do not raise a runtime exception when a choice state does not match

lib/floe/workflow/choice_rule/data.rb

@@ -19,7 +19,7 @@ def true?(context, input)
           lhs = variable_value(context, input)
           rhs = compare_value(context, input)
 
-          validate!(lhs)
+          return unless valid?(lhs)
 
           case compare_key
           when "IsNull" then is_null?(lhs)
@@ -58,8 +58,8 @@ def true?(context, input)
 
         private
 
-        def validate!(value)
-          raise "No such variable [#{variable}]" if value.nil? && !%w[IsNull IsPresent].include?(compare_key)
+        def valid?(value)
+          !value.nil? || %w[IsNull IsPresent].include?(compare_key)
         end
 
         def is_null?(value) # rubocop:disable Naming/PredicateName

lib/floe/workflow/states/choice.rb

@@ -24,7 +24,7 @@ def finish
           output     = output_path.value(context, input)
 
           context.next_state = next_state
-          context.output     = output
+          context.output     = next_state.nil? ? {"Error" => "States.NoChoiceMatched"} : output
           super
         end
 

spec/workflow/choice_rule_spec.rb

@@ -71,7 +71,7 @@
         let(:input) { {} }
 
         it "raises an exception" do
-          expect { subject }.to raise_exception(RuntimeError, "No such variable [$.foo]")
+          expect(subject).to be_nil
         end
       end
 

spec/workflow/states/choice_spec.rb

@@ -2,6 +2,7 @@
   let(:input)    { {} }
   let(:ctx)      { Floe::Workflow::Context.new(:input => input) }
   let(:state)    { workflow.current_state }
+  let(:use_default) { true}
   let(:workflow) do
     make_workflow(
       ctx, {
@@ -19,7 +20,7 @@
               "Next"          => "SecondMatchState"
             },
           ],
-          "Default" => "DefaultState"
+          "Default" => (use_default ? "DefaultState" : nil)
         },
         "FirstMatchState"  => {"Type" => "Succeed"},
         "SecondMatchState" => {"Type" => "Succeed"},
@@ -53,9 +54,19 @@
   end
 
   describe "#run_nonblock!" do
-    context "with a missing variable" do
-      it "raises an exception" do
-        expect { state.run_nonblock! }.to raise_error(RuntimeError, "No such variable [$.foo]")
+    context "with a missing variable and no default" do
+      let(:use_default) { false }
+      it "returns an error" do
+        state.run_nonblock!
+        expect(ctx.next_state).to be_nil
+        expect(ctx.output).to eq({"Error" => "States.NoChoiceMatched"})
+      end
+    end
+
+    context "with a missing variable and a default" do
+      it "uses the default" do
+        state.run_nonblock!
+        expect(ctx.next_state).to eq("DefaultState")
       end
     end