Fail when accessing registry with invalid key

features/registry.feature

@@ -34,3 +34,15 @@ Feature: key/value registry
       """
     When I successfully execute the recipe
     Then the output must contain "some_value"
+
+  Scenario: `get' keyword should trigger an error when given invalid key
+    Given a recipe with:
+      """
+      task :some_task do
+        echo get :no_key
+        echo 'after_fail'
+      end
+      """
+    When I execute the recipe
+    Then the output must not contain "after_fail"
+    And the output must match /\A\w+Error:\s+:no_key/

lib/producer/core/env.rb

@@ -18,7 +18,9 @@ module Producer
       end
 
       def [](key)
-        @registry[key]
+        @registry.fetch key
+      rescue KeyError
+        fail RegistryKeyError, key.inspect
       end
       alias get []
 

lib/producer/core/errors.rb

@@ -4,5 +4,6 @@ module Producer
     RuntimeError                = Class.new(RuntimeError)
     ConditionNotMetError        = Class.new(Error)
     RemoteCommandExecutionError = Class.new(RuntimeError)
+    RegistryKeyError            = Class.new(RuntimeError)
   end
 end

spec/producer/core/env_spec.rb

@@ -107,6 +107,10 @@ module Producer::Core
       it 'returns the value indexed by given key from the registry' do
         expect(env[:some_key]).to eq :some_value
       end
+
+      it 'raises an error when given invalid key' do
+        expect { env[:no_key] }.to raise_error RegistryKeyError
+      end
     end
 
     describe '#[]=' do