Macros are without a doubt the most advanced feature in Haxe. They are often perceived as dark magic that only a select few are capable of mastering, yet there is nothing magical (and certainly nothing dark) about them.
Define: Abstract Syntax Tree (AST)
The AST is the result of parsing Haxe code into a typed structure. This structure is exposed to macros through the types defined in the file
haxe/macro/Expr.hxof the Haxe Standard Library.
Figure: The role of macros during compilation.
A basic macro is a syntax-transformation. It receives zero or more expressions and also returns an expression. If a macro is called, it effectively inserts code at the place it was called from. In that respect, it could be compared to a preprocessor like #define in C++, but a Haxe macro is not a textual replacement tool.
We can identify different kinds of macros, which are run at specific compilation stages:
--macro compiler parameter. They are executed after the compiler arguments were processed and the typer context has been created, but before any typing was done (see Initialization Macros).@:build or @:autoBuild metadata. They are executed per type, after the type has been set up (including its relation to other types, such as inheritance for classes) but before its fields are typed (see Type Building).