+-- | Pickle a 'Double' that can be missing its leading zero (for
+-- values less than one). For example, we've seen,
+--
+-- <TrackLength KPH=".805">0.5</TrackLength>
+--
+-- Which 'xpPrim' can't handle without the leading
+-- zero. Unfortunately there's no way pickle/unpickle can be
+-- inverses of each other here, since \"0.5\" and \".5\" should
+-- unpickle to the same 'Double'.
+--
+-- Examples:
+--
+-- >>> let tn = text_node "0.5"
+-- >>> unpickleDoc xp_fracpart_only_double tn
+-- Just 0.5
+--
+-- >>> let tn = text_node ".5"
+-- >>> unpickleDoc xp_fracpart_only_double tn
+-- Just 0.5
+--
+-- >>> let tn = text_node "foo"
+-- >>> unpickleDoc xp_fracpart_only_double tn
+-- Nothing
+--
+xp_fracpart_only_double :: PU Double
+xp_fracpart_only_double =
+ (to_double, from_double) `xpWrapMaybe` xpText
+ where
+ -- | Convert a 'String' to a 'Double', maybe. We always prepend a
+ -- zero, since it will fix the fraction-only values, and not hurt
+ -- the ones that already have a leading integer.
+ to_double :: String -> Maybe Double
+ to_double s = readMaybe ("0" ++ s)
+
+ from_double :: Double -> String
+ from_double = show
+
+
+