Define state constants

README.md

@@ -390,6 +390,24 @@ Machine.successors
 }
 ```
 
+## Class constants
+Adding a state to a state machine will automatically create a constant for the value, for example:
+```ruby
+class OrderStateMachine
+  include Statesman::Machine
+
+  state :pending, initial: true
+  state :checking_out
+  state :cancelled
+
+  # Constants created as a side effect of adding state
+  transition from: PENDING, to: [CHECKING_OUT, CANCELLED]
+end
+
+OrderStateMachine::PENDING #=> "pending"
+OrderStateMachine::CHECKING_OUT # => "checking_out"
+```
+
 ## Instance methods
 
 ### `Machine#current_state`

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

@@ -39,6 +39,8 @@ def state(name, options = { initial: false })
           validate_initial_state(name)
           @initial_state = name
         end
+        define_state_constant(name)
+
         states << name
       end
 
@@ -163,6 +165,16 @@ def validate_from_and_to_state(from, to)
 
       private
 
+      def define_state_constant(state_name)
+        constant_name = state_name.upcase
+
+        if const_defined?(constant_name)
+          raise StateConstantConflictError, "Name conflict: '#{self.class.name}::#{constant_name}' is already defined"
+        else
+          const_set(constant_name, state_name)
+        end
+      end
+
       def add_callback(callback_type: nil, callback_class: nil,
                        from: nil, to: nil, &block)
         validate_callback_type_and_class(callback_type, callback_class)

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

@@ -11,10 +11,14 @@
 
     specify { expect(machine.states).to eq(%w[x y]) }
 
+    specify { expect(machine::X).to eq "x" }
+
+    specify { expect(machine::Y).to eq "y" }
+
     context "initial" do
-      before { machine.state(:x, initial: true) }
+      before { machine.state(:z, initial: true) }
 
-      specify { expect(machine.initial_state).to eq("x") }
+      specify { expect(machine.initial_state).to eq("z") }
 
       context "when an initial state is already defined" do
         it "raises an error" do
@@ -23,6 +27,16 @@
         end
       end
     end
+
+    context "when state name constant is already defined" do
+      it "warns about name conflict" do
+        machine.const_set(:SOME_CONST, "some const")
+
+        expect { machine.state(:some_const) }.to raise_error(
+          Statesman::StateConstantConflictError, "Name conflict: 'Class::SOME_CONST' is already defined"
+        )
+      end
+    end
   end
 
   describe ".remove_state" do