So how is this useful? There is some power here that can be exploited.
Running this program again produces the desired result of 2.6. To avoid this issue, the content of the PRICE macro should be wrapped in a grouping: #macro PRICE ( 5 + 8 ) This can be problematic if not identified by the developer, as this behaviour can lead to difficult to diagnose bugs.
The compiler will expand the expression 0.2 * PRICE into 0.2 * 5 + 8, not 0.2 * (5 + 8). In fact, the value of tax ends up being 4! The reason for this is that the macro PRICE interferes with the precedence of the expression. Now suppose it is used in an expression such as var tax = 0.2 * PRICE show_message ( "VAT: " + string ( tax )) Īt first glance, the expected value of tax might be 2.6 since 20% of 13 ( 5 + 8) is 2.6. I'm a fan of this term because it clearly describes what the issue involves! Consider the following macro #macro PRICE 5 + 8 The term "precedence problem" has been borrowed from the official GNU C docs 1. The following sub-sections detail common pitfalls to macros that can be exploited in GML. Upon compilation, all occurrences of MESSAGE will be replaced with "hello world". The symbol MESSAGE is the macro identifier and "hello world" is the piece of source code that the macro represents. Consider the following macro definition #macro MESSAGE "hello world" The most common application of macros in GML is to give synonyms to constants. Macros in GML use the #macro directive, and function similarly to C macros. As the name suggests, these constructs perform their tasks before or during compile-time, hence their disappearance once the build process of the program is complete. More specifically, macros are a form of compiler directive or preprocessor directive that enable developers to assign symbols to pieces of (commonly used) source code. Macros do not exist once the program is compiled, as they disappear once their job is done. This post assumes basic knowledge of what macros are, but for those that want a quick introduction: macros act as instructions to search for and replace some piece of code with another. Note: although macros are styled in SCREAMING_SNAKE_CASE by convention, all macro definitions in this post that resemble keywords will be in snake_case. Basic approaches to creating such syntax extensions are outlined throughout the post. Rather, this post is designed to make these ideas known to those who are interested. The aim of this post isn't to explore whether using macros as syntax extensions is practical, beneficial, or even a good idea. Currently GameMaker will treat them as interchangeable, but this may change in the future and your code is cleaner and more obvious when using the correct operators for comparisons and assignments.Using bad practices, careful design and creativity, macros can become a powerful feature for introducing new syntax into GameMaker Language (GML). NOTE: When comparing two values to see if they are equal, you should use the " =" operator, and only use the " =" one for assignment. However, the initial example may not, as explained on the section in the Expressions And Operators page. Note that while this is slightly more verbose, it means that there is no ambiguity in the code and that it will compile correctly on ail platforms at all times. If test = true variable = false else variable = true It is a good habit to always put brackets around the expressions and curly brackets around the statements in the if (otherwise only the first statement will be executed), and take a new line in the block for each statement, for example: NOTE In the GameMaker language any value that is less than or equal to 0 will evaluate as false, while any value that is greater than 0 will evaluate as true. In this case the expression will be evaluated, and if it evaluates as false, the statement after else is executed, otherwise the initial statement is executed (it's true). Note that the "then" part of the condition is implicit, but there is a then keyword that can be used (although it's almost always omitted), so you can also create conditionals like this:Īpart from if and then, you can also use the else keyword to do something else if the expression being checked evaluates as false. Here you are saying that if an expression resolves as true then do something. A simple if condition takes an expression and will perform one or more statement s if the expression resolves as true, with the following basic form: A fundamental feature of most programming languages is the ability to ask a simple question that gives a boolean true or false answer, and in GML this is achieved using the if keyword.
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