Add the FromXmlFk class, like FromXml except it requires an FK (old idea).

[dead/htsn-import.git] / src / Xml.hs

diff --git a/src/Xml.hs b/src/Xml.hs
index 2e9c18bfad5fec0cd9599667c07ae7ed060aae0e..bbb72be0348d1864ddf3094fda6173769ef247f1 100644 (file)
--- a/src/Xml.hs
+++ b/src/Xml.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE TypeFamilies #-}
 
 -- | General XML stuff.
@@ -6,6 +7,7 @@ module Xml (
   DtdName(..),
   FromXml(..),
   FromXmlFk(..),
+  ToDb(..),
   parse_opts,
   pickle_unpickle,
   unpickleable )
@@ -13,7 +15,7 @@ where
 
 -- System imports.
 import Control.Exception ( SomeException(..), catch )
-import Database.Groundhog.Core ( DefaultKey )
+import Database.Groundhog.Core ( PersistEntity(..) )
 import Text.XML.HXT.Core (
   (>>>),
   (/>),
@@ -29,6 +31,13 @@ import Text.XML.HXT.Core (
   xunpickleVal,
   yes )
 
+-- | 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
@@ -41,11 +50,8 @@ import Text.XML.HXT.Core (
 --   to get the database type from the XML type that we have to define
 --   for HXT.
 --
-class FromXml a where
-  -- | Each instance @a@ must declare its associated database type @Db a@.
-  type Db a :: *
-
-  -- | And provide a function for getting a @Db a@ out of an @a@.
+class (ToDb a) => FromXml a where
+  -- | A function for getting a @Db a@ out of an @a@.
   from_xml :: a -> Db a
 
 
@@ -54,17 +60,15 @@ class FromXml a where
 --   first. Members of this class can be converted from an XML
 --   representation to a database representation in this manner.
 --
-class FromXmlFk a where
-  -- | Each instance @a@ must declare its associated database type @DbFk a@.
-  type DbFk a :: *
-
+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 @DbFk a@ out of an @a@ and a foreign
-  --   key.
-  from_xml_fk :: a -> DefaultKey (Parent a) -> DbFk 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