Add the FromXml type family.

diff --git a/src/Xml.hs b/src/Xml.hs
index 129c6e9cebe4236b7af99a7988c168b662eefb44..21a806847ca1761845260f1a2d4f92344d07916d 100644 (file)
--- a/src/Xml.hs
+++ b/src/Xml.hs
@@ -1,6 +1,9 @@
+{-# LANGUAGE TypeFamilies #-}
+
 -- | General XML stuff.
 --
 module Xml (
+  FromXml(..),
   parse_opts,
   pickle_unpickle )
 where
@@ -22,6 +25,28 @@ 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 FromXml a where
+  -- | Each instance a must declare its associated XML type (Xml a)
+  type Xml a :: *
+
+  -- | And provide a function for getting an (Xml a) out of an "a."
+  to_xml :: a -> Xml a
+
+
 -- | 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