Add 'unpickleable' to Xml for use in tests.

diff --git a/src/Xml.hs b/src/Xml.hs
index 96ba0116e184986b7b0a1942a5d96c002eb860db..2f68dd09c661de44048f78fe0a45314fe4d38b98 100644 (file)
--- a/src/Xml.hs
+++ b/src/Xml.hs
@@ -6,13 +6,16 @@ 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,
@@ -91,6 +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
@@ -112,3 +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