<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class="">I do not feel `AnySequence` for these overloads being a problem TBH, at least conceptually. IIUC a subsequence is expected to be a view to a consecutive range of the sequence, and require no extra storage since it is just about index manipulation.</div><div class=""><br class=""></div><div class="">On the other hand, filtering or mapping a collection involves transformations with non-trivial computational cost, in which case the transformed collections are generally expected to be further manipulated, instead of just iteration. That is likely why these are by default eager, since being lazy would mean the transformation has to happen every time an element is touched, given the constraint of no extra storage and no mutation as a read-only view (i.e. cannot cache the results).</div><div class=""><br class=""></div><div class="">Anyway, I remembered in the early days of swift-evolution, a similar topic had been brought up and the response was that the transforming methods e.g. `map` are deliberately chosen to be eager. Please correct me if my memory serves me wrong.</div><div class=""><br class=""></div><div class=""><br class=""></div><div class="">Regards</div><div class="">Anders</div><br class=""><div><blockquote type="cite" class=""><div class="">On 26 Apr 2017, at 4:26 AM, Pavol Vaskovic 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=utf-8" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class="">Hello,</div><div class=""><br class=""></div>I was planning to wait pitching this after I have tested the implementation, but I’m getting anxious, that I’ll miss the 4.0 boat, so here goes. This is early draft. I would especially appreciate analysis of the impact on source compatibility and ABI stability. <div class=""><br class=""></div><div class=""><div class=""><a href="https://github.com/palimondo/swift-evolution/blob/sequence-cleanup/proposals/NNNN-SequenceCleanup.md" class="">https://github.com/palimondo/swift-evolution/blob/sequence-cleanup/proposals/NNNN-SequenceCleanup.md</a></div></div><div class=""><br class=""></div><div class=""><h1 style="box-sizing: border-box; margin-right: 0px; margin-bottom: 16px; margin-left: 0px; line-height: 1.25; padding-bottom: 0.3em; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(234, 236, 239); color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-variant-ligatures: normal; orphans: 2; widows: 2; margin-top: 0px !important;" class="">Sequence Cleanup</h1><ul style="box-sizing: border-box; padding-left: 2em; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class=""><li style="box-sizing: border-box;" class="">Proposal: <a href="https://github.com/palimondo/swift-evolution/blob/sequence-cleanup/proposals/NNNN-SequenceCleanup.md" style="box-sizing: border-box; color: rgb(3, 102, 214); text-decoration: none;" class="">SE-NNNN</a></li><li style="box-sizing: border-box; margin-top: 0.25em;" class="">Authors: <a href="https://github.com/palimondo" style="box-sizing: border-box; color: rgb(3, 102, 214); text-decoration: none;" class="">Pavol Vaskovic</a>, <a href="https://github.com/swiftdev" style="box-sizing: border-box; color: rgb(3, 102, 214); text-decoration: none;" class="">Author 2</a></li><li style="box-sizing: border-box; margin-top: 0.25em;" class="">Review Manager: TBD</li><li style="box-sizing: border-box; margin-top: 0.25em;" class="">Status: <span style="box-sizing: border-box; font-weight: 600;" class="">Pitch</span></li></ul><h2 style="box-sizing: border-box; margin-top: 24px; margin-bottom: 16px; line-height: 1.25; padding-bottom: 0.3em; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(234, 236, 239); color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-variant-ligatures: normal; orphans: 2; widows: 2;" class=""><a id="user-content-introduction" class="anchor" href="https://github.com/palimondo/swift-evolution/blob/sequence-cleanup/proposals/NNNN-SequenceCleanup.md#introduction" aria-hidden="true" style="box-sizing: border-box; color: rgb(3, 102, 214); text-decoration: none; float: left; padding-right: 4px; margin-left: -20px; line-height: 1;"><svg aria-hidden="true" class="octicon octicon-link" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Introduction</h2><p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">This proposal is to change the default implementation of <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.0470588); border-radius: 3px;" class="">protocol Sequence</code> methods so that all operations are eager (not lazy) and return <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.0470588); border-radius: 3px;" class="">Array<Element></code>. This removes the need to return type erased <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.0470588); border-radius: 3px;" class="">AnySequence</code> that negatively impacts performance in order to hide the heterogenous implementation and provides Swift users with more homogenous interface. The lazy computation of <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.0470588); border-radius: 3px;" class="">prefix</code> and <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.0470588); border-radius: 3px;" class="">dropFirst</code> methods will be moved to <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.0470588); border-radius: 3px;" class="">LazySequence</code>.