Macro methods

Macro defs allow you to define a method for a class hierarchy which is then instantiated for each concrete subtype.

A def is implicitly considered a macro def if it contains a macro expression which refers to @type. For example:

  1. class Object
  2.   def instance_vars_names
  3.     {{ @type.instance_vars.map &.name.stringify }}
  4.   end
  5. end
  7. class Person
  8.   def initialize(@name : String, @age : Int32)
  9.   end
  10. end
  12. person = Person.new "John", 30
  13. person.instance_vars_names # => ["name", "age"]

In macro definitions, arguments are passed as their AST nodes, giving you access to them in macro expansions ({{some_macro_argument}}). However that is not true for macro defs. Here the parameter list is that of the method generated by the macro def. You cannot access the call arguments during compile-time.

  1. class Object
  2.   def has_instance_var?(name) : Bool
  3.     # We cannot access name inside the macro expansion here,
  4.     # instead we need to use the macro language to construct an array
  5.     # and do the inclusion check at runtime.
  6.     {{ @type.instance_vars.map &.name.stringify }}.includes? name
  7.   end
  8. end
  10. person = Person.new "John", 30
  11. person.has_instance_var?("name")     # => true
  12. person.has_instance_var?("birthday") # => false