LSL 101/The Structure of a Script

The Structure of a Script

Here is the simplest possible valid LSL script. It doesn't actually ask the computer to do anything but all scripts have, at minimum, this structure:

<lsl> default {

    state_entry() {
    }

} </lsl>

In order to explain even this short piece of code we need to introduce you to some terms that will probably be new to you.

LSL scripts use two concepts called states and events.

I did not write the above text but I would like to say that while the structure above is valid and does work, the way it is formatted can sometimes be confusing and can cause problems when you get into longer scripts later on. I chose to leave the above script in to help show this. The way I suggest that anyone who is new to LSL (Linden Scripting Language) use this structure which is virtually the same but can help later if you get into the habit now. I prefer to move the bracket down so that the script is easier to read later and you can see the opening and closing brackets line up.

<lsl> //The only changes made are the brackets being moved //I move the brackets to make it easier to read the script //and find errors later when your scripts get bigger

default { //Opening bracket

    state_entry()
    { //Opening bracket
    } //Closing bracket

} //Closing bracket </lsl>

States

State is actually a very good name for what states do in LSL. If you think of a car, it can either be moving or stopped. We can say its state is moving (when it is moving) and its state is stopped (when it is stopped). Another example is your own state of being: you can be awake, asleep, active, sitting, standing, hungry, bored, confused, etc.

All LSL scripts have at least one state: the default state. This is the state when no other states are active. You can see in the code above the word default is used to tell the script about what happens in the default state.

Here is an example with two states:

<lsl> default {

    state_entry() {
        llOwnerSay("Switching to the hungry state...");
        state hungry;
    }

}

state hungry {

   state_entry() {
       llOwnerSay("I am very hungry! Does anyone have any spam?");
   }

} </lsl>

The first thing to notice is that the hungry state needs the word state, so that the script knows this describes a state rather than something else. The default state does not need to be proceeded by the word state.

Next, notice the curly braces '{' and '}'. These tell the script which lines are part of the default state, which are part of the hungry state, and which belong to the state_entry() event handler, which we describe next.

Events

When something happens we can say an event has happened. LSL knows about many kinds of events and can respond to them depending on what kind of event happened.

LSL scripts don't run on your PC, they run on the server where the sim you are in is running. The server takes care of seeing when something changes - an avatar moves, you click something, the clock ticks, someone types something in text chat, you create an object, save a notecard, give someone a landmark, etc. - and it passes on information about those changes to the viewer running on your PC, which then displays those changes or shows a dialog or whatever is needed. The server also passes those events on to any scripts that have asked to know about that particular kind of event.

Your script can tell the server to inform it of events by including an event handler. In the example above we have added event handlers called state_entry(), which requires the server to tell it when the script enters that particular state. When the script receives the state_entry() event it runs the instructions inside the curly braces belonging to the state_entry() event handler.

Some events also pass other information from the server; for instance the listen() event receives a channel number (to indicate which channel the chat was heard on), the name of the avatar or object that sent the chat, their (or its) UUID key, and the text of the message that was typed or sent. The listen() event handler is declared like this:

<lsl> listen(integer channel, string name, key id, string message) {

   // some actions to take when something is heard in text chat

} </lsl>

We will discuss more about events but before we do, and in order to explain the code above, we need to introduce another concept: variables.

Variables

We said that the server can pass information about events and in our example above you can see that we refer to the different pieces of information by giving them different names: channel, name, id, message. Not only are these separate pieces of information, they are different types of information: channel is a number, name is text, id is a special kind of number called a UUID key, message is text.

Variables, then, are declared by giving their type and their name. But variables would not be very useful if they did not also have a value. When we give a value to a variable it can be said we defined the variable.

Before you use a variable in LSL you must declare it and usually you will want to define it as well. These can both be done on separate lines or on the same line of code. Here are two examples:

<lsl> string myName; myName = "An Avatar";

// or

string myName = "An Avatar"; </lsl>

So what different types of variables does LSL know about?

integer

Integers are numbers, but only a limited set of numbers. Integers in LSL are any numbers between −2,147,483,648 and +2,147,483,647, so long as they are 'whole' numbers (that is, they don't have a decimal point, like 1.5).

integers are declared and defined like this

<lsl> integer myNumber = 42; </lsl>

You are used to using the decimal number system (called base 10), where numbers are counted using the digits 0 to 9, but you should also know there are other number systems that can be used with LSL, such as hexadecimal (base 16), which uses digits 0 to 9 and letters A to F. You don't need to know about the hexadecimal system to write scripts but you may well come across hexadecimal numbers if you are modifying scripts someone else has written and you may later find that there are some places where it makes sense to use hexadecimal numbers instead of decimals.

Hexadecimal numbers are written as in this example (which does exactly the same as the example above):

<lsl> integer myNumber = 0x2a; </lsl>

2a in hexidecimal = 42 [(2 * 16) + 10 is the same as (4 * 10) + 2]

A note to those who have used other programming languages before: LSL does not have a binary variable type. TRUE and FALSE are stored using integers with TRUE having a value of 1 and FALSE having a value of 0. We will discuss this in more detail later.