<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div><br><br>Sent from my iPad</div><div><br>On Mar 17, 2017, at 5:17 PM, Michael LeHew <<a href="mailto:lehewjr@apple.com">lehewjr@apple.com</a>> wrote:<br><br></div><blockquote type="cite"><div><meta http-equiv="Content-Type" content="text/html charset=utf-8"><br class=""><div><blockquote type="cite" class=""><div class="">On Mar 17, 2017, at 3:08 PM, Matthew Johnson <<a href="mailto:matthew@anandabits.com" class="">matthew@anandabits.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space; position: relative;" class=""><blockquote type="cite" class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px;"><div class=""><br class="Apple-interchange-newline">On Mar 17, 2017, at 12:04 PM, Michael LeHew via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><div dir="auto" class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;">Hi friendly swift-evolution folks,<div class=""><br class=""></div><div class="">The Foundation and Swift team would like for you to consider the following proposal:</div></div></div></div></blockquote><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><br class=""></div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class="">This proposal is really incredible! It is an invaluable addition to the language - far better than simple first-class properties. I really can’t wait to see it implemented! The design looks very solid. I’m especially happy to see that a path to eventually get away from using classes has already been identified and planned for.</div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><br class=""></div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class="">Thank you so much for bringing this forward in Swift 4. It is a wonderful (and rather unexpected) surprise!</div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><br class=""></div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class="">Seeing this makes me *really* wish we had a way to get at a collection of `PartialKeyPath<Self>` for all the (visible) properties of a type. I guess the <i class="">visible</i> part of that makes it tricky. We can always work around it in the meantime.</div></div></div></blockquote><div><br class=""></div><div>We had discussed that a future application where KeyPath's would make a lot of sense is with the Mirror API. Of course in the interest of the finiteness of time, we aren't pursuing that right now. </div><div><br class=""></div><div>One thing that gets interesting with the scope-restricted visibility of KeyPaths, is what happens if an fileprivate KeyPath gets leaked out of the file? That's a scary/maybe useful thing? But a complication that emerges pretty quick and thus another reason not to pursue that just now. </div></div></div></blockquote><div><br></div>Yep, totally understand. :) The interaction with access control will definitely have some subtleties to consider.<div><br></div><div>Is the plan to allow a type to manually vend a KeyPath wider than the visibility of properties in the path? It looks that way and that's probably the right call.<div><div><br><blockquote type="cite"><div><div><br class=""><blockquote type="cite" class=""><div class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space; position: relative;" class=""><br class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;"><blockquote type="cite" class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px;"><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><div dir="auto" class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><div class=""><br class=""></div><div class="">Many thanks,</div><div class="">-Michael</div><div class=""><br class=""></div><div class=""><h1 id="toc_0" class="" 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);">Smart KeyPaths: Better Key-Value Coding for Swift</h1><ul class="" 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);"><li class="" style="-webkit-print-color-adjust: exact; margin: 0px;">Proposal: SE-NNNN</li><li class="" style="-webkit-print-color-adjust: exact; margin: 0px;">Authors: <a href="https://github.com/Catfish-Man" class="" style="-webkit-print-color-adjust: exact; color: rgb(65, 131, 196); margin-top: 0px;">David Smith</a>, <a href="https://github.com/mlehew" class="" style="-webkit-print-color-adjust: exact; color: rgb(65, 131, 196);">Michael LeHew</a>, <a href="https://github.com/jckarter" class="" style="-webkit-print-color-adjust: exact; color: rgb(65, 131, 196);">Joe Groff</a></li><li class="" style="-webkit-print-color-adjust: exact; margin: 0px;">Review Manager: TBD</li><li class="" style="-webkit-print-color-adjust: exact; margin: 0px;">Status: <strong class="" style="-webkit-print-color-adjust: exact; margin-top: 0px;">Awaiting Review</strong></li><li class="" style="-webkit-print-color-adjust: exact; margin: 0px;">Associated PRs:<ul class="" style="-webkit-print-color-adjust: exact; margin: 0px 0px 15px; padding-left: 30px;"><li class="" style="-webkit-print-color-adjust: exact; margin: 0px;"><a href="https://github.com/apple/swift-evolution/pull/644" class="" style="-webkit-print-color-adjust: exact; color: rgb(65, 131, 196); margin-top: 0px;">#644</a></li></ul></li></ul><h2 id="toc_1" class="" 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);">Introduction</h2><p class="" style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);">We propose a family of concrete <em class="" style="-webkit-print-color-adjust: exact;">Key Path</em> types that represent uninvoked references to properties that can be composed to form paths through many values and directly get/set their underlying values.</p><h2 id="toc_2" class="" 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);">Motivation</h2><h4 id="toc_3" class="" style="-webkit-print-color-adjust: exact; margin: 20px 0px 10px; padding: 0px; -webkit-font-smoothing: antialiased; cursor: text; position: relative; font-size: 16px; font-family: Helvetica, arial, sans-serif; background-color: rgb(255, 255, 255);">We Can Do Better than String</h4><p class="" style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);">On Darwin platforms Swift's existing <code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">#keyPath()</code> syntax provides a convenient way to safely <em class="" style="-webkit-print-color-adjust: exact;">refer</em> to properties. Unfortunately, once validated, the expression becomes a <code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">String</code> which has a number of important limitations:</p><ul class="" 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);"><li class="" style="-webkit-print-color-adjust: exact; margin: 0px;">Loss of type information (requiring awkward <code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">Any</code> APIs)</li><li class="" style="-webkit-print-color-adjust: exact; margin: 0px;">Unnecessarily slow to parse</li><li class="" style="-webkit-print-color-adjust: exact; margin: 0px;">Only applicable to <code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">NSObjects</code></li><li class="" style="-webkit-print-color-adjust: exact; margin: 0px;">Limited to Darwin platforms</li></ul><h4 id="toc_4" class="" style="-webkit-print-color-adjust: exact; margin: 20px 0px 10px; padding: 0px; -webkit-font-smoothing: antialiased; cursor: text; position: relative; font-size: 16px; font-family: Helvetica, arial, sans-serif; background-color: rgb(255, 255, 255);">Use/Mention Distinctions</h4><p class="" style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);">While methods can be referred to without invoking them (<code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">let x = foo.bar</code> instead of <code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">let x = foo.bar()</code>), this is not currently possible for properties and subscripts.