Completion support

[jchapuis]

This PR contains a CompletionSupport trait which can be mixed-in to enable completion for a grammar, i.e. parsers which are 'augmented' with a completions method returning possible entry completions for a certain input. This can be used to elaborate as-you-type completions menus or tab-completion experiences, and is e.g. easy to plug with readline to implement a console application. This is ideal to develop e.g. search query grammars, since the flexibility granted by parser-combinators is very valuable in such a context.

All completion-related types, combinator overloads and tests are located in a separate completions package and there no impact to existing code. As a result, a small amount of duplication was required for some combinators (but it's very limited).

Note that in addition to this base set of definitions, I could also add a fuzzy completion mix-in to this PR, which allows defining as-you-type experiences supporting fuzzy string matching in an easy way.

Also, let me know of any required adaptation to the copyright notice for inclusion consideration. Thanks!

[gourlaysama]

Thanks for working on this @jchapuis. That looks like a great addition to parser-combinators! I'll try and review and comment soon :)

[SethTisue]

I asked on Gitter (scala/contributors) if anyone else would like to help review this.

[jchapuis]

Hi all, if there is any way I can help in making this move forward, let me know! This extension has proven very valuable for us, and I find that it is an ideal use case for parser combinators. Indeed, when developing interactive and dynamic grammars, flexibility is much more important than performance - and flexibility is where combinators shine. Completions with dynamic grammars and fuzzy string matching allow for slick natural language search experiences for instance (which is what we use it for).

[olafurpg]

This is cool! Would it be possible to use completions to generate helpful parse error messages? For example,

Parse error!
animal plane
       ^
found: plane
expected: bear | lion

[jchapuis]

@olafurpg thanks! This is a great idea, I will look into this. It should be indeed feasible to "augment" error messages for parsers with CompletionSupport.

[jchapuis]

For information, I've made this available in a separate library: https://github.com/jchapuis/scala-parser-combinators-completion. It works in the same way, mixing-in the CompletionSupport trait augments parsers with completion support.

So I'm closing this pull request for now (let's see if that library gains some traction and interest).