yield instead of calling instance_eval in Builder.new if block has an ar...

lib/middleware/builder.rb

@@ -8,20 +8,25 @@ module Middleware
   #
   # Building a middleware stack is very easy:
   #
-  #     app = Middleware::Builder.new do
-  #       use A
-  #       use B
+  #     app = Middleware::Builder.new do |b|
+  #       b.use A
+  #       b.use B
   #     end
   #
   #     # Call the middleware
   #     app.call(7)
   #
   class Builder
     # Initializes the builder. An optional block can be passed which
-    # will be evaluated in the context of the instance.
+    # will either yield the builder or be evaluated in the context of the instance.
     #
     # Example:
     #
+    #     Builder.new do |b|
+    #       b.use A
+    #       b.use B
+    #     end
+    #
     #     Builder.new do
     #       use A
     #       use B
@@ -35,7 +40,14 @@ class Builder
     def initialize(opts=nil, &block)
       opts ||= {}
       @runner_class = opts[:runner_class] || Runner
-      instance_eval(&block) if block_given?
+
+      if block_given?
+        if block.arity == 1
+          yield self
+        else
+          instance_eval(&block)
+        end
+      end
     end
 
     # Returns a mergeable version of the builder. If `use` is called with

spec/middleware/builder_spec.rb

@@ -11,6 +11,38 @@ def appender_proc(data)
     Proc.new { |env| env[:data] << data }
   end
 
+  context "initialized with a block" do
+    context "without explicit receiver" do
+      it "instance evals the block" do
+        data = {}
+        proc = Proc.new { |env| env[:data] = true }
+
+        app = described_class.new do
+          use proc
+        end
+
+        app.call(data)
+
+        data[:data].should == true
+      end
+    end
+
+    context "with explicit receiver" do
+      it "yields self to the block" do
+        data = {}
+        proc = Proc.new { |env| env[:data] = true }
+
+        app = described_class.new do |b|
+          b.use proc
+        end
+
+        app.call(data)
+
+        data[:data].should == true
+      end
+    end
+  end
+
   context "basic `use`" do
     it "should add items to the stack and make them callable" do
       data = {}