Merge pull request #314 from pboling/dictionary

StrictKeyAccess Extension

2 files changed, 72 insertions, 0 deletions

diff --git a/lib/hashie.rb b/lib/hashie.rb
index b0f771c..59a7c25 100644
--- a/lib/hashie.rb
+++ b/lib/hashie.rb
@@ -26,6 +26,7 @@ module Hashie
     autoload :PrettyInspect,     'hashie/extensions/pretty_inspect'
     autoload :KeyConversion,     'hashie/extensions/key_conversion'
     autoload :MethodAccessWithOverride, 'hashie/extensions/method_access'
+    autoload :StrictKeyAccess, 'hashie/extensions/strict_key_access'
 
     module Parsers
       autoload :YamlErbParser, 'hashie/extensions/parsers/yaml_erb_parser'
diff --git a/lib/hashie/extensions/strict_key_access.rb b/lib/hashie/extensions/strict_key_access.rb
new file mode 100644
index 0000000..6972252
--- /dev/null
+++ b/lib/hashie/extensions/strict_key_access.rb
@@ -0,0 +1,71 @@
+module Hashie
+  module Extensions
+    # SRP: This extension will fail an error whenever a key is accessed that does not exist in the hash.
+    #
+    #   EXAMPLE:
+    #
+    #     class StrictKeyAccessHash < Hash
+    #       include Hashie::Extensions::StrictKeyAccess
+    #     end
+    #
+    #     >> hash = StrictKeyAccessHash[foo: "bar"]
+    #     => {:foo=>"bar"}
+    #     >> hash[:foo]
+    #     => "bar"
+    #     >> hash[:cow]
+    #       KeyError: key not found: :cow
+    #
+    # NOTE: For googlers coming from Python to Ruby, this extension makes a Hash behave like a "Dictionary".
+    #
+    module StrictKeyAccess
+      class DefaultError < StandardError
+        def initialize(msg = 'Setting or using a default with Hashie::Extensions::StrictKeyAccess does not make sense', *args)
+          super
+        end
+      end
+
+      # NOTE: This extension would break the default behavior of Hash initialization:
+      #
+      #     >> a = StrictKeyAccessHash.new(a: :b)
+      #     => {}
+      #     >> a[:a]
+      #       KeyError: key not found: :a
+      #
+      # Includes the Hashie::Extensions::MergeInitializer extension to get around that problem.
+      # Also note that defaults still don't make any sense with a StrictKeyAccess.
+      def self.included(base)
+        # Can only include into classes with a hash initializer
+        base.send(:include, Hashie::Extensions::MergeInitializer)
+      end
+
+      def [](key)
+        fetch(key)
+      end
+
+      def default(_ = nil)
+        fail DefaultError
+      end
+
+      def default=(_)
+        fail DefaultError
+      end
+
+      def default_proc
+        fail DefaultError
+      end
+
+      def default_proc=(_)
+        fail DefaultError
+      end
+
+      def key(value)
+        result = super
+        if result.nil? && (!key?(result) || self[result] != value)
+          fail KeyError, "key not found with value of #{value.inspect}"
+        else
+          result
+        end
+      end
+    end
+  end
+end