C# Language Design Notes for Mar 19, 2018

Warning: These are raw notes, and still need to be cleaned up. Read at your own peril!

Agenda

Triage

7.3

Moved undone 7.3 features to 8.0.

Allow default in deconstruction

We allowed for ==, so it seems we should also allow it for assignment and initialization.

Should probably not require ValueTuple, but that's more of a compiler thing.

Conclusion

The feature is a good idea. Even though it's simple, we should not rush it into 7.3. 8.0.

case Customer { MiddleName: "" or null }

and, or and not patterns 1350

not feels useful. and is hard to come up with scenarios for. or has more scenarios, but may be better served by a in { 1, 2, 3 } style pattern.

if (x is not string s) { ... }
else { ... /* s */ }

not would cause definite assignment to "flip". Should s be disallowed in a not pattern.

The {} pattern for not null is a bit cryptic. not null would certainly be more direct.

Conclusion

Punt to 8.x, when we know more of the rest of the pattern story.

partial type inference #1349

We've talked about this many times in the past, but couldn't settle on a syntax that was nice enough and worth its while.

A 4th option: allow M<string>(...) to match an M with more type parameters. But that probably goes against intuition and would complicate overload resolution.

For 2: Just commas is known from typeof, so that concept is already in the language, though you can't mix "there" and "not there" today.

Conclusion

8.X is when we'll look at it again.

Permit t is null for unconstrained type parameter #1284

For null we special case type parameters in all other kinds of places, so this is the only place where you can't.

Also, t is 3. We allow type patterns, but not constant patterns.

Conclusion

Absolutely should do this. Mark as 8.0.

Implicitly scoped using #114 #1174

We have sympathy for this syntactic sugar, and we'd be interested in allowing it. It seems related to similar features, such as "defer" statements (avoids nesting try/finally).

Also fits nicely with the work we already did with expression variable scoping.

Also namespace without curlies, where you just say namespace X.Y; and it's in force for the remainder of the file.

Conclusion

Discuss at 8.X. Could get pulled up to 8.0. Let's also put defer and namespace into 8.X for consideration then.

User-defined positional patterns

A next step from recursive patterns.

Conclusion

Let's revisit in 8.X when recursive patterns have played out.