feat: Raise error on state constant name conflict

lib/statesman/exceptions.rb

@@ -11,6 +11,8 @@ class TransitionConflictError < StandardError; end
 
   class MissingTransitionAssociation < StandardError; end
 
+  class StateConstantConflictError < StandardError; end
+
   class TransitionFailedError < StandardError
     def initialize(from, to)
       @from = from

lib/statesman/machine.rb

@@ -169,7 +169,7 @@ def define_state_constant(state_name)
         constant_name = state_name.upcase
 
         if const_defined?(constant_name)
-          warn "Name conflict: '#{self.class.name}::#{constant_name}' is already defined"
+          raise StateConstantConflictError, "Name conflict: '#{self.class.name}::#{constant_name}' is already defined"
         else
           const_set(constant_name, state_name)
         end

spec/statesman/exceptions_spec.rb

@@ -51,6 +51,16 @@
     end
   end
 
+  describe "StateConstantConflictError" do
+    subject(:error) { Statesman::StateConstantConflictError.new }
+
+    its(:message) { is_expected.to eq("Statesman::StateConstantConflictError") }
+
+    its "string matches its message" do
+      expect(error.to_s).to eq(error.message)
+    end
+  end
+
   describe "TransitionFailedError" do
     subject(:error) { Statesman::TransitionFailedError.new("from", "to") }
 

spec/statesman/machine_spec.rb

@@ -31,9 +31,10 @@
     context "when state name constant is already defined" do
       it "warns about name conflict" do
         machine.const_set(:SOME_CONST, "some const")
-        warning_message = "Name conflict: 'Class::SOME_CONST' is already defined\n"
 
-        expect { machine.state(:some_const) }.to output(warning_message).to_stderr
+        expect { machine.state(:some_const) }.to raise_error(
+          Statesman::StateConstantConflictError, "Name conflict: 'Class::SOME_CONST' is already defined"
+        )
       end
     end
   end