Hey everyone,

I’m trying to write a parser for todo.txt using NimbleParsec, it’s my first time writing a parser as well as first time being exposed to the idea of parser combinators, so I’m not really sure how to express what I want. At the moment, I have a pretty basic parser which correctly does done state as well as priority of a task. I’m trying to parse dates next.

The todo.txt specification says that two dates can be provided, a start date and an end date. A line item with both a start and end date might look like this

x 2021-01-02 2021-01-01 Make a new years resolution

If two dates are provided, the end date must come first followed by the start date. If only one date is given, it should be the start date. Right now, I have my dates defined like this

  date =
    integer(4)
    |> ignore(string("-"))
    |> integer(2)
    |> ignore(string("-"))
    |> integer(2)

  start_date =
    date
    |> post_traverse({TodoTex.ParserHelper, :map_date, [:end]})
    |> lookahead_not(date)

  end_date =
    date
    |> post_traverse({TodoTex.ParserHelper, :map_date, [:end]})

the function ParserHelper.map_date turns the three integers into a %Date{} struct, as well as adding :start or :end respectively. It looks like this:

  def map_date(_rest, results, context, _line, _offset, tag) do
    {[{:date, tag, apply(Date, :new!, Enum.reverse(results))}], context}
  end

and finally my parser is created like so:

  defparsec(
    :todo,
    optional(done)
    |> optional(ignore(string(" ")))
    |> optional(priority)
    |> optional(ignore(string(" ")))
    |> optional(choice([start_date, end_date]))
    |> optional(ignore(string(" ")))
    |> optional(end_date)
    |> optional(ignore(string(" "))),
    debug: true
  )

When I run this parser with the string with two dates, both of them are tagged with :end. If I run the parser on a string with only one date it’s tagged with :end as well. How can I get the date to be correctly tagged as start if it’s either the only date or the last date given? Thanks.