<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class="">I started a thread earlier this week about strong type-aliases and object aliases. Here’s a fuller proposal on object aliases.</div><div class=""><br class=""></div><div class=""><h1 id="toc_0" style="-webkit-print-color-adjust: exact; margin: 0px 0px 10px; padding: 0px; -webkit-font-smoothing: antialiased; cursor: text; position: relative; font-size: 28px; font-family: Helvetica, arial, sans-serif; background-color: rgb(255, 255, 255);" class="">Feature name</h1><ul style="-webkit-print-color-adjust: exact; margin: 15px 0px; padding-left: 30px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class=""><li style="-webkit-print-color-adjust: exact; margin: 0px;" class="">Proposal: <a href="file:///Users/daryle/NNNN-local-object-aliases.md" style="-webkit-print-color-adjust: exact; color: rgb(65, 131, 196); margin-top: 0px;" class="">SE-NNNN</a></li><li style="-webkit-print-color-adjust: exact; margin: 0px;" class="">Authors: <a href="https://github.com/CTMacUser" style="-webkit-print-color-adjust: exact; color: rgb(65, 131, 196); margin-top: 0px;" class="">Daryle Walker</a>, <a href="https://github.com/swiftdev" style="-webkit-print-color-adjust: exact; color: rgb(65, 131, 196);" class="">Author 2</a></li><li style="-webkit-print-color-adjust: exact; margin: 0px;" class="">Review Manager: TBD</li><li style="-webkit-print-color-adjust: exact; margin: 0px;" class="">Status: <strong style="-webkit-print-color-adjust: exact; margin-top: 0px;" class="">Awaiting review</strong></li></ul><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class=""><em style="-webkit-print-color-adjust: exact;" class="">During the review process, add the following fields as needed:</em></p><ul style="-webkit-print-color-adjust: exact; margin: 15px 0px; padding-left: 30px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class=""><li style="-webkit-print-color-adjust: exact; margin: 0px;" class="">Decision Notes: <a href="https://lists.swift.org/pipermail/swift-evolution/" style="-webkit-print-color-adjust: exact; color: rgb(65, 131, 196); margin-top: 0px;" class="">Rationale</a>, <a href="https://lists.swift.org/pipermail/swift-evolution/" style="-webkit-print-color-adjust: exact; color: rgb(65, 131, 196);" class="">Additional Commentary</a></li><li style="-webkit-print-color-adjust: exact; margin: 0px;" class="">Bugs: <a href="https://bugs.swift.org/browse/SR-NNNN" style="-webkit-print-color-adjust: exact; color: rgb(65, 131, 196); margin-top: 0px;" class="">SR-NNNN</a>, <a href="https://bugs.swift.org/browse/SR-MMMM" style="-webkit-print-color-adjust: exact; color: rgb(65, 131, 196);" class="">SR-MMMM</a></li><li style="-webkit-print-color-adjust: exact; margin: 0px;" class="">Previous Revision: <a href="https://github.com/apple/swift-evolution/blob/...commit-ID.../proposals/NNNN-filename.md" style="-webkit-print-color-adjust: exact; color: rgb(65, 131, 196); margin-top: 0px;" class="">1</a></li><li style="-webkit-print-color-adjust: exact; margin: 0px;" class="">Previous Proposal: <a href="file:///Users/daryle/XXXX-filename.md" style="-webkit-print-color-adjust: exact; color: rgb(65, 131, 196); margin-top: 0px;" class="">SE-XXXX</a></li></ul><h2 id="toc_1" style="-webkit-print-color-adjust: exact; margin: 20px 0px 10px; padding: 0px; -webkit-font-smoothing: antialiased; cursor: text; position: relative; font-size: 24px; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(204, 204, 204); font-family: Helvetica, arial, sans-serif; background-color: rgb(255, 255, 255);" class="">Introduction</h2><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class="">This is a proposal to define aliases to objects.</p><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class="">Swift-evolution thread: <a href="https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170619/037631.html" style="-webkit-print-color-adjust: exact; color: rgb(65, 131, 196);" class="">1</a></p><h2 id="toc_2" style="-webkit-print-color-adjust: exact; margin: 20px 0px 10px; padding: 0px; -webkit-font-smoothing: antialiased; cursor: text; position: relative; font-size: 24px; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(204, 204, 204); font-family: Helvetica, arial, sans-serif; background-color: rgb(255, 255, 255);" class="">Motivation</h2><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class="">Aliasing allows a named object to actually refer to another object instead of newly-allocated storage. Referring to an object with a simple name isn't very useful, but referring to an object needing a complex expression to point to it can help with reducing typing.</p><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class="">However, aliasing has a cost. Compilers have to make certain assumptions if objects can have multiple names referring to them, and these assumptions reduce what kinds of optimizations can be made.</p><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class="">Language design can make a difference in how code can be optimized. Languages like C and C++ assume aliasing is allowed by default, limiting how many optimizations can be done. More recent versions of C have a keyword ("<code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">restrict</code>") to ban certain objects from aliasing. Other languages go the other way; you need to take extra measures to alias objects, since object handling bars aliasing by default.