</p><p class="" style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);">Making indirect references to a properties' concrete types also lets us expose metadata about the property, and in the future additional behaviors.</p><h4 id="toc_5" class="" style="-webkit-print-color-adjust: exact; margin: 20px 0px 10px; padding: 0px; -webkit-font-smoothing: antialiased; cursor: text; position: relative; font-size: 16px; font-family: Helvetica, arial, sans-serif; background-color: rgb(255, 255, 255);">More Expressive KeyPaths</h4><p class="" style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);">We would also like to support being able to use <em class="" style="-webkit-print-color-adjust: exact;">Key Paths</em> to access into collections, which is not currently possible.</p><h2 id="toc_6" class="" 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);">Proposed solution</h2><p class="" style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);">We propose introducing a new expression akin to <code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">Type.method</code>, but for properties and subscripts. These property reference expressions produce <code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">KeyPath</code> objects, rather than <code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">Strings</code>. <code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">KeyPaths</code> are a family of generic classes <em class="" style="-webkit-print-color-adjust: exact;">(structs and protocols here would be ideal, but requires generalized existentials)</em> which encapsulate a property reference or chain of property references, including the type, mutability, property name(s), and ability to set/get values.</p><p class="" style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);">Here's a sample of it in use:</p><div class="" style="-webkit-print-color-adjust: exact; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);"><div class="prism-show-language" style="-webkit-print-color-adjust: exact; position: relative;"><div class="prism-show-language-label" style="-webkit-print-color-adjust: exact; background-color: rgb(207, 207, 207); display: inline-block; position: absolute; bottom: auto; left: auto; top: 0px; right: 0px; width: auto; height: auto; font-size: 0.9em; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 5px; padding: 0px 0.5em; text-shadow: none; z-index: 1; -webkit-box-shadow: none; box-shadow: none; transform: none;">Swift</div></div><pre class=" language-swift" style="-webkit-print-color-adjust: exact; margin-top: 0.5em; margin-bottom: 0.5em; background-color: rgb(245, 242, 240); border: 1px solid rgb(204, 204, 204); font-size: 13px; line-height: 1.5; white-space: pre-wrap; overflow: auto; padding: 1em; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; text-shadow: white 0px 1px; font-family: Consolas, Monaco, 'Andale Mono', 'Ubuntu Mono', monospace; word-spacing: normal; word-break: normal; word-wrap: normal; tab-size: 4; -webkit-hyphens: none;"><code class=" language-swift" style="-webkit-print-color-adjust: exact; margin: 0px; padding: 0px; border: none; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-image: none; text-shadow: white 0px 1px; font-family: Consolas, Monaco, 'Andale Mono', 'Ubuntu Mono', monospace; word-spacing: normal; word-break: normal; word-wrap: normal; line-height: 1.5; tab-size: 4; -webkit-hyphens: none;"><span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">struct</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Person</span> <span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">{</span>
<span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">var</span> name<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">String</span>
<span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">var</span> friends<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">[</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Person</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">]</span>
<span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">var</span> bestFriend<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Person</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">?</span>
<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">}</span>
<span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">var</span> han <span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">=</span> <span class="token function" style="-webkit-print-color-adjust: exact; color: rgb(221, 74, 104);">Person</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">(</span>name<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token string" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">"Han Solo"</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">,</span> friends<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">[</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">]</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">)</span>
<span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">var</span> luke <span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">=</span> <span class="token function" style="-webkit-print-color-adjust: exact; color: rgb(221, 74, 104);">Person</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">(</span>name<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token string" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">"Luke Skywalker"</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">,</span> friends<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">[</span>han<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">]</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">)</span>
<span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">let</span> firstFriendsNameKeyPath <span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">=</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Person</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">.</span>friends<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">[</span><span class="token number" style="-webkit-print-color-adjust: exact; color: rgb(153, 0, 85);">0</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">]</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">.</span>name
<span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">let</span> firstFriend <span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">=</span> luke<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">[</span>path<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">]</span> <span class="token comment" spellcheck="true" style="-webkit-print-color-adjust: exact; color: slategray;">// han</span>
<span class="token comment" spellcheck="true" style="-webkit-print-color-adjust: exact; color: slategray;">// or equivalently, with type inferred from context</span>
