+{-# LANGUAGE TypeFamilies #-}
+
-- | General XML stuff.
--
module Xml (
+ ToFromXml(..),
parse_opts,
pickle_unpickle )
where
+import Database.Groundhog ( AutoKey )
import Text.XML.HXT.Core (
(>>>),
(/>),
xunpickleVal,
yes )
+
+-- | A typeclass for types which can be converted into an associated
+-- XML type. The story behind this is long, but basically, we need
+-- to different types for each XML thingie 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. So this
+-- typeclass gives us a way to get the XML type from the Groundhog
+-- type.
+--
+-- At first there appears to be an equally-valid approach, getting the
+-- Groundhog type from the XML one. But Groundhog won't use type family
+-- instances, so here we are.
+--
+class ToFromXml a where
+ -- | Each instance a must declare its associated XML type (Xml a)
+ type Xml a :: *
+ type Container a :: *
+
+ -- | And provide a function for getting an (Xml a) out of an "a."
+ to_xml :: a -> Xml a
+
+ -- | And provide a function for getting an "a" out of an (Xml a).
+ from_xml :: Xml a -> a
+
+ -- | Often we need to provide a key to use as a foreign key into
+ -- some container. If the instance "belongs" to some other object,
+ -- then it might need to be passed a key before it can un-XML
+ -- itself. For example, the XML version of 'NewsTeam' doesn't
+ -- contain a message ID which is part of its database type.
+ from_xml_fk :: AutoKey (Container a) -> Xml a -> a
+ from_xml_fk _ = from_xml
+
-- | A list of options passed to 'readDocument' when we parse an XML
-- document. We don't validate because the DTDs from TSN are
-- wrong. As a result, we don't want to keep useless DTDs