Chapter 9: Macros and Metaprogramming

We love Ruby because of its’ dynamic nature and metaprogramming! Unlike Ruby, Crystal is a compiled language. That’s why there are some key differences.
  • There’s no eval.
  • There’s no send.
In Crystal we use Macros to achieve this kind of behaviour and metaprogramming. You can think of Macros as ‘Code that writes/modifies code’.
P.S: Macros are expanded into code at compile-time.
Check this.
macro define_method(name, content)
def {{name}}
define_method foo, 1
# This generates:
# def foo
# 1
# end
foo # => 1
In the example we created a macro named define_method and we just called that macro like a normal method. That macro expanded into
def foo
Pretty cool! We got eval behaviour at compile-time.
Macros are really powerful but there’s one rule that you can’t break.
A macro should expand into a valid Crystal program