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:
class Object
def instance_vars_names
{{ @type.instance_vars.map &.name.stringify }}
end
end
class Person
def initialize(@name : String, @age : Int32)
end
end
person = Person.new "John", 30
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 argument list is that of the method generated by the macro def. You cannot access their compile-time value.
class Object
def has_instance_var?(name) : Bool
# We cannot access name inside the macro expansion here,
# instead we need to use the macro language to construct an array
# and do the inclusion check at runtime.
{{ @type.instance_vars.map &.name.stringify }}.includes? name
end
end
person = Person.new "John", 30
person.has_instance_var?("name") # => true
person.has_instance_var?("birthday") # => false