Clean commented-out code, expose functions to handle errors and prepare FromXML implementation

lib/Text/InvisiXML/Namespace.hs

@@ -3,13 +3,12 @@ module Text.InvisiXML.Namespace (
       uRI
     , prefix
     , ixml
-    --, at
-    --, to
     , addAttr
+    , getAttr
     , setChildren
   ) where
 
---import Data.PositionTree (Position(..))
+import Data.List (find)
 import Text.XML.Light (Attr(..), Content(..), Element(..), QName(..), add_attr)
 
 uRI :: String
@@ -21,14 +20,6 @@ prefix = "ixml"
 ixml :: String -> QName
 ixml qName = QName {qName, qURI = Just uRI, qPrefix = Just prefix}
 
-{-
-at :: Position -> Attr
-at (Position p) = Attr (ixml "at") (show p)
-
-to :: Position -> Attr
-to (Position p) = Attr (ixml "to") (show p)
--}
-
 onContent :: (Element -> Element) -> Content -> Content
 onContent f (Elem e) = Elem (f e)
 onContent _ x = x
@@ -36,5 +27,8 @@ onContent _ x = x
 addAttr :: Attr -> Content -> Content
 addAttr = onContent . add_attr
 
+getAttr :: QName -> Element -> Maybe String
+getAttr key = fmap attrVal . find ((==) key . attrKey) . elAttribs
+
 setChildren :: [Content] -> Content -> Content
 setChildren elContent = onContent $ \e -> e {elContent}

lib/Text/XML/Light/Serializer.hs

 module Text.XML.Light.Serializer (
       FromXML(..)
     , ToXML(..)
-    , (.=)
     , decode
+    , eitherDecode
     , encode
+    , (.=)
+    , expected
   ) where
 
 import Data.List (intercalate)
+import Text.Printf (printf)
 import Text.XML.Light (Attr(..), Content, QName, parseXML, ppContent)
 import Text.XML.Light.Lexer (XmlSource)
 
@@ -16,14 +19,18 @@ class FromXML a where
 class ToXML a where
   toXML :: a -> [Content]
 
-encode :: ToXML a => a -> String
-encode = intercalate "\n" . fmap ppContent . toXML
-
 decode :: (XmlSource s, FromXML a) => s -> Maybe a
 decode = either (\_ -> Nothing) Just . eitherDecode
 
 eitherDecode :: (XmlSource s, FromXML a) => s -> Either String a
 eitherDecode = fromXML . parseXML
 
+encode :: ToXML a => a -> String
+encode = intercalate "\n" . fmap ppContent . toXML
+
 (.=) :: Show v => QName -> v -> Attr
 k .= v = Attr k $ show v
+
+expected :: String -> [Content] -> Either String a
+expected elemType =
+  Left . printf "Expected %s but got %s" elemType . concat . fmap ppContent