah) initiate a conversation, posing a question to the player character (such as 'What are you doing here?') just before the very first turn, we might could do something like this:

OneTimePromptDaemon, InitObject
  execute() { construct(self, &beforePrompt); }
    sarah.initiateConversation(sarahTalking, 'query-presence');

Don't worry about how initiateConversation works for now, we'll be going into that later. For now it suffices to know that this is the statement we need to execute to get Sarah chatting just before the first turn, so that the player's first command can be a require to her question. The point of OneTimePromptDaemon is to provide us somewhere where we can put this statement and have it execute when we need it.

Note that there's nothing magical about the name beforePrompt() here; we could have called it anything we liked, although beforePrompt() is at least descriptive of when the method is invoked, and might be useful if we wanted to define a custom subclass for this kind of situation:

class FirstTurnPromptDaemon: OneTimePromptDaemon, InitObject
  execute() { construct(self, &beforePrompt); }
  beforePrompt() {}

Rather more noteworthy here is the combination of OneTimePromptDaemon with InitObject, which we'll discuss next.