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}}
    {{content}}
  end
end
​
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
    1
  end
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
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