From 1313369f569e4a1eecd96a4ac09f533e940d02fa Mon Sep 17 00:00:00 2001
From: Michael Orlitzky <michael@orlitzky.com>
Date: Fri, 4 Jul 2014 02:38:48 -0400
Subject: [PATCH] Split the (Parent a) associated type into a new Child class.

---
 src/Xml.hs | 21 +++++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)

diff --git a/src/Xml.hs b/src/Xml.hs
index 5c84664..7cfe6eb 100644
--- a/src/Xml.hs
+++ b/src/Xml.hs
@@ -4,6 +4,7 @@
 -- | General XML stuff.
 --
 module Xml (
+  Child(..),
   DtdName(..),
   FromXml(..),
   FromXmlFk(..),
@@ -66,16 +67,24 @@ class (ToDb a) => FromXml a where
   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.
+-- | A class for XML representations which are children of other
+--   elements. The foal is to associate a child XML element with its
+--   parent element's database type. This is required to construct the
+--   database analogue of @a@ in the 'FromXmlFk' and 'FromXmlFkTeams'
+--   classes.
 --
-class (ToDb a) => FromXmlFk a where
+class Child a where
   -- | The type of our parent object, i.e. to the type to whom our
   --   foreign key will point.
   type Parent 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 (Child a, ToDb a) => FromXmlFk a where
   -- | 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.
@@ -87,7 +96,7 @@ class (ToDb a) => FromXmlFk a where
 --   conversion, but also two foreign keys to away/home teams (as
 --   represented in "TSN.Team").
 --
-class (ToDb a) => FromXmlFkTeams a where
+class (Child a, ToDb a) => FromXmlFkTeams a where
   -- | The function that produces a @Db a@ out of a parent foreign
   -- key, two team foreign keys, and an @a@. The parameter order makes
   -- it easier to map this function over a bunch of things.
-- 
2.43.2