feat(core): add XOR type for mutual exclusion typing

[damienwebdev]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/graycoreio/daffodil/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Currently, we don't have any legible way to describe a Disjoint type.

Fixes: N/A

What is the new behavior?

This adds a new XOR type that we can use to describe our interfaces a bit better.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

Derived from microsoft/TypeScript#14094 (comment)

[yard2010]

Honest question - why is this closed and not merged? Can I do anything about it?

[griest024]

I don't remember exactly but I believe we didn't have a use case for it.

[griest024]

@damienwebdev Do you remember why this was closed? Were there problems with the type behavior?

@yard2010 The code is MIT licensed so feel free to use it somewhere else if you want. Just be sure to give proper credit (note the comment in the TS repo).

[yard2010]

This is embarrassing - I have just realised that this is not the official typescript repo! I got here from this typescript issue and I didn't understand I left the repo. Anyway, thanks for the permission to use it and good luck with what you're doing, it looks fantastic.

[griest024]

good luck with what you're doing, it looks fantastic.

Tell your friends 😁

[damienwebdev]

A comment here, because I came to this conclusion long ago. The XOR type is an anti-pattern that should instead be replaced with a discriminable union. You should "kind" your types with a string kind and discriminate on the kind.

My XOR type works, but goes against what the typescript team is in favor of.