Add 'unpickleable' to Xml for use in tests.

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

diff --git a/src/Xml.hs b/src/Xml.hs
index 129c6e9cebe4236b7af99a7988c168b662eefb44..2f68dd09c661de44048f78fe0a45314fe4d38b98 100644 (file)
--- a/src/Xml.hs
+++ b/src/Xml.hs
@@ -1,13 +1,21 @@
+{-# LANGUAGE TypeFamilies #-}
+
 -- | General XML stuff.
 --
 module Xml (
+  DtdName(..),
+  ToFromXml(..),
   parse_opts,
-  pickle_unpickle )
+  pickle_unpickle,
+  unpickleable )
 where
 
+import Control.Exception ( SomeException(..), catch )
+import Database.Groundhog ( AutoKey )
 import Text.XML.HXT.Core (
   (>>>),
   (/>),
+  PU,
   SysConfigList,
   XmlPickler(..),
   hasName,
@@ -22,6 +30,44 @@ 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
+
+
+-- | Represents the DTD filename ("SYSTEM") part of the DOCTYPE
+--   definition.
+newtype DtdName = DtdName String
+
 -- | 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
@@ -48,6 +94,11 @@ parse_opts =
 --   to 'xpickle' would be ambiguous. By returning some @a@s, we allow
 --   the caller to annotate its type.
 --
+--   Note that this will happily pickle nothing to nothing and then
+--   unpickle it back to more nothing. So the fact that the
+--   before/after results from this function agree does not mean that
+--   the document was successfully unpickled!
+--
 pickle_unpickle :: XmlPickler a
                 => String
                 -> FilePath
@@ -69,3 +120,20 @@ pickle_unpickle root_element filepath = do
                    hasName root_element
                    >>>
                    xunpickleVal xpickle
+
+
+
+-- | Is the given XML file unpickleable? Unpickling will be attempted
+--   using the @unpickler@ argument. If we unilaterally used the
+--   generic 'xpickle' function for our unpickler, a type ambiguity
+--   would result. By taking the unpickler as an argument, we allow
+--   the caller to indirectly specify a concrete type.
+--
+unpickleable :: XmlPickler a => FilePath -> PU a -> IO Bool
+unpickleable filepath unpickler = do
+  xmldoc <- try_unpickle `catch` (\(SomeException _) -> return [])
+  return $ (not . null) xmldoc
+  where
+    try_unpickle = runX $ readDocument parse_opts filepath
+                          >>>
+                          xunpickleVal unpickler