push reduction visitors to a reduction base class

this lets our old depth first and dot visitors to work normally

lib/arel/visitors/depth_first.rb

@@ -5,22 +5,11 @@ def initialize block = nil
         @block = block || Proc.new
       end
 
-      def accept object
-        visit object
-      end
-
       private
 
-      def visit object
-        send dispatch[object.class], object
-      rescue NoMethodError => e
-        raise e if respond_to?(dispatch[object.class], true)
-        superklass = object.class.ancestors.find { |klass|
-          respond_to?(dispatch[klass], true)
-        }
-        raise(TypeError, "Cannot visit #{object.class}") unless superklass
-        dispatch[object.class] = dispatch[superklass]
-        retry
+      def visit o
+        super
+        @block.call o
       end
 
       def unary o

lib/arel/visitors/dot.rb

@@ -23,11 +23,12 @@ def initialize
       end
 
       def accept object
-        super
+        visit object
         to_dot
       end
 
       private
+
       def visit_Arel_Nodes_Ordering o
         visit_edge o, "expr"
       end

lib/arel/visitors/reduce.rb

@@ -0,0 +1,25 @@
+require 'arel/visitors/visitor'
+
+module Arel
+  module Visitors
+    class Reduce < Arel::Visitors::Visitor
+      def accept object, collector
+        visit object, collector
+      end
+
+      private
+
+      def visit object, collector
+        send dispatch[object.class], object, collector
+      rescue NoMethodError => e
+        raise e if respond_to?(dispatch[object.class], true)
+        superklass = object.class.ancestors.find { |klass|
+          respond_to?(dispatch[klass], true)
+        }
+        raise(TypeError, "Cannot visit #{object.class}") unless superklass
+        dispatch[object.class] = dispatch[superklass]
+        retry
+      end
+    end
+  end
+end

lib/arel/visitors/to_sql.rb

@@ -1,9 +1,10 @@
 require 'bigdecimal'
 require 'date'
+require 'arel/visitors/reduce'
 
 module Arel
   module Visitors
-    class ToSql < Arel::Visitors::Visitor
+    class ToSql < Arel::Visitors::Reduce
       ##
       # This is some roflscale crazy stuff.  I'm roflscaling this because
       # building SQL queries is a hotspot.  I will explain the roflscale so that

lib/arel/visitors/visitor.rb

@@ -1,8 +1,8 @@
 module Arel
   module Visitors
     class Visitor
-      def accept object, collector
-        visit object, collector
+      def accept object
+        visit object
       end
 
       private
@@ -18,8 +18,8 @@ def dispatch
         DISPATCH[self.class]
       end
 
-      def visit object, collector
-        send dispatch[object.class], object, collector
+      def visit object
+        send dispatch[object.class], object
       rescue NoMethodError => e
         raise e if respond_to?(dispatch[object.class], true)
         superklass = object.class.ancestors.find { |klass|

test/visitors/test_to_sql.rb

@@ -23,7 +23,7 @@ def compile node
 
       it 'can define a dispatch method' do
         visited = false
-        viz = Class.new(Arel::Visitors::Visitor) {
+        viz = Class.new(Arel::Visitors::Reduce) {
           define_method(:hello) do |node, c|
             visited = true
           end