</p><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class="">Swift is currently an alias-adverse language. The model for the equivalent of pointers is supposed to be for short-term use, and not persisted. Other constructs that would use references: read-write properties, read-write subscripts, and <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">inout</code> function parameters, can all be implemented by copy-in-then-copy-out, presumably to avoid alias dynamics and its anti-optimizations. So the scope of aliases here will be limited to local-scale renaming of object locations that the compiler can connect statically.</p><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class="">Yes, the use case is currently weak, but it is a stepping stone for stronger cases, like changing the interface of an object with (currently not in the language) strong type-aliases without copies.</p><h2 id="toc_3" style="-webkit-print-color-adjust: exact; margin: 20px 0px 10px; padding: 0px; -webkit-font-smoothing: antialiased; cursor: text; position: relative; font-size: 24px; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(204, 204, 204); font-family: Helvetica, arial, sans-serif; background-color: rgb(255, 255, 255);" class="">Proposed solution</h2><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class="">The solution is to introduce a new kind of object declaration. It uses a new keyword <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">pose</code> in the same place as <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">let</code> or <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">var</code>. It must be initialized with an expression that specifies an object, and be typed with a layout-compatible type (like the <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">unsafeBitCast</code> function).</p><div style="-webkit-print-color-adjust: exact; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class=""><pre style="-webkit-print-color-adjust: exact; margin-top: 15px; margin-bottom: 15px; background-color: rgb(248, 248, 248); border: 1px solid rgb(204, 204, 204); font-size: 13px; line-height: 19px; overflow: auto; padding: 6px 10px; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class=""><code class="language-swift" style="-webkit-print-color-adjust: exact; margin: 0px; padding: 0px; border: none; background-color: transparent; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">struct Sample {
var test1 = (1, 2, 3, "apple")
//...
func trial1() {
pose firstTestNumber = test1.0
print(firstTestNumber) // prints "1"
//...
firstTestNumber = 4
print(test1.0) // prints "4"
}
}</code></pre></div><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class="">When an object is used, the compiler associates the object with some sort of location ID. An alias just reuses its original's ID instead of having one of its own.</p><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class="">Here, the substitution is simple, but longer chains are imaginable. With a local-scope limitation, aliases work kind-of like macro constants in C.</p><h2 id="toc_4" style="-webkit-print-color-adjust: exact; margin: 20px 0px 10px; padding: 0px; -webkit-font-smoothing: antialiased; cursor: text; position: relative; font-size: 24px; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(204, 204, 204); font-family: Helvetica, arial, sans-serif; background-color: rgb(255, 255, 255);" class="">Detailed design</h2><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class="">Add to the "Grammar of a Declaration":</p><blockquote style="-webkit-print-color-adjust: exact; margin: 15px 0px; border-left-width: 4px; border-left-style: solid; border-left-color: rgb(221, 221, 221); padding: 0px 15px; color: rgb(119, 119, 119); font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class=""><div style="-webkit-print-color-adjust: exact; margin: 0px;" class=""><em style="-webkit-print-color-adjust: exact;" class="">declaration</em> → <em style="-webkit-print-color-adjust: exact;" class="">alias-declaration</em></div></blockquote><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class="">Add a new section "Grammar of an Alias Declaration":</p><blockquote style="-webkit-print-color-adjust: exact; margin: 15px 0px; border-left-width: 4px; border-left-style: solid; border-left-color: rgb(221, 221, 221); padding: 0px 15px; color: rgb(119, 119, 119); font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class=""><div style="-webkit-print-color-adjust: exact; margin: 0px;" class=""><em style="-webkit-print-color-adjust: exact;" class="">alias-declaration</em> → <em style="-webkit-print-color-adjust: exact;" class="">attributes_opt</em> <em style="-webkit-print-color-adjust: exact;" class="">declaration-modifiers_opt</em><strong style="-webkit-print-color-adjust: exact;" class="">pose</strong> <em style="-webkit-print-color-adjust: exact;" class="">pattern-initializer-list</em></div></blockquote><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class="">An alias declaration can only be in the local scope of a function. Expressions that describe source