</p><p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">Swift-evolution thread: <a href="https://lists.swift.org/pipermail/swift-evolution/" style="box-sizing: border-box; color: rgb(3, 102, 214); text-decoration: none;" class="">Discussion thread topic for that proposal</a></p><h2 style="box-sizing: border-box; margin-top: 24px; margin-bottom: 16px; line-height: 1.25; padding-bottom: 0.3em; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(234, 236, 239); color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-variant-ligatures: normal; orphans: 2; widows: 2;" class=""><a id="user-content-motivation" class="anchor" href="https://github.com/palimondo/swift-evolution/blob/sequence-cleanup/proposals/NNNN-SequenceCleanup.md#motivation" aria-hidden="true" style="box-sizing: border-box; color: rgb(3, 102, 214); text-decoration: none; float: left; padding-right: 4px; margin-left: -20px; line-height: 1;"><svg aria-hidden="true" class="octicon octicon-link" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Motivation</h2><p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">The API of <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.0470588); border-radius: 3px;" class="">Sequence</code> protocol evolved over time to include irregular mixture of eager and lazy methods. In particular, it included lazy implementation of <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.0470588); border-radius: 3px;" class="">prefix</code> and <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.0470588); border-radius: 3px;" class="">dropFirst</code> from before the lazy sequence views found their home in <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.0470588); border-radius: 3px;" class="">LazySequence</code> protocol. They are implemented by internal classes that wrap the underlying sequence and delay the computation until its elements are requested through iteration. <a href="https://github.com/palimondo/swift-evolution/blob/sequence-cleanup/proposals/SE-0045.md" style="box-sizing: border-box; color: rgb(3, 102, 214); text-decoration: none;" class="">SE-0045</a> added third internal class to implement <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.0470588); border-radius: 3px;" class="">drop(while:_)</code>, that eagerly drops the required element and wraps the partially used <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.0470588); border-radius: 3px;" class="">Iterator</code> from the original sequence.</p><p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">All other default implementations of <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.0470588); border-radius: 3px;" class="">Sequence</code> methods are realized on top of <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.0470588); border-radius: 3px;" class="">Array</code>s and perform their computation eagerly. Due to the requirements of the associated type <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.0470588); border-radius: 3px;" class="">SubSequence</code> returned from many <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.0470588); border-radius: 3px;" class="">Sequence</code> protocol methods, all these heterogeneous implementations need to be hidden using type erasure by wrapping the results in <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.0470588); border-radius: 3px;" class="">AnySequence</code>. The heterogeneity of implementation is currently well documented, if the user notices the different complexity of the methods that return lazy wrappers -- Complexity: O(1), instead of O(n). But given the existence of other lazy sequence views on <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.0470588); border-radius: 3px;" class="">LazySequence</code> one might be surprised by this irregularity and overlook this historical quirk of implementation.</p><p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">Furthermore, the presence of <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.0470588); border-radius: 3px;" class="">AnySequence</code> and <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.0470588); border-radius: 3px;" class="">AnyIterator</code> it returns, present serious optimization obstacle for the Swift compiler, when we chain the calls to these sequence methods. This results is non-specialized generic iterator, that ends up in a tight for-in loop. Currently there is no <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.0470588); border-radius: 3px;" class="">prefix</code> implementation on <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.0470588); border-radius: 3px;" class="">LazySequence</code> - preventing the workaround for avoiding <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.0470588); border-radius: 3px;" class="">AnySequence</code> by going through <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.0470588); border-radius: 3px;" class="">.lazy</code> call.</p><div class="highlight highlight-source-swift" style="box-sizing: border-box; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;"><pre style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 13.6px; margin-top: 0px; margin-bottom: 0px; font-variant-ligatures: normal; font-variant-east-asian: normal; font-variant-position: normal; font-stretch: normal; line-height: 1.45; word-wrap: normal; padding: 16px; overflow: auto; background-color: rgb(246, 248, 250); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; word-break: normal;" class="">s.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">lazy</span>.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">prefix</span>(maxIterations).<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">prefix</span>(<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">while</span>:{ <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">...</span> })</pre></div><h2 style="box-sizing: border-box; margin-top: 24px; margin-bottom: 16px; line-height: 1.25; padding-bottom: 0.3em; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(234, 236, 239); color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-variant-ligatures: normal; orphans: 2; widows: 2;" class=""><a id="user-content-proposed-solution" class="anchor" href="https://github.com/palimondo/swift-evolution/blob/sequence-cleanup/proposals/NNNN-SequenceCleanup.md#proposed-solution" aria-hidden="true" style="box-sizing: border-box; color: rgb(3, 102, 214); text-decoration: none; float: left; padding-right: 4px; margin-left: -20px; line-height: 1;"><svg aria-hidden="true" class="octicon octicon-link