How to get meaningfull getMessage() of a parse result if parsing fails in optional parser?

[meier-rene]

Hi, this is more a question than an issue. I can not find out how to get a nice failure message in case of an parse failure in an optional parser. Instead I get the message that the following non optional parser fails.
Example:
A string to parse:

line1 value
line2 !
line3 value

My parser definition:

def("start",
ref("line1")
.seq(ref("line2").optional)
.seq(ref("line3"))
.end()

def("line1", StringParser.of("line1 ") 
.seq(CharacterParser.word().plusLazy(Token.NEWLINE_PARSER)
 .seq(Token.NEWLINE_PARSER));

def("line2", StringParser.of("line2 ") 
.seq(CharacterParser.word().plusLazy(Token.NEWLINE_PARSER)
 .seq(Token.NEWLINE_PARSER));

def("line3", StringParser.of("line3 ") 
.seq(CharacterParser.word().plusLazy(Token.NEWLINE_PARSER)
 .seq(Token.NEWLINE_PARSER));

If I parse the string I get line3 expected at the beginning of line 2 from getMessages(). This is in principle correct, but it would be more usefull if I would get something like letter or digit expected at the end of the line 2.

Can someone give me an advice how to get better Messages in case of failure of an optional parser?

[renggli]

You can get the desired behavior by reordering the choices. That is you try first line1 followed by line3; and only then line1 followed by line2 followed by line3:

def("start", ref("line1")
    .seq(ref("line3")
        .or(ref("line2").seq(ref("line3"))))
    .end());

[meier-rene]

Thank you for your suggestions. Unfortunately this will get a bit too complicated, because I have multiple lines with optional components and if I understand it correctly I would have to put all possible combinations in the parser definition. Never mind, I will augment the messages from the parser with some additional logging to make the identification of the actual syntax problem a bit easier.

I would like to thank you for this very nice library. It makes parsing of complicated data really easy.