Additional test cases and bug fixes for subtle issues

src/Parser.hs

@@ -20,6 +20,7 @@ data SaplingAST
   deriving (Show, Eq, Ord)
 
 parseSapling :: String -> SaplingAST
+parseSapling "" = error "Empty input provided to parseSapling"
 parseSapling input = case runParser parseExpression "" (lexSapling input) of
   Left err -> error "Failed to parse input"
   Right ast -> ast
@@ -122,8 +123,14 @@ parseListLiteral = do
   satisfy (== LCloseBracket)
   return (SList elements)
 
+--parseListItem :: Parser SaplingAST
+--parseListItem = parseGroupedItem <|> parseSingleItem
 parseListItem :: Parser SaplingAST
-parseListItem = parseGroupedItem <|> parseSingleItem
+parseListItem = choice
+  [ parseGroupedItem  -- Handle expressions inside parentheses
+  , parseListLiteral  -- Allow nested lists
+  , parseSingleItem   -- Handle single tokens like `t` or identifiers
+  ]
 
 parseGroupedItem :: Parser SaplingAST
 parseGroupedItem = do