New Parser: exactly

[jeffrose]

I found myself in a situation where based on the results of a lookAhead() I need to take either x or x-1 characters. I couldn't find a convenient way to do that with the current API, so I wrote a new parser, exactly.

exactly( 3, digit ).run( '12345' );
//=> { result: [ '1', '2', '3' ] ... }

Now this does open the door to some anti-patterns. While exactly( 3, letter ) makes sense, exactly( 3, letters ) does not.

First of all, is there a way I could've done this with the current API that I simply didn't see?
Secondly, would you want a PR for the new parser? Admittedly it's not very complicated but it works.

[francisrstokes]

Hey Jeff,

exactly() would be an excellent addition to the library. I've also found myself needing it, and normally i use:

const exactly = (n, parser) => sequenceOf(Array.from({ length: n}, () => parser);

But it would be good to have a proper parser with descriptive error messages. So I'd welcome a PR.

As for anti-patterns - since these are already possible with things like many(letters) I would say it's on the library user to understand what they're constructing.

[francisrstokes]

Merged and published in 3.1.0 🎉, thanks @jeffrose !