+-- | Common associated type shared by 'FromXml' and 'FromXmlFk'. This
+-- basically just forces the client to define the \"database
+-- version\" of his type.
+--
+class ToDb a where
+ -- | Each instance @a@ must declare its associated database type @Db a@.
+ type Db a :: *
+
+-- | A typeclass for XML types that can be converted into an
+-- associated database type. The story behind this is long, but
+-- basically, we need to different types most XML thingies we're
+-- going to import: a database type and an XML type.
+--
+-- Both Groundhog and HXT are very particular about the types that
+-- they can use, and there's no way to reuse e.g. a type that HXT
+-- can pickle in Groundhog. This typeclass gives us a standard way
+-- to get the database type from the XML type that we have to define
+-- for HXT.
+--
+class (ToDb a) => FromXml a where
+ -- | A function for getting a @Db a@ out of an @a@.
+ from_xml :: a -> Db a
+
+
+-- | Some database types cannot be constructed from the XML type
+-- alone; they must be supplied a foreign key to a parent object
+-- first. Members of this class can be converted from an XML
+-- representation to a database representation in this manner.
+--
+class (ToDb a) => FromXmlFk a where
+ -- | The type of our parent object, i.e. to the type to whom our
+ -- foreign key will point.
+ type Parent a :: *
+
+ -- | The function that produces a @Db a@ out of a foreign key and an
+ -- @a@. The parameter order makes it easier to map this function
+ -- over a bunch of things.
+ from_xml_fk :: DefaultKey (Parent a) -> a -> Db a
+
+
+-- | Represents the DTD filename (\"SYSTEM\") part of the DOCTYPE
+-- definition.
+newtype DtdName = DtdName String
+