What does Collection.subscript._read do in Swift 5?

DevAndArtist · January 31, 2019, 9:20am

I don't know if this is still up-to-date, as in this thread John McCall and others mentioned that __shared is slightly different than originally described in the manifesto, but the idea behind read is summed up there nicely:

github.com

apple/swift/blob/main/docs/OwnershipManifesto.md#generalized-accessors

# Ownership

## Introduction

Adding "ownership" to Swift is a major feature with many benefits
for programmers.  This document is both a "manifesto" and a
"meta-proposal" for ownership: it lays out the basic goals of
the work, describes a general approach for achieving those goals,
and proposes a number of specific changes and features, each of
which will need to be separately discussed in a smaller and more
targeted proposal.  This document is intended to provide a framework
for understanding the contributions of each of those changes.

### Problem statement

The widespread use of copy-on-write value types in Swift has generally
been a success.  It does, however, come with some drawbacks:

* Reference counting and uniqueness testing do impose some overhead.

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Since read is not officially proposed yet, it's still prefixed with an underscore.

So instead of return you must yield, but I don't know if it's available in the beta or maybe as _yield, which is also not officially proposed yet.