<span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">let</span> firstFriendName <span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">=</span> luke<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">[</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">.</span>friends<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">[</span><span class="token number" style="-webkit-print-color-adjust: exact; color: rgb(153, 0, 85);">0</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">]</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">.</span>name<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">]</span>
<span class="token comment" spellcheck="true" style="-webkit-print-color-adjust: exact; color: slategray;">// rename Luke's first friend</span>
luke<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">[</span>firstFriendsNameKeyPath<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">]</span> <span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">=</span> <span class="token string" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">"A Disreputable Smuggler"</span>
<span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">let</span> bestFriendsName <span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">=</span> luke<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">[</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">.</span>bestFriend<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">]</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">?</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">.</span>name <span class="token comment" spellcheck="true" style="-webkit-print-color-adjust: exact; color: slategray;">// nil, if he is the last jedi</span></code></pre></div><h2 id="toc_7" class="" 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);">Detailed design</h2><h3 id="toc_8" class="" style="-webkit-print-color-adjust: exact; margin: 20px 0px 10px; padding: 0px; -webkit-font-smoothing: antialiased; cursor: text; position: relative; font-size: 18px; font-family: Helvetica, arial, sans-serif; background-color: rgb(255, 255, 255);">Core KeyPath Types</h3><p class="" style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);"><code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">KeyPaths</code> are a hierarchy of progressively more specific classes, based on whether we have prior knowledge of the path through the object graph we wish to traverse. </p><h5 id="toc_9" class="" style="-webkit-print-color-adjust: exact; margin: 20px 0px 10px; padding: 0px; -webkit-font-smoothing: antialiased; cursor: text; position: relative; font-size: 14px; font-family: Helvetica, arial, sans-serif; background-color: rgb(255, 255, 255);">Unknown Path / Unknown Root Type</h5><p class="" style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);"><code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">AnyKeyPath</code> is fully type-erased, referring to 'any route' through an object/value graph for 'any root'. Because of type-erasure many operations can fail at runtime and are thus nillable. </p><div class="" style="-webkit-print-color-adjust: exact; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);"><div class="prism-show-language" style="-webkit-print-color-adjust: exact; position: relative;"><div class="prism-show-language-label" style="-webkit-print-color-adjust: exact; background-color: rgb(207, 207, 207); display: inline-block; position: absolute; bottom: auto; left: auto; top: 0px; right: 0px; width: auto; height: auto; font-size: 0.9em; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 5px; padding: 0px 0.5em; text-shadow: none; z-index: 1; -webkit-box-shadow: none; box-shadow: none; transform: none;">Swift</div></div><pre class=" language-swift" style="-webkit-print-color-adjust: exact; margin-top: 0.5em; margin-bottom: 0.5em; background-color: rgb(245, 242, 240); border: 1px solid rgb(204, 204, 204); font-size: 13px; line-height: 1.5; white-space: pre-wrap; overflow: auto; padding: 1em; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; text-shadow: white 0px 1px; font-family: Consolas, Monaco, 'Andale Mono', 'Ubuntu Mono', monospace; word-spacing: normal; word-break: normal; word-wrap: normal; tab-size: 4; -webkit-hyphens: none;"><code class=" language-swift" style="-webkit-print-color-adjust: exact; margin: 0px; padding: 0px; border: none; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-image: none; text-shadow: white 0px 1px; font-family: Consolas, Monaco, 'Andale Mono', 'Ubuntu Mono', monospace; word-spacing: normal; word-break: normal; word-wrap: normal; line-height: 1.5; tab-size: 4; -webkit-hyphens: none;"><span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">class</span> <span class="token class-name" style="-webkit-print-color-adjust: exact;">AnyKeyPath</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">CustomDebugStringConvertible</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">,</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Hashable</span> <span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">{</span>
<span class="token comment" spellcheck="true" style="-webkit-print-color-adjust: exact; color: slategray;">// MARK - Composition</span>
<span class="token comment" spellcheck="true" style="-webkit-print-color-adjust: exact; color: slategray;">// Returns nil if path.rootType != self.valueType</span>
<span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">func</span> <span class="token function" style="-webkit-print-color-adjust: exact; color: rgb(221, 74, 104);">appending</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">(</span>path<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">AnyKeyPath</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">)</span> <span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">-</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">AnyKeyPath</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">?</span>
<span class="token comment" spellcheck="true" style="-webkit-print-color-adjust: exact; color: slategray;">// MARK - Runtime Information </span>
<span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">class</span> <span class="token class-name" style="-webkit-print-color-adjust: exact;">var</span> rootType<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Any</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">.</span><span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">Type</span>
<span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">class</span> <span class="token class-name" style="-webkit-print-color-adjust: exact;">var</span> valueType<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Any</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">.</span><span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">Type</span>
<span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">static</span> <span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">func</span> <span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">==</span> <span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">(</span>lhs<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">AnyKeyPath</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">,</span> rhs<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">AnyKeyPath</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">)</span> <span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">-</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Bool</span>
<span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">var</span> hashValue<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Int</span>
