Designing Games with Game Maker - YoYo Games

Game Maker contains a built-in programming language. This programming language gives you
much more flexibility and control than the standard actions. This language we will refer to as
GML (the Game Maker Language). There are a number of different places where you can type
programs in this language. First of all, when you define scripts. A script is a program in GML.
Secondly, when you add a code action to an event. In a code action you again have to provide
a program in GML. Thirdly, in the room creation code. And finally, wherever you need to
specify a value in an action, you can also use an expression in GML. An expression, as we will
see below is not a complete program, but a piece of code resulting in a value.
In this chapter we will describe the basic structure of programs in GML. When you want to use
programs in GML, there are a couple of things you have to be careful about. First of all, for all
your resources (sprites, objects, sounds, etc.) you must use names that start with a letter and
only consist of letters, digits and the underscore '_' symbol. Otherwise you cannot refer to
them from within the program. Make sure all resources have different names. Also be careful
not to name resources self, other, global, or all because these have special meaning in the
language. Also you should not use any of the keywords, indicated below.
A program
A program consists of a set of instructions, called statements. A program must start with the
symbol '{' and end with the symbol '}'. Between these symbols there are the statements.
Statements must be separated with a ';' symbol. So the global structure of every program is:
{
}
;
;
...
There are a number of different types of statements, which will be discussed below.
Variables
Like any programming language GML contains variables. Variables are memory locations that
store information. They have a name so that you can refer to them. A variable in GML can