Defining macros

In Crystal, a macro has two meanings. Generally, it refers to any code that runs or expands at compile time. However, more specifically, it can refer to a type of method that accepts AST nodes at compile time, whose body is pasted into the program at the point the macro is used. An example of the latter is the property macro, which you saw in previous chapters, which is an easy way to define both a getter and a setter method for a given instance variable:

class Example
  property age : Int32
  
  def initialize(@age : Int32); end
end

The preceding code is equivalent to the following:

class Example
  @age : Int32
 
  def initialize(@age : Int32); end
 
  def age : Int32
    @age
  end
 
  def age=(@age : Int32)
  end
end

As we mentioned earlier, macros accept AST nodes at compile time and output Crystal code that is added to the program as if it was...