<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">}</span></code></pre></div><h5 id="toc_10" class="" style="-webkit-print-color-adjust: exact; margin: 20px 0px 10px; padding: 0px; -webkit-font-smoothing: antialiased; cursor: text; position: relative; font-size: 14px; font-family: Helvetica, arial, sans-serif; background-color: rgb(255, 255, 255);">Unknown Path / Known Root Type</h5><p class="" style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);">If we know a little more type information (what kind of thing the key path is relative to), then we can use <code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">PartialKeyPath <Root></code>, which refers to an 'any route' from a known root:</p><div class="" style="-webkit-print-color-adjust: exact; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);"><div class="prism-show-language" style="-webkit-print-color-adjust: exact; position: relative;"><div class="prism-show-language-label" style="-webkit-print-color-adjust: exact; background-color: rgb(207, 207, 207); display: inline-block; position: absolute; bottom: auto; left: auto; top: 0px; right: 0px; width: auto; height: auto; font-size: 0.9em; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 5px; padding: 0px 0.5em; text-shadow: none; z-index: 1; -webkit-box-shadow: none; box-shadow: none; transform: none;">Swift</div></div><pre class=" language-swift" style="-webkit-print-color-adjust: exact; margin-top: 0.5em; margin-bottom: 0.5em; background-color: rgb(245, 242, 240); border: 1px solid rgb(204, 204, 204); font-size: 13px; line-height: 1.5; white-space: pre-wrap; overflow: auto; padding: 1em; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; text-shadow: white 0px 1px; font-family: Consolas, Monaco, 'Andale Mono', 'Ubuntu Mono', monospace; word-spacing: normal; word-break: normal; word-wrap: normal; tab-size: 4; -webkit-hyphens: none;"><code class=" language-swift" style="-webkit-print-color-adjust: exact; margin: 0px; padding: 0px; border: none; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-image: none; text-shadow: white 0px 1px; font-family: Consolas, Monaco, 'Andale Mono', 'Ubuntu Mono', monospace; word-spacing: normal; word-break: normal; word-wrap: normal; line-height: 1.5; tab-size: 4; -webkit-hyphens: none;"><span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">class</span> <span class="token class-name" style="-webkit-print-color-adjust: exact;">PartialKeyPath</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);"><</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Root</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">AnyKeyPath</span> <span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">{</span>
<span class="token comment" spellcheck="true" style="-webkit-print-color-adjust: exact; color: slategray;">// MARK - Composition</span>
<span class="token comment" spellcheck="true" style="-webkit-print-color-adjust: exact; color: slategray;">// Returns nil if Value != self.valueType</span>
<span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">func</span> <span class="token function" style="-webkit-print-color-adjust: exact; color: rgb(221, 74, 104);">appending</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">(</span>path<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">AnyKeyPath</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">)</span> <span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">-</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">PartialKeyPath</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);"><</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Root</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">?</span>
<span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">func</span> appending<span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);"><</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Value</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">,</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">AppendedValue</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">(</span>path<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">KeyPath</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);"><</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Value</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">,</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">AppendedValue</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">)</span> <span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">-</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">KeyPath</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);"><</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Root</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">,</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">AppendedValue</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">?</span>
<span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">func</span> appending<span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);"><</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Value</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">,</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">AppendedValue</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">(</span>path<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">ReferenceKeyPath</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);"><</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Value</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">,</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">AppendedValue</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">)</span> <span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">-</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">ReferenceKeyPath</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);"><</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Root</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">,</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">AppendedValue</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">?</span>
<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">}</span></code></pre></div><h5 id="toc_11" class="" style="-webkit-print-color-adjust: exact; margin: 20px 0px 10px; padding: 0px; -webkit-font-smoothing: antialiased; cursor: text; position: relative; font-size: 14px; font-family: Helvetica, arial, sans-serif; background-color: rgb(255, 255, 255);">Known Path / Known Root Type</h5><p class="" style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);">When we know both what the path is relative to and what it refers to, we can use <code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">KeyPath</code>. Thanks to the knowledge of the Root and Value types, all of the failable operations lose their Optional. </p><div class="" style="-webkit-print-color-adjust: exact; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);"><div class="prism-show-language" style="-webkit-print-color-adjust: exact; position: relative;"><div class="prism-show-language-label" style="-webkit-print-color-adjust: exact; background-color: rgb(207, 207, 207); display: inline-block; position: absolute; bottom: auto; left: auto; top: 0px; right: 0px; width: auto; height: auto; font-size: 0.9em; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 5px; padding: 0px 0.5em; text-shadow: none; z-index: 1; -webkit-box-shadow: