Actually added Pattern.hs

This file should have been in the earlier patch at 22:01 on Mon Aug 20, but it got missed out by accident.

Pattern.hs

@@ -0,0 +1,34 @@
+module Pattern (Pattern(..)) where
+
+import Data.Generics
+
+data Pattern =
+    -- | We don't care what the value is -- will match against any item
+    DontCare
+    -- | A named "item of interest".  Given the specified key, all data items with the same key should be identical (otherwise match errors will be given)
+    | Named String Pattern
+    -- | A constructed item.  This is special because the sub-items may not be of the right type for the constructor,
+    --   because they may be special items (such as DontCare)
+    | Match Constr [Pattern]
+  deriving (Typeable,Show,Eq)
+
+--No proper gunfold, as I still can't figure out to implement it (Constr is problematic)
+instance Data Pattern where
+  toConstr (DontCare) = constr_DontCare
+  toConstr (Named {}) = constr_Named
+  toConstr (Match {}) = constr_Match
+
+  gunfold _ _ _ = error "gunfold Pattern"
+  
+  dataTypeOf _ = ty_Pattern
+  
+ty_Pattern = mkDataType "TreeUtil.Pattern"
+   [
+    constr_DontCare
+    ,constr_Named
+    ,constr_Match
+   ]
+ 
+constr_DontCare = mkConstr ty_Pattern "DontCare" [] Prefix
+constr_Named = mkConstr ty_Pattern "Named" [] Prefix
+constr_Match = mkConstr ty_Pattern "Match" [] Prefix