objects must be:</p><ul style="-webkit-print-color-adjust: exact; margin: 15px 0px; padding-left: 30px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class=""><li style="-webkit-print-color-adjust: exact; margin: 0px;" class="">a named object, including function parameters</li><li style="-webkit-print-color-adjust: exact; margin: 0px;" class="">a member of a qualifying tuple object</li><li style="-webkit-print-color-adjust: exact; margin: 0px;" class="">a stored property of a qualifying <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">struct</code> (or <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">class</code>?) object</li></ul><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class="">A source object must have a lifetime at least as long as any aliases to it. A source object cannot have <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">willSet</code> and/or <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">didSet</code>observers. The alias poses as an object of its type annotation, defaulting to the source object's type if omitted. An annotation must be of the source object's type or a layout-compatible type. An alias has the same mutability status as its source object.</p><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class="">An alias has the same operations as its annotated type, using the storage of the source object. An alias used as an <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">inout</code> function argument is banned if it and at least one other <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">inout</code> argument share memory (in whole or in part).</p><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class="">Since source objects are restricted to have their storage established statically, the compiler can reuse a source object's location ID when an alias to that source is referenced. Since an alias doesn't escape its containing function (any returns or <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">inout</code> action would copy to/from the source object), additional global aliasing checks are avoided.</p><h2 id="toc_5" style="-webkit-print-color-adjust: exact; margin: 20px 0px 10px; padding: 0px; -webkit-font-smoothing: antialiased; cursor: text; position: relative; font-size: 24px; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(204, 204, 204); font-family: Helvetica, arial, sans-serif; background-color: rgb(255, 255, 255);" class="">Source compatibility</h2><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class="">Besides the new keyword <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">pose</code>, which should be conditional if possible, the changes are additive. I don't think it is legal to currently use an identifier <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">pose</code> in its planned syntax, so there should be no code to migrate.</p><h2 id="toc_6" style="-webkit-print-color-adjust: exact; margin: 20px 0px 10px; padding: 0px; -webkit-font-smoothing: antialiased; cursor: text; position: relative; font-size: 24px; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(204, 204, 204); font-family: Helvetica, arial, sans-serif; background-color: rgb(255, 255, 255);" class="">Effect on ABI stability</h2><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class="">The effects of aliases happen only during translation, reusing locations of either named objects or a sub-objects of named objects. Since they shouldn't escape the function containing them (and can't be sub-objects of another type), the ABI should be unaffected.</p><h2 id="toc_7" style="-webkit-print-color-adjust: exact; margin: 20px 0px 10px; padding: 0px; -webkit-font-smoothing: antialiased; cursor: text; position: relative; font-size: 24px; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(204, 204, 204); font-family: Helvetica, arial, sans-serif; background-color: rgb(255, 255, 255);" class="">Effect on API resilience</h2><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class="">Since aliases shouldn't leak out from being a function implementation aid, there should be no effect on the API.</p><h2 id="toc_8" style="-webkit-print-color-adjust: exact; margin: 20px 0px 10px; padding: 0px; -webkit-font-smoothing: antialiased; cursor: text; position: relative; font-size: 24px; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(204, 204, 204); font-family: Helvetica, arial, sans-serif; background-color: rgb(255, 255, 255);" class="">Alternatives considered</h2><p style="-webkit-print-color-adjust: exact; margin-top: 15px; margin-right: 0px; margin-left: 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255); margin-bottom: 0px !important;" class="">The alternative is to do nothing. This would currently make reuse of a sub-object for read-write a bit more wordy. But this facility may be more useful if making interface-wise different but layout-compatible types (like strong type-aliases) is added.</p></div><br class=""><div class="">
<div style="color: rgb(0, 0, 0); letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class="">— </div><div class="">Daryle Walker<br class="">Mac, Internet, and Video Game Junkie<br class="">darylew AT mac DOT com </div></div>
</div>
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