The latest information on `reasync`?

Evolution Discussion
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1

I saw this comment the other day:

and felt inspired to make a thread in which the people who know more than I could perhaps give some insight into the current situation with the concept of reasync. Are there doubts about its usefulness? If use cases are what's missing I've got plenty:

Example of real use case for `reasync`

The mutated(by:) method defined below is extremely convenient for a certain style of programming in which expressions are progressively built-up by stacking method calls, like in SwiftUI.

protocol ExpressionErgonomic { }
extension ExpressionErgonomic {
    func mutated (by mutation: (inout Self) throws->()) rethrows -> Self {
        var copy = self
        try mutation(&copy)
        return copy
    }
    
    func mutated (by mutation: (inout Self)async throws->()) async rethrows -> Self {
        var copy = self
        try await mutation(&copy)
        return copy
    }
}

With reasync this would be written as just:

protocol ExpressionErgonomic { }
extension ExpressionErgonomic {
    func mutated (by mutation: (inout Self)async throws->()) reasync rethrows -> Self {
        var copy = self
        try await mutation(&copy)
        return copy
    }
}

Has it just been a matter of strategically deferring implementation costs, but that reasync is sure to be added at some point?

Thanks in advance for the time taken to provide any information!

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2

Would be great to see this. Amongst other things (typed throws, primary associated types), it seems like this would be a prerequisite to including an AsyncSource dual to AsyncSequence within the standard library.

@reasync @rethrows
protocol AsyncSource {
  ...
  func send(_ element: Element) async throws
  ...
}
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3

+1. This and typed throws (or at least some way to get (a) primary associated type(s) for AsyncSequence) are IMO the two pieces that I think are most sorely missing when looking at the current API surface of Swift's standard library.

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4

In this vein, I have written a slightly more elaborate use case for reasync.

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