none; box-shadow: none; transform: none;">Swift</div></div><pre class=" language-swift" style="-webkit-print-color-adjust: exact; margin-top: 0.5em; margin-bottom: 0.5em; background-color: rgb(245, 242, 240); border: 1px solid rgb(204, 204, 204); font-size: 13px; line-height: 1.5; white-space: pre-wrap; overflow: auto; padding: 1em; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; text-shadow: white 0px 1px; font-family: Consolas, Monaco, 'Andale Mono', 'Ubuntu Mono', monospace; word-spacing: normal; word-break: normal; word-wrap: normal; tab-size: 4; -webkit-hyphens: none;"><code class=" language-swift" style="-webkit-print-color-adjust: exact; margin: 0px; padding: 0px; border: none; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-image: none; text-shadow: white 0px 1px; font-family: Consolas, Monaco, 'Andale Mono', 'Ubuntu Mono', monospace; word-spacing: normal; word-break: normal; word-wrap: normal; line-height: 1.5; tab-size: 4; -webkit-hyphens: none;"><span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">public</span> <span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">class</span> <span class="token class-name" style="-webkit-print-color-adjust: exact;">KeyPath</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);"><</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Root</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">,</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Value</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">PartialKeyPath</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);"><</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Root</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span> <span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">{</span>
<span class="token comment" spellcheck="true" style="-webkit-print-color-adjust: exact; color: slategray;">// MARK - Composition</span>
<span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">func</span> appending<span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);"><</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">AppendedValue</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">(</span>path<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">KeyPath</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);"><</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Value</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">,</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">AppendedValue</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">)</span> <span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">-</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">KeyPath</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);"><</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Root</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">,</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">AppendedValue</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span>
<span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">func</span> appending<span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);"><</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">AppendedValue</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">(</span>path<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">WritableKeyPath</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);"><</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Value</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">,</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">AppendedValue</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">)</span> <span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">-</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span> <span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">Self</span>
<span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">func</span> appending<span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);"><</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">AppendedValue</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">(</span>path<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">ReferenceWritableKeyPath</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);"><</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Value</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">,</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">AppendedValue</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">)</span> <span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">-</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">ReferenceWritableKeyPath</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);"><</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Root</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">,</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">AppendedValue</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span>
<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">}</span></code></pre></div><h5 id="toc_12" class="" style="-webkit-print-color-adjust: exact; margin: 20px 0px 10px; padding: 0px; -webkit-font-smoothing: antialiased; cursor: text; position: relative; font-size: 14px; font-family: Helvetica, arial, sans-serif; background-color: rgb(255, 255, 255);">Value/Reference Mutation Semantics Mutation</h5><p class="" style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);">Finally, we have a pair of subclasses encapsulating value/reference mutation semantics. These have to be distinct because mutating a copy of a value is not very useful, so we need to mutate an inout value.</p><div class="" style="-webkit-print-color-adjust: exact; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);"><div class="prism-show-language" style="-webkit-print-color-adjust: exact; position: relative;"><div class="prism-show-language-label" style="-webkit-print-color-adjust: exact; background-color: rgb(207, 207, 207); display: inline-block; position: absolute; bottom: auto; left: auto; top: 0px; right: 0px; width: auto; height: auto; font-size: 0.9em; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 5px; padding: 0px 0.5em; text-shadow: none; z-index: 1; -webkit-box-shadow: none; box-shadow: none; transform: none;">Swift</div></div><pre class=" language-swift" style="-webkit-print-color-adjust: exact; margin-top: 0.5em; margin-bottom: 0.5em; background-color: rgb(245, 242, 240); border: 1px solid rgb(204, 204, 204); font-size: 13px; line-height: 1.5; white-space: pre-wrap; overflow: auto; padding: 1em; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; text-shadow: white 0px 1px; font-family: Consolas, Monaco, 'Andale Mono', 'Ubuntu Mono', monospace; word-spacing: normal; word-break: normal; word-wrap: normal; tab-size: 4; -webkit-hyphens: none;"><code class=" language-swift" style="-webkit-print-color-adjust: exact; margin: 0px; padding: 0px; border: none; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-image: none; text-shadow: white 0px 1px; font-family: Consolas, Monaco, 'Andale Mono', 'Ubuntu Mono', monospace; word-spacing: normal; word-break: normal; word-wrap: normal; line-height: 1.5; tab-size: 4; -webkit-hyphens: none;"><span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">class</span> <span class="token class-name" style="-webkit-print-color-adjust: exact;">WritableKeyPath</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);"><</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Root</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">,</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Value</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">KeyPath</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);"><</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Root</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">,</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Value</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span> <span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">{</span>
