Improve list parser

Current parser for lists, like `packages:` in `cabal.project.local`,
fails to process some input, for example:

packages:
    foo/foo.cabal
  , bar/bar.cabal
  , baz/baz.cabal

This commit simplifies the parser by using `sepBy` directly and by
handling indented fields more uniformly.

src/Hie/Cabal/Parser.hs

@@ -6,6 +6,7 @@ import Control.Applicative
 import Control.Monad
 import Data.Attoparsec.Text
 import Data.Char
+import Data.Functor
 import Data.Maybe
 import Data.Text (Text)
 import qualified Data.Text as T
@@ -92,19 +93,24 @@ parseString = parseQuoted <|> unqualName
 unqualName :: Parser Text
 unqualName = takeWhile1 (not . (\c -> isSpace c || c == ','))
 
+-- | Skip spaces and if enf of line is reached, skip it as well and require that
+-- next one starts with indent.
+--
+-- Used for parsing fields.
+optSkipToNextLine :: Indent -> Parser ()
+optSkipToNextLine i = do
+  skipMany $ satisfy (\c -> isSpace c && not (isEndOfLine c))
+  mChar <- peekChar
+  case mChar of
+    Just c | isEndOfLine c ->
+      char c *> indent i $> ()
+    _ -> pure ()
+
+-- | Comma or space separated list, with optional new lines.
 parseList :: Indent -> Parser [Text]
 parseList i = items <|> (emptyOrComLine >> indent i >> items)
   where
-    items = do
-      skipMany tabOrSpace
-      h <- parseString
-      skipMany tabOrSpace
-      skipMany (char ',')
-      t <-
-        items
-          <|> (skipToNextLine >> indent i >> parseList i)
-          <|> pure []
-      pure $ h : t
+    items = sepBy parseString (optSkipToNextLine i *> skipMany (char ',') *> optSkipToNextLine i)
 
 pathMain :: Indent -> [Text] -> Text -> [Text] -> [Text] -> Parser [Text]
 pathMain i p m o a =

test/Spec.hs

@@ -86,6 +86,10 @@ spec = do
     $ it "quoted list"
     $ ("\"one\"\n two\n three3" :: Text) ~> parseList 1
       `shouldParse` ["one", "two", "three3"]
+  describe "Should Succeed"
+    $ it "list with leading commas"
+    $ ("one\n , two\n , three3" :: Text) ~> parseList 1
+      `shouldParse` ["one", "two", "three3"]
 
 exeSection :: Text
 exeSection =