Parser overhaul

[erikedin]

The parser did not turn out as well as I'd hoped. In order to add Rules, and become less brittle, I think it needs an overhaul. I made an experimental prototype in a separate repository, and I believe that parser combinators will work well for this.

I'll use this issue to track progress, aiming to parse a full Feature, with limited but correct functionality.

• Rules
• Only Given step
• Block texts in Givens
• No data tables
• Scenario but not Outlines or Backgrounds
• Should skip empty lines and comments

Once this is done, I'll create additional issues for fixing the missing pieces.

Basic parser combinators unrelated to Gherkin specifics

• [x] Line parser that matches a full line
• [x] An Optionally parser that only optionally parses something.
• [x] A Sequence parser
• [x] A logical Or parser
• [x] Repeating parser
• [x] LineIfNot, for recognizing any line that isn't a given parser
• [x] Transformer, converts one parse result to another type using a transform function
• [x] Joined, joins many strings
• [x] StartsWith, recognizing a line that starts with a given word

Gherkin specific things

• [x] An operation that skips empty lines and comments
• [x] A Keyword parser that recognizes keywords like Scenario:, Background:, or Given.
• [x] Block text parser
• [x] Given step parser
• [x] Rule parser
• [x] Scenario parser
• [x] Feature parser

[mkschulze]

that sounds like a lot of work

[erikedin]

Note as much as you'd think. Most of the tasks in the list above are actually trivial. It's a lot less work than the current parser, for sure.

[erikedin]

The new experimental parser can now parse a very simple Feature file with Rules, Scenarios, and Given steps. I'll create additional issues for the missing features until the new parser is complete.