add outline of Church option type

[billpmurphy]

Simple implementation of an option type encoded as functions. Feel free to reject if you believe this doesn't fit in with the rest of what's in the church module.

I didn't add anything to impl Term, I'm not sure what (if anything) should be added there for options.

[ljedrz]

I'm all for adding other Church data types (especially when they are as useful as Option<T>), but I always try to provide some reference backing them, at least in the module's documentation; could you provide one?

[billpmurphy]

This took me down a fun research rabbit hole.

The Wikipedia article on Church encoding mentions "tagged unions" but doesn't elaborate. There are few papers (e.g. 1, 2, 3) that mention the Church encoding of algebraic data types as "more common" than Morgensen-Scott encoding or Boehm-Berarducci encoding but don't provide a reference. The Wikipedia article on Morgensen-Scott encoding mentions this but also lacks a reference.

This is the earliest paper I could find that describes how to Church-encode arbitrary algebraic data types. There also are a bunch of blog posts (e.g. 1, 2, 3, 4) that demonstrate the same concept as well with Option/Maybe as an example.

Since there's no drop-in Wikipedia reference, do you have a preference for what to cite?

[ljedrz]

Hmm, there's no clear winner, but I guess we could leave it for later. There are some minor details in this commit that I'd like to adjust when I have time, but overall I think this is a very nice addition to the Church collection.

[ljedrz]

As for adding related impl Term methods I'm not sure myself; on one hand, there are some of them available for Church lists, numerals and pairs and they are much faster than their Church equivalents; on the other, they aren't the core functionality of this library and most users probably aren't concerned with the performance. They may even be dropped altogether in version 2.0.