Macros

Macros for Ruby

gem install macros

Any source file that contains macro definitions, or that requires macro expansion, should be loaded with Macros.require.

require 'macros'

Macros.require 'my_macros'
Macros.require 'my_code_using_macros'

Macros look like normal method definitions, but they are defined inside a Macros do ; end block.

# my_macros.rb

Macros do
  # Replace the +output+ method with the +puts+ method
  def with_output(ast)
    treemap(ast) do |node|
      if smatch? node, s(:send, nil, :output)
        s(:send, nil, :puts, *node.children.drop(2))
      else
        node
      end
    end
  end
end

Now this code

# my_code_using_macros.rb

with_output do
  output "foo"
  output "bar"
end

Will be transformed into

puts "foo"
puts "bar"

AST stands for Abstract Syntax Tree, a way of representing the structure of code as data.

The macro will receive an instance of Parser::AST::Node, and must return an instance of Parser::AST::Node. A Node consists of a type and zero or more children. You can use Macros.parse and Macros.unparse to experiment.

For example

obj.do_thing(x, y)

parses to

s(:send,
  s(:send, nil, :obj), :do_thing,
  s(:send, nil, :x),
  s(:send, nil, :y))

Inside the macro definition the following convenience functions are available:

Construct an AST node of given type, with specific children.

Is the given object an AST node?

Similar to Enumerable#map, but performs a full tree walk, passing any AST::Node to the block.

Returns an array of any node in the tree that satisfies the predicate

spec is an Array of symbols, integers, and arrays. It is used a bit like XPath or CSS locators.

node = s(:def, :a_name, s(:args, s(:arg, :x), s(:arg, :y)))
sfind(node, [:def, 1, :args, [:arg, 0]])
# => [:x, y]

Checks if the node matches the "pattern"

node = s(:def, :a_name, s(:args, s(:arg, :x), s(:arg, :y)))
smatch?(node, s(:def, :a_name))
# => true

Well, it works, but it's a toy. Working with Ruby syntax trees is pretty awkward, and macros can easily lead to a mess. You have been warned!

© Arne Brasseur 2015

Mozilla Public License Version 2.0. See LICENSE file.