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