Added barrier

dataflow.rb

@@ -31,6 +31,10 @@ def by_need(&block)
     Variable.new &block
   end
 
+  def barrier(*variables)
+    variables.each{|v| v.__wait__ }
+  end
+
   def need_later(&block)
     local do |future|
       Thread.new { unify future, block.call }
@@ -71,17 +75,21 @@ def __activate_trigger__
       @__trigger__ = nil # GC
     end
 
-    def method_missing(name, *args, &block)
+    def __wait__
       LOCK.synchronize do
-        unless @__bound__
-          return "#<Dataflow::Variable:#{__id__} unbound>" if name == :inspect
+        unless @__bound__          
           if @__trigger__
             __activate_trigger__
           else
             __binding_condition__.wait
           end
         end
       end unless @__bound__
+    end
+
+    def method_missing(name, *args, &block)
+      return "#<Dataflow::Variable:#{__id__} unbound>" if !@__bound__ && name == :inspect
+      __wait__
       @__value__.__send__(name, *args, &block)
     end
 

spec/barrier_spec.rb

@@ -0,0 +1,25 @@
+require "#{File.dirname(__FILE__)}/spec_helper"
+
+describe 'A barrier' do
+  it 'waits for variables to be bound before continuing' do
+    local do |x, y, barrier_broken|
+      Thread.new do 
+        barrier x, y
+        unify barrier_broken, true
+      end
+      Thread.new { unify x, :x }
+      Thread.new { unify y, :y }
+      barrier_broken.should be_true
+    end
+  end
+
+  it 'continues for variables that are already bound' do
+    local do |x, y, barrier_broken|
+      unify x, :x
+      unify y, :y
+      barrier x, y
+      unify barrier_broken, true
+      barrier_broken.should be_true
+    end
+  end
+end