<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><br class=""><div><blockquote type="cite" class=""><div class="">On Mar 17, 2017, at 5:38 PM, Joe Groff 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=""><meta http-equiv="Content-Type" content="text/html charset=us-ascii" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><br class=""><div class=""><blockquote type="cite" class=""><div class="">On Mar 17, 2017, at 12:34 PM, David Hart 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 style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; position: relative;" class=""><span 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-stroke-width: 0px; float: none; display: inline !important;" class="">Sent off-list by mistake:</span><div 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-stroke-width: 0px;"><br class=""></div><div 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-stroke-width: 0px;">Nice proposal. I have a few comments inline:</div><div 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-stroke-width: 0px;"><br class=""><div class=""><blockquote type="cite" class=""><div class="">On 17 Mar 2017, at 18:04, 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;"><div dir="auto" class="" style="word-wrap: break-word; -webkit-nbsp-mode: 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 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></div></div></div></div></blockquote>What metadata is attached? How is it accessed? What future features are you thinking about?</div></div></div></div></blockquote><div class=""><br class=""></div><div class="">To begin with, you'd have limited ability to stringify a key path. Eventually we'd like to support other reflectiony things, including:</div><div class=""><br class=""></div><div class="">- Asking for the primary key paths a type supports</div><div class="">- Asking for a key by name or index</div><div class="">- Breaking a key path down by components</div><div class=""><br class=""></div><div class="">I also see key path objects as a good way of eventually addressing some of the design problems we ran up against with property behaviors (<a href="https://github.com/apple/swift-evolution/blob/master/proposals/0030-property-behavior-decls.md" class="">https://github.com/apple/swift-evolution/blob/master/proposals/0030-property-behavior-decls.md</a> from last year), including the problem of what exactly a property behavior declaration *is* (a type? a protocol? a function-like thing? something completely new?), and the problem of handling "out-of-band" operations on a property beyond getting and setting, such as clearing a cached lazy value, registering for notifications on an observable property, and so on. I think it would be natural to express property behaviors as a user-defined key path type; the key path type can provide the get/set logic for the property as well as any other interesting operations the property supports. This answers the questions of both what behaviors look like (they're just types that conform to KeyPath) and how they extend properties with new actions (they're just methods of the key path value) fairly nicely.</div></div></div></div></blockquote><div><br class=""></div><div>Would these user-defined key path types be unbound like the ones described in the current proposal? That would be really cool. </div><div><br class=""></div><div>I am thinking through how this would impact the design of a library I’m working on. It is a use case where I would want a single unbound instance per property rather than an instance of the key path (and its storage) for each instance of the type. Storage of the property value itself would not be affected by the behavior.</div><div><br class=""></div><div>I would use reflection to discover the key paths of a type that my library recognizes and use them to interact with the property. I would have the user initialize the custom key path for a property with a key path into a related type where both have the same value type. The library would use the key paths to synchronize data between the related instances. There is no need to change the per-instance storage or access (the library would not rely on intercepting specific accesses to the property to synchronize the data). It would only be used as a powerful tool for working with arbitrary types.</div><div><br class=""></div><blockquote type="cite" class=""><div class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class=""><blockquote type="cite" class=""><div class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; position: relative;" class=""><div 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-stroke-width: 0px;"><div class=""><blockquote type="cite" class=""><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space;"><div dir="auto" class="" style="word-wrap: break-word; -webkit-nbsp-mode: space;"><div class=""><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></p></div></div></div></div></blockquote>How different would the design be with generalized existentials? Are they plans to migrate to that design once we do get generalized existentials?<br class=""><blockquote type="cite" class=""><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space;"><div dir="auto" class="" style="word-wrap: break-word; -webkit-nbsp-mode: space;"><div class=""><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 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: rgb(112, 128, 144);">// han</span>
<span class="token comment" spellcheck="true" style="-webkit-print-color-adjust: exact; color: rgb(112, 128, 144);">// 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: rgb(112, 128, 144);">// 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: rgb(112, 128, 144);">// 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: rgb(112, 128, 144);">// MARK - Composition</span>
<span class="token comment" spellcheck="true" style="-webkit-print-color-adjust: exact; color: rgb(112, 128, 144);">// 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: rgb(112, 128, 144);">// 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: rgb(112, 128, 144);">// MARK - Composition</span>
<span class="token comment" spellcheck="true" style="-webkit-print-color-adjust: exact; color: rgb(112, 128, 144);">// 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: rgb(112, 128, 144);">// 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: rgb(112, 128, 144);">// 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: rgb(112, 128, 144);">// Self.type is inferred</span></code></pre></div></div></div></div></div></blockquote><div class=""><br class=""></div>Perhaps I'm missing something, but what does that syntax bring compared to person.name? I see quite a few examples of the key paths being used literally in the subscript syntax but fail to see the usefulness of doing that. Can you give use cases?<br class=""></div></div></div></div></blockquote><div class=""><br class=""></div><div class="">The value comes from `.name` being a separate value from `person`. In the same way that closures let you abstract over functions and methods as plain old values independent of their original context, keypaths should let you do the same with a property.</div><div class=""><br class=""></div><div class="">-Joe</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="">https://lists.swift.org/mailman/listinfo/swift-evolution<br class=""></div></blockquote></div><br class=""></body></html>