<span class="token comment" spellcheck="true" style="-webkit-print-color-adjust: exact; color: slategray;">// MARK - Composition</span>
<span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">func</span> appending<span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);"><</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">AppendedPathValue</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">(</span>path<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">WritableKeyPath</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);"><</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Value</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">,</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">AppendedPathValue</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">)</span> <span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">-</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">WritableKeyPath</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);"><</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Root</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">,</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">AppendedPathValue</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span>
<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">}</span>
<span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">class</span> <span class="token class-name" style="-webkit-print-color-adjust: exact;">ReferenceWritableKeyPath</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);"><</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Root</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">,</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Value</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">WritableKeyPath</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);"><</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Root</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">,</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Value</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span> <span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">{</span>
<span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">override</span> <span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">func</span> appending<span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);"><</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">AppendedPathValue</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">(</span>path<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">WritableKeyPath</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);"><</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Value</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">,</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">AppendedPathValue</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">)</span> <span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">-</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">ReferenceWritableKeyPath</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);"><</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Root</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">,</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">AppendedPathValue</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span>
<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">}</span></code></pre></div><h3 id="toc_13" class="" style="-webkit-print-color-adjust: exact; margin: 20px 0px 10px; padding: 0px; -webkit-font-smoothing: antialiased; cursor: text; position: relative; font-size: 18px; font-family: Helvetica, arial, sans-serif; background-color: rgb(255, 255, 255);">Access and Mutation Through KeyPaths</h3><p class="" style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);">To get or set values for a given root and key path we effectively add the following subscripts to all Swift types. </p><div class="" style="-webkit-print-color-adjust: exact; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);"><div class="prism-show-language" style="-webkit-print-color-adjust: exact; position: relative;"><div class="prism-show-language-label" style="-webkit-print-color-adjust: exact; background-color: rgb(207, 207, 207); display: inline-block; position: absolute; bottom: auto; left: auto; top: 0px; right: 0px; width: auto; height: auto; font-size: 0.9em; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 5px; padding: 0px 0.5em; text-shadow: none; z-index: 1; -webkit-box-shadow: none; box-shadow: none; transform: none;">Swift</div></div><pre class=" language-swift" style="-webkit-print-color-adjust: exact; margin-top: 0.5em; margin-bottom: 0.5em; background-color: rgb(245, 242, 240); border: 1px solid rgb(204, 204, 204); font-size: 13px; line-height: 1.5; white-space: pre-wrap; overflow: auto; padding: 1em; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; text-shadow: white 0px 1px; font-family: Consolas, Monaco, 'Andale Mono', 'Ubuntu Mono', monospace; word-spacing: normal; word-break: normal; word-wrap: normal; tab-size: 4; -webkit-hyphens: none;"><code class=" language-swift" style="-webkit-print-color-adjust: exact; margin: 0px; padding: 0px; border: none; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-image: none; text-shadow: white 0px 1px; font-family: Consolas, Monaco, 'Andale Mono', 'Ubuntu Mono', monospace; word-spacing: normal; word-break: normal; word-wrap: normal; line-height: 1.5; tab-size: 4; -webkit-hyphens: none;"><span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">extension</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Any</span> <span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">{</span>
<span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">subscript</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">(</span>path<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">AnyKeyPath</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">)</span> <span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">-</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Any</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">?</span> <span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">{</span> <span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">get</span> <span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">}</span>
<span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">subscript</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);"><</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Root</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">Self</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">(</span>path<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">PartialKeyPath</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);"><</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Root</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">)</span> <span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">-</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Any</span> <span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">{</span> <span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">get</span> <span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">}</span>
