File: Traversal — RuboCop-AST master

The ::RuboCop::AST::Traversal module provides a simple way to walk an AST and process specific node types. Unlike Parser::AST::Processor, it does not transform the AST — it is read-only.

Basic Usage

Include the Traversal module and define <code>on_<type></code> methods for the node types you care about:

class MyVisitor
  include RuboCop::AST::Traversal

  def on_send(node)
    puts "Method call: #{node.method_name}"
    super # continue walking child nodes
  end
end

source = {RuboCop::AST::ProcessedSource.new}(code, 3.4)
visitor = {MyVisitor.new}
visitor.walk(source.ast)

How It Works

The walk method starts traversal from a given node. For each node, it calls the corresponding <code>on_<type></code> method (e.g. on_send for <code>:send</code> nodes, on_if for <code>:if</code> nodes).

The default <code>on_<type></code> callbacks automatically recurse into child nodes. If you override a callback, call super to continue the traversal into that node’s children. Omitting super will skip the subtree.

def on_class(node)
  # process the class node but skip its children
  record_class(node)
  # no super -- children are not visited
end

Combining with ProcessedSource

A common pattern is to process every node in a source file:

source = {RuboCop::AST::ProcessedSource.new}(code, 3.4)
visitor = {MyVisitor.new}
visitor.walk(source.ast)

You can also use Node#each_node if you just need to iterate without the visitor pattern:

source.ast.each_node(:send) do |send_node|
  puts send_node.method_name
end