Implement the visitor pattern

Rakefile

@@ -85,3 +85,12 @@ namespace :props do
     write_hash_to_file.call(short_names_to_tokens, "#{dir}/short.yml")
   end
 end
+
+namespace :visitor do
+  task :generate do
+    require 'regexp_parser/tools'
+    generator = Regexp::Tools::VisitorGenerator.new
+    File.write('lib/regexp_parser/visitor.rb', generator.visitor_code)
+    File.write('lib/regexp_parser/visitor_nodes.rb', generator.node_code)
+  end
+end

lib/regexp_parser/tools.rb

@@ -0,0 +1 @@
+require 'regexp_parser/tools/visitor_generator'

lib/regexp_parser/tools/visitor_generator.rb

@@ -0,0 +1,74 @@
+require 'erb'
+require 'regexp_parser'
+
+module Regexp::Tools
+  class VisitorGenerator
+    attr_reader :root_mod
+
+    def initialize(root_mod = Regexp::Expression)
+      @root_mod = root_mod
+    end
+
+    def visitor_code
+      ERB.new(<<~END).result(binding).gsub(/^\s+\n/, "\n")
+        require 'regexp_parser/visitor_nodes'
+
+        class Regexp::Visitor
+          def visit_children(node)
+            node.expressions.each do |exp|
+              exp.accept(self)
+            end
+          end
+
+          def visit(node)
+            node.accept(self)
+          end
+        <% each_class do |c| %>
+          def <%= visitor_mtd_name(c) %>(node)
+            puts "Visiting \#{node.type}" if $debug
+          <% if c.instance_methods.include?(:expressions) %>
+            visit_children(node)
+          <% end %>end
+        <% end %>end
+      END
+    end
+
+    def node_code
+      ERB.new(<<~END).result(binding)
+        module Regexp::Expression<% each_class do |c| %>
+          class <%= c.name.split('::')[2..-1].join('::') %>
+            def accept(visitor)
+              visitor.<%= visitor_mtd_name(c) %>(self)
+            end
+          end
+        <% end %>end
+      END
+    end
+
+    private
+
+    def visitor_mtd_name(klass)
+      name = klass.name
+        .split('::')[2..-1].join('_')
+        .gsub(/([a-z])([A-Z])/) { "#{$1}_#{$2}" }
+        .downcase
+
+      "visit_#{name}"
+    end
+
+    def each_class(mod = root_mod, &block)
+      return to_enum(__method__, mod) unless block
+
+      case mod
+        when Class
+          return if mod.name.split('::').last == 'Base'
+          return unless mod < Regexp::Expression::Base
+          yield mod
+        when Module
+          mod.constants.each do |c|
+            each_class(mod.const_get(c), &block)
+          end
+      end
+    end
+  end
+end