When writing scripts in Game Maker Language projects, I have come to realize a couple things that I want to share for anyone else who might be working on GML projects:
GML parsing is a bit loose. This makes the language fault tolerant, which is nice for newbie programmers who don’t need to get beaten up for not following strict syntax. But by enabling you to get away with imprecise syntax, it lets you get in to trouble that isn’t easy to detect.
Strict syntax is your friend, if you have it as an option in the language you’re using, I recommend following it as soon as you’re comfortable. Strive to get comfortable as quickly as possible. Unfortunately, GML does not have a strict mode…so I have the following tips to offer:
- Scripts (sometimes referred to as “programs” in the official Game Maker documentation) are supposed to begin and end with curly braces. The interpreter is forgiving if you don’t do this. But you should always do it.
- GML terminates statements with a semi-colon. The semi-colon is semi-optional, but I recommend using them wherever you intend a line of code to end, so that the interpreter doesn’t have to do extra work guessing that for you. Doing so will make it look like you know Pascal, C, or Java more than you know VB, which is enough reason to do it;) (Usually a statement is more or less the same thing as a line of code in your script file, but some structures may incorporate blocks of code, such as loops or branches.)
- Strings in GML can include line break characters (there’s no escaping with \n or using a character code like CRLF, like there is in most languages), so if you want to have a string with a line break in it, you simply hit the enter key and put a literal line break in your string. The interpreter handles this OK since the string is bounded by double or single quotes, but it’s still a weird thing about the language that they don’t really explain adequately in the documentation. If you’re not using semi-colons where you intend to end a line, it can get confusing to look at a multi-line string declaration that incorporates the line break as part of the string.
- It’s easy to not scope your variables appropriately without realizing it. GML allows global vars, and has two kinds of locality: locality with respect to an instance of an object, and locality with respect to a script. You shouldn’t use globals unless you really need to; according to the language doc they are slower to access than local variables. I haven’t noticed any difference that I can measure, but I’m sure that it is something that adds up, and at any rate proper scoping is a good habit to get into.When I refactored my project to pull drag-n-drop code out and replace it with scripts that were more re-usable, this caused some of my instance variables to turn into script variables. I had to go back and turn them into instance variables once I discovered that this had happened, and caused some issues with the way the code was executed by the interpreter.If you’re using variables in a script that were not declared in the object somewhere (normally the best place for this is in the object’s
create()event), and you are declaring the variable in the script, but you want to make it an instance-local variable, you can do so. The code to do this is:
That dot between
MyLocalInstanceVaris actually an operator. Knowing this is nifty, but I am not prepared to expound on why just yet.
Keep reading the language specification doc! Every time I go back to it and read, I pick up something I didn’t know, or refine something I thought I knew. One of the really nice things about GML is that it’s a small enough language that you can pick it up and do things with the basic features right away, without needing or being overwhelmed by all the other stuff that you’ll discover as you continue to progress as a developer. And there are enough built-in features in GML that you don’t have to spend a lot of time figuring out how to build sophisticated stuff out of primitives all by yourself.
The more I work in GML, the more respect and appreciation I gain for it. It is not the purest language, nor is it the most powerful, nor is the development environment as rich and sophisticated as a “serious” IDE like Visual Studio or Eclipse, but it is extremely accessible for a non-programmer to pick up and start working with and being effective quickly.
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Every one of these tips was golden. I can’t believe a blog post from 2011 was so helpful. We clearly share some perspective on GML. Good read!
Actually, almost all of this information is out of date and wrong.
1) GML scripts do NOT require starting and beginning with curly braces.
2) Line termination with semi-colons is absolutely something you should do, always.
3) GML strings can use # as a line break character, or you can use literal line breaks if you want. I wrote a better article on string handling that you should read instead.
4) Variable scoping is still easy to get tripped up on when you're new to GML, but the scoping rules have changed a bit in GM:Studio from what they were when I wrote this in 2011.
Actually in GMS2, they are introducing escape characters for strings, so \n for a newline will work in 2.0.
this is all wrong :c
This article is very old, and I’ve learned a lot since writing it.