<span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">subscript</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);"><</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Root</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">Self</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">,</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Value</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">(</span>path<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">KeyPath</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);"><</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Root</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">,</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Value</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">)</span> <span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">-</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Value</span> <span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">{</span> <span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">get</span> <span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">}</span>
<span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">subscript</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);"><</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Root</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">Self</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">,</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Value</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">(</span>path<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">WritableKeyPath</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);"><</span><span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Root</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">,</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Value</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">)</span> <span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">-</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Value</span> <span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">{</span> <span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">set</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">,</span> <span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">get</span> <span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">}</span>
<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">}</span></code></pre></div><p class="" style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);">This allows for code like</p><div class="" style="-webkit-print-color-adjust: exact; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);"><div class="prism-show-language" style="-webkit-print-color-adjust: exact; position: relative;"><div class="prism-show-language-label" style="-webkit-print-color-adjust: exact; background-color: rgb(207, 207, 207); display: inline-block; position: absolute; bottom: auto; left: auto; top: 0px; right: 0px; width: auto; height: auto; font-size: 0.9em; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 5px; padding: 0px 0.5em; text-shadow: none; z-index: 1; -webkit-box-shadow: none; box-shadow: none; transform: none;">Swift</div></div><pre class=" language-swift" style="-webkit-print-color-adjust: exact; margin-top: 0.5em; margin-bottom: 0.5em; background-color: rgb(245, 242, 240); border: 1px solid rgb(204, 204, 204); font-size: 13px; line-height: 1.5; white-space: pre-wrap; overflow: auto; padding: 1em; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; text-shadow: white 0px 1px; font-family: Consolas, Monaco, 'Andale Mono', 'Ubuntu Mono', monospace; word-spacing: normal; word-break: normal; word-wrap: normal; tab-size: 4; -webkit-hyphens: none;"><code class=" language-swift" style="-webkit-print-color-adjust: exact; margin: 0px; padding: 0px; border: none; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-image: none; text-shadow: white 0px 1px; font-family: Consolas, Monaco, 'Andale Mono', 'Ubuntu Mono', monospace; word-spacing: normal; word-break: normal; word-wrap: normal; line-height: 1.5; tab-size: 4; -webkit-hyphens: none;">person<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">[</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">.</span>name<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">]</span> <span class="token comment" spellcheck="true" style="-webkit-print-color-adjust: exact; color: slategray;">// Self.type is inferred</span></code></pre></div><p class="" style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);">which is both appealingly readable, and doesn't require read-modify-write copies (subscripts access <code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">self</code> inout). Conflicts with existing subscripts are avoided by using generic subscripts to specifically only accept key paths with a <code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">Root</code> of the type in question.</p><h3 id="toc_14" class="" style="-webkit-print-color-adjust: exact; margin: 20px 0px 10px; padding: 0px; -webkit-font-smoothing: antialiased; cursor: text; position: relative; font-size: 18px; font-family: Helvetica, arial, sans-serif; background-color: rgb(255, 255, 255);">Referencing Key Paths</h3><p class="" style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);">Forming a <code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">KeyPath</code> borrows from the same syntax used to reference methods and initializers,<code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">Type.instanceMethod</code> only now working for properties and collections. Optionals are handled via optional-chaining. Multiply dotted expressions are allowed as well, and work just as if they were composed via the <code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">appending</code> methods on <code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">KeyPath</code>.</p><p class="" style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);">There is no change or interaction with the #keyPath() syntax introduced in Swift 3. </p><h3 id="toc_15" class="" style="-webkit-print-color-adjust: exact; margin: 20px 0px 10px; padding: 0px; -webkit-font-smoothing: antialiased; cursor: text; position: relative; font-size: 18px; font-family: Helvetica, arial, sans-serif; background-color: rgb(255, 255, 255);">Performance</h3><p class="" style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);">The performance of interacting with a property via <code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">KeyPaths</code> should be close to the cost of calling the property directly.