ignore but retain trailing or leading blank lines

Author: elaforge

TODO

@@ -4,8 +4,7 @@ string-literal
   * ensure single line works
   * real tests, empty line handling and round trip
   - documentation
-  - ignore but retain trailing or leading blank lines
-    . only when not --wrapped though
+  * ignore but retain trailing or leading blank lines
   - I might want to be able to ignore but preserve leading gunk on the first
     line, e.g. for
     ( "blah\

src/StringLiteral.hs

@@ -27,7 +27,7 @@ main :: IO ()
 main = do
     let showUsage = putStr usage >> Exit.exitFailure
     args <- maybe showUsage return . parseArgs =<< Environment.getArgs
-    Text.IO.interact $ Text.unlines . process args . Text.lines
+    Text.IO.interact $ Text.unlines . focusNonEmpty (process args) . Text.lines
 
 data Operation = Add | Remove deriving (Eq, Show)
 data Kind = Backslash | Lines deriving (Eq, Show)
@@ -64,6 +64,17 @@ process (wrapped, op, kind) = case (wrapped, op, kind) of
 indentation :: Text
 indentation = "    "
 
+-- | Pass through leading and trailing blank lines.
+focusNonEmpty :: ([Text] -> [Text]) -> [Text] -> [Text]
+focusNonEmpty f lines = pre ++ f within ++ post
+    where (pre, within, post) = span2 (Text.null . Text.strip) lines
+
+span2 :: (a -> Bool) -> [a] -> ([a], [a], [a])
+span2 f xs = (pre, in2, post)
+    where
+    (pre, in1) = span f xs
+    (in2, post) = List.Extra.spanEnd f in1
+
 -- * backslashes
 
 inferBackslashed :: [Text] -> Bool