Add a rangeAt function to construct a Range at an index.

src/Range.hs

@@ -11,6 +11,10 @@ import Data.Option
 data Range = Range { start :: !Int, end :: !Int }
   deriving (Eq, Show)
 
+-- | Make a range at a given index.
+rangeAt :: Int -> Range
+rangeAt a = Range a a
+
 -- | Return the length of the range.
 rangeLength :: Range -> Int
 rangeLength range = end range - start range