add memoized subject

code/implicit_subject/account_spec.rb

@@ -8,6 +8,6 @@
   end
 
   context "when first created" do
-    it { should have_a_balance_of(Money.new(0, :BLR))}
+    it { should have_a_balance_of(0, :BLR)}
   end
 end

code/lib/account.rb

@@ -5,11 +5,15 @@ class Account
 
   def initialize(amount=nil, currency=nil)
     if amount && currency
-      @balance = Money.new(amount, currency)
+      deposit(amount, currency)
     else
       @balance = Money.new(0, :BLR)
     end
   end
+
+  def deposit(amount, currency)
+    @balance = Money.new(amount, currency)
+  end
 
   def inspect
     "Account (#{balance.inspect})"

code/memoized_subject/account_spec.rb

@@ -0,0 +1,10 @@
+require 'spec_helper'
+
+describe Account do
+  context "with a zero balance" do
+    it "accepts deposits" do
+      subject.deposit(12, :USD)
+      subject.should have_a_balance_of(12, :USD)
+    end
+  end
+end

code/spec/support/matchers.rb

@@ -1,6 +1,6 @@
-Spec::Matchers.define :have_a_balance_of do |money|
+Spec::Matchers.define :have_a_balance_of do |amount, currency|
   match do |account|
-    account.balance == money
+    account.balance == Money.new(amount, currency)
   end
 end