Latest revision as of 00:22, 9 May 2011

Prog Structure and Practice

"It's just a puzzle box." 
  -- Kirstie, Hellraiser

Firstly, there are three classes of entities in the game which can have progs: Mobiles,

Objects, and Rooms. Broadly speaking, the syntax used on each of these prog types is the

same. Respectively, these are referred to as mobprogs, objprogs, or room progs. Over time,

the prog system has been rationalized so that the syntax for mpcommands and if checks is

the same across the type of entity. "Mobprog" is therefore often used to refer to the general

class of all prog types, and this is the usage which will be employed in this document. Every

prog has the following structure:

  [prog_trigger] [modifier] [argument]
  Body of Prog: 
  {
  (Control structures)
  (Commands)
  }

It isn’t immediately obvious by what we mean by all these terms, so let’s try to clarify them:

prog_trigger

The prog_trigger reflects a central fact about progs: All progs are, in fact, a set of actions

which occur in response to a triggering event. What a trigger really says is, "Execute the

commands that are in my body when conditions matching my trigger are met".

To take a concrete example, consider the following prog_trigger:

Greet_prog:

Syntax: greet_prog <Percentage value from 1-100>

A greet_prog triggers whenever a pc enters the room with the object/room/mob. The chance that

the greet prog triggers is equal to the percentage value given as an argument.

Ex:

greet_prog 50

say Hello there!

So, this mob has a 50% chance any time a pc enters the room of triggering the prog, at which

point the pc will see the mob saying "Hello there!"

Another example of a prog trigger which does not have a percentage argument is the speech_prog.

Syntax: speech_prog [p] [argument]

Speech_prog triggers if [argument] is said aloud in the same room as mobile or object. The "p"

determines whether or not the match must be exact or simply a substring match. If no argument is

specified, the speech_prog will trigger on any speech at all.

Ex:

speech_prog p Hello

say Hi there!

Now, a mobile with this program would respond whenever someone said "hello" to the mob.

The Prog Body:

The body of the prog is where the ‘meat’ of the prog resides. Broadly speaking, you will find

two types of things within the body:

Commands

Control Structures

(i) Commands

Commands are actions that the mob can perform. As a general rule, mobiles can perform any

action a player can that does not involve the use of a skill or spell. [They can do these

as well, but it requires a special other thing to be done first.]

In addition to these normal commands, there are a suite of prog-specific commands. These

commands are designed to allow progs to duplicate imm commands, player skills or spells, or to

just generally do interesting things. To take a concrete example, consider the following prog

specific command:

mpgoto:

Syntax: mpgoto <vnum>

When executed from within a mob prog, mpgoto causes the mobile in question to goto the room

with vnum specified. Taking the examples we’ve seen, we can construct a very simple prog:

greet_prog 100

say Oh no, people!

mpgoto 5000

In the above prog, "greet_prog" is the prog trigger, and the say and the mpgoto are examples

of commands. This prog basically says, "Whenever someone enters the same room as this mob,

have it say "Oh no, people!", then have the mob goto room 5000".

This is a very basic prog, but might be used on a mischievous imp or forest spirit. [And a

cunning player would go to room 5000, and ambush the imp there!] There are many, many different

mpcommands, so we’ll detail those in their own chapter.

(ii) Control Structures

Normally, commands in the body of a prog are executed in linear order. I.e., if we have a prog:

prog_trigger

command 1

command 2

command 3

When the prog triggers, it will execute command 1, then command 2, then command 3, and so on, line

by line, from top to bottom.

Control structures allow us to control the natural order of prog execution. Structures exist to

allow for logical branching, multiple conditionals, looping, subroutines, and numerous other

functions. To take a concrete example, consider the "loop" statement:

Loop:

Syntax: loop <lower number> to <higher number>

[Body of loop]

endloop

The loop structure, like its name implies, will loop through the body of the loop, executing the

lines within for every iteration of the loop. So, consider the following prog:

greet_prog 100

loop 1 to 3

say Hey Hey!

endloop

This prog will trigger when someone enters the room. It will then hit the loop, so it will

execute its contents three times. So, if I enter a room with a mob who had this prog, I will

see:

Mob says, ‘Hey Hey!’ [this is when the loop is on 1]

Mob says, ‘Hey Hey!’ [this is when the loop is on 2]

Mob says, ‘Hey Hey!’ [this is when the loop is on 3]

Later, we’ll give control structures their own chapter. But for right now, the important thing

is to realize is that the default behavior for a prog is to execute its commands in order, and

control structures exist to change the order in which commands are executed.