Add custom error setter

src/parser/parser.nim

@@ -320,12 +320,22 @@ proc `+`*(parserFnA: Parser -> ParserResult, parserFnB: Parser -> ParserResult):
   return proc(parser: Parser): ParserResult =
     parserFnA(parser).flatMap(parserFnB)
 
+proc setErrorExpectedField(err: ParserError, expected: string): ParserError =
+  ParserError(
+    kind: err.kind,
+    unexpected: err.unexpected,
+    expected: expected,
+    index: err.index,
+    parser: err.parser,
+  )
 
 let newlineParser = choice(@[
   ch(NewLines),
   endOfStream,
 ])
-proc newline*(parser: Parser): ParserResult = newlineParser(parser)
+proc newline*(parser: Parser): ParserResult =
+  newlineParser(parser)
+  .mapErr((x: ParserError) => x.setErrorExpectedField("Newline"))
 
 proc parseSeq*(parser: ParserResult, xs: seq[Parser -> ParserResult]): ParserResult =
   xs.foldl(a.flatMap(b), parser)