Warn when replacing placeholders

lib/turnip/placeholder.rb

@@ -4,6 +4,11 @@ class Match < Struct.new(:regexp, :block);end
 
     class << self
       def add(name, &block)
+        if placeholders.key?(name)
+          location = caller_locations.detect { |l| l.to_s !~ /lib\/turnip\/dsl\.rb/ }
+          warn "Placeholder :#{name} was replaced at #{location}."
+        end
+
         placeholders[name] = Placeholder.new(name, &block)
       end
 

spec/placeholder_spec.rb

@@ -9,6 +9,8 @@
       end
     end
 
+    after { Turnip::Placeholder.send(:placeholders).clear }
+
     it 'returns a regexp for the given placeholder' do
       resolved = described_class.resolve(:test)
 
@@ -150,4 +152,19 @@
       end
     end
   end
+
+  describe 'replacing placeholders' do
+    before do
+      described_class.add(:test) do
+        match(/foo/)
+      end
+    end
+
+    it 'issues a warning' do
+      expect(described_class).to receive(:warn).with(/Placeholder :test was replaced/)
+      described_class.add(:test) do
+        match(/bar/)
+      end
+    end
+  end
 end

spec/step_definition_spec.rb

@@ -3,6 +3,8 @@
 describe Turnip::StepDefinition do
   let(:all_steps) { [] }
 
+  after { Turnip::Placeholder.send(:placeholders).clear }
+
   describe "#match" do
     it "matches a simple step" do
       step = Turnip::StepDefinition.new("there are monsters") {}