X-Git-Url: http://gitweb.michael.orlitzky.com/?a=blobdiff_plain;f=src%2FXml.hs;h=dfb6d055ac0e297b94d42d92eff810e1464d42ec;hb=76cf3eee776d35ba2b18dd0d07df7496a083ae3a;hp=129c6e9cebe4236b7af99a7988c168b662eefb44;hpb=988f693ce7f1abb6566e75d539ac312b627c31d5;p=dead%2Fhtsn-import.git

diff --git a/src/Xml.hs b/src/Xml.hs
index 129c6e9..dfb6d05 100644
--- a/src/Xml.hs
+++ b/src/Xml.hs
@@ -1,10 +1,14 @@
+{-# LANGUAGE TypeFamilies #-}
+
 -- | General XML stuff.
 --
 module Xml (
+  ToFromXml(..),
   parse_opts,
   pickle_unpickle )
 where
 
+import Database.Groundhog ( AutoKey )
 import Text.XML.HXT.Core (
   (>>>),
   (/>),
@@ -22,6 +26,39 @@ 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