</p><h2 id="toc_16" class="" 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);">Source compatibility</h2><p class="" style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);">This change is additive and there should no affect on existing source. </p><h2 id="toc_17" class="" 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);">Effect on ABI stability</h2><p class="" style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);">This feature adds the following requirements to ABI stability: </p><ul class="" 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);"><li class="" style="-webkit-print-color-adjust: exact; margin: 0px;">mechanism to access key paths of public properties</li></ul><p class="" style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);">We think a protocol-based design would be preferable once the language has sufficient support for generalized existentials to make that ergonomic. By keeping the class hierarchy closed and the concrete implementations private to the implementation it should be tractable to provide compatibility with an open protocol-based design in the future.</p><h2 id="toc_18" class="" 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);">Effect on API resilience</h2><p class="" style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);">This should not significantly impact API resilience, as it merely provides a new mechanism for operating on existing APIs.</p><h2 id="toc_19" class="" 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);">Alternatives considered</h2><h4 id="toc_20" class="" style="-webkit-print-color-adjust: exact; margin: 20px 0px 10px; padding: 0px; -webkit-font-smoothing: antialiased; cursor: text; position: relative; font-size: 16px; font-family: Helvetica, arial, sans-serif; background-color: rgb(255, 255, 255);">More Features</h4><p class="" style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);">Various drafts of this proposal have included additional features (decomposable key paths, prefix comparisons, support for custom <code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">KeyPath</code> subclasses, creating a <code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">KeyPath</code> from a <code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">String</code> at runtime, <code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">KeyPaths</code> conforming to <code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">Codable</code>, bound key paths as a concrete type, etc.). We anticipate approaching these enhancements additively once the core <code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">KeyPath</code> functionality is in place. </p><h4 id="toc_21" class="" style="-webkit-print-color-adjust: exact; margin: 20px 0px 10px; padding: 0px; -webkit-font-smoothing: antialiased; cursor: text; position: relative; font-size: 16px; font-family: Helvetica, arial, sans-serif; background-color: rgb(255, 255, 255);">Spelling</h4><p class="" style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);">We also explored many different spellings, each with different strengths. We have chosen the current syntax due to the balance with existing function type references.</p><table class="" style="-webkit-print-color-adjust: exact; margin: 15px 0px; padding: 0px; border-collapse: collapse; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);"><thead class="" style="-webkit-print-color-adjust: exact;"><tr class="" style="-webkit-print-color-adjust: exact; border-top-width: 1px; border-top-style: solid; border-top-color: rgb(204, 204, 204); margin: 0px; padding: 0px;"><th class="" style="-webkit-print-color-adjust: exact; border: 1px solid rgb(204, 204, 204); margin: 0px; padding: 6px 13px;">Current</th><th class="" style="-webkit-print-color-adjust: exact; border: 1px solid rgb(204, 204, 204); margin: 0px; padding: 6px 13px;"><code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">#keyPath</code></th><th class="" style="-webkit-print-color-adjust: exact; border: 1px solid rgb(204, 204, 204); margin: 0px; padding: 6px 13px;">Lisp-style</th></tr></thead><tbody class="" style="-webkit-print-color-adjust: exact;"><tr class="" style="-webkit-print-color-adjust: exact; border-top-width: 1px; border-top-style: solid; border-top-color: rgb(204, 204, 204); margin: 0px; padding: 0px;"><td class="" style="-webkit-print-color-adjust: exact; border: 1px solid rgb(204, 204, 204); margin: 0px; padding: 6px 13px;"><code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">Person.friends[0].name</code></td><td class="" style="-webkit-print-color-adjust: exact; border: 1px solid rgb(204, 204, 204); margin: 0px; padding: 6px 13px;"><code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">#keyPath(Person, .friends[0].name)</code></td><td class="" style="-webkit-print-color-adjust: exact; border: 1px solid rgb(204, 204, 204); margin: 0px; padding: 6px 13px;">`<code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">Person.friend.name</code></td></tr><tr class="" style="-webkit-print-color-adjust: exact; border-top-width: 1px; border-top-style: solid; border-top-color: rgb(204, 204, 204); background-color: rgb(248, 248, 248); margin: 0px; padding: 0px;"><td class="" style="-webkit-print-color-adjust: exact; border: 1px solid rgb(204, 204, 204); margin: 0px; padding: 6px 13px;"><code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">luke[.friends[0].name]</code></td><td class="" style="-webkit-print-color-adjust: exact; border: 1px solid rgb(204, 204, 204); margin: 0px; padding: 6px 13px;"><code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">#keyPath(luke, .friends[0].name)</code></td><td class="" style="-webkit-print-color-adjust: exact; border: 1px solid rgb(204, 204, 204); margin: 0px; padding: 6px 13px;"><code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">luke</code>`<code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">.friends[0].name</code></td></tr><tr class="" style="-webkit-print-color-adjust: exact; border-top-width: 1px; border-top-style: solid; border-top-color: rgb(204, 204, 204); margin: 0px; padding: 0px;"><td class="" style="-webkit-print-color-adjust: exact; border: 1px solid rgb(204, 204, 204); margin: 0px; padding: 6px 13px;"><code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">luke.friends[0][.name]</code></td><td class="" style="-webkit-print-color-adjust: exact; border: 1px solid rgb(204, 204, 204); margin: 0px; padding: 6px 13px;"><code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">#keyPath(luke.friends[0], .name)</code></td><td class="" style="-webkit-print-color-adjust: exact; border: 1px solid rgb(204, 204, 204); margin: 0px; padding: 6px 13px;"><code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">luke.friends[0]</code>`<code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">.name</code></td></tr></tbody></table><p class="" style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);">While the crispness is very appealing, the spelling of the 'escape' character was hard to agree upon (along with the fact that it requires parentheses to reduce ambiguity). <code class="" style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: pre-wrap; 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;">#keyPath</code> was very specific, but verbose especially when composing multiple key paths together.</p></div></div></div>_______________________________________________<br class="">swift-evolution mailing list<br class=""><a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a><br class=""><a href="https://lists.swift.org/mailman/listinfo/swift-evolution" class="">https://lists.swift.org/mailman/listinfo/swift-evolution</a></div></blockquote></div></div></blockquote></div><br class=""></div></blockquote></div></div></div></body></html>