The Trees The Fork Oak Day6 - Pando Parser Completed

Wraping up the parser

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Yesterday we knocked out the difficult portions of the Pando file parser. We built parse functions for the status, the task identifier, and the dependencies list. Today we will tie the parsers together into a full task parser.

Todo Entry

  - Command Line [ IP, DG ]
    status: parse_status >>
    identifier: parse_identifier >>
    dependencies: parse_dependencies >>
    (Todo::new(status, identifier, dependencies))

To combine the individual components of the line parser I used a new macro: do_parse! which threads a series of parsers together, allowing for naming individual results at each step, and finally combining the named results into a final result.

This gets us most of the way there however given that the dependencies parser is optional, we still get an error if we try to parse > todo without a dependencies list. We need to tell the parser what to expect to end the parsing of the task. We turn to the terminated! macro to do this, which takes a base parser, and a terminator parser.

  named!(todo<CompleteStr, Todo>, terminated!(
        status: parse_status >>
        identifier: parse_identifier >>
        dependencies: parse_dependencies >>
        (Todo::new(status, identifier, dependencies))
alt!(eof!() | nom::eol)));

The two things which will for sure finish a todo item are the end of stream, marked by the eof!() macro, and the new line, marked by the eol parser.

Finishing it up

Finally we introduce one last combinator: many1! which produces a parser that can be one or more iterations of another parser. The basic implementation matches our previous parsers:

  named!(pando<CompleteStr, Vec<Todo>, many1!(todo));

However we would prefer to pass the combined parser a &str, so we can unwrap the named! macro into an equivalent function and construct a CompleteStr for the input to the many1! macro.

  fn parse_pando(input: &str) -> IResult<CompleteStr, Vec<Todo>> {
    many1!(input, todo)

Similarly, instead of returning an IResult, which would require dependence and understanding of nom types, it would be better if our parser returned an Option. This isn't really how the types are meant to be used, but it simplifies things for us here so I'm going for it. Sue me.

  fn parse_pando(input: &str) -> Option<Vec<Todo>> {
    match many1!(CompleteStr(input), todo) {
        Ok((_, todos)) => Some(todos),
        Err(_) => None

And voila! We have our file parser. I suspect there are some bugs with regards to line endings (the identifier looks for \n to finish for example), but it will do for now. Here is the test I used to verify things are working.

fn pando_works() {
"x Brainstorm
> Specify Format [ B ]
- Implement Parser [ SF ]
- DOT Generator [ B ]
- Command Line [ IP, DG ]"),
        Ok((CompleteStr(""), vec![
            Todo::new(TaskStatus::Completed, "Brainstorm", vec![]),
            Todo::new(TaskStatus::InProgress, "Specify Format", vec!["B"]),
            Todo::new(TaskStatus::Waiting, "Implement Parser", vec!["SF"]),
            Todo::new(TaskStatus::Waiting, "DOT Generator", vec!["B"]),
            Todo::new(TaskStatus::Waiting, "Command Line", vec!["IP", "DG"])

Next on the docket is the second half of the translator; first we parsed the format into a data structure, now we have to convert that data structure into a DOT file. I will save this step for next time.

Till tomorrow,