[Pitch] Property delegate composability, backing storage, and $

[DaveZ (David Zarzycki)]

@Douglas_Gregor

Why is it important that this feature use the attribute syntax? What alternatives were considered?

The proposal has some alternatives---the Kotlin-inspired by syntax and the original property behavior's [behavior-name] syntax. However, custom attributes were by far the most popular, and I like them as a general extension point (vs. inventing new-new syntax for everything).

What about my proposal to use underscores to make type variables explicit? To repeat our conversation from SE-0258, instead of @A @B var x = y, one would write: var x : A<B<_>> = y and sidestep the unordered nature of attributes (and not "burn through" the attribute namespace). Here is a link to our earlier conversation:

SE-0258: Property Delegates

I think this would be a nice improvement to the language that composes well with property delegates. It stands alone for, e.g. something like: let d: Dictionary<_, Double> = ["one": 1, "pi": 3.14159] // infer the Key type to be String I don't know that we could intercept the reference-counting operations well enough, but it's possible that we could pull some Builtin tricks in the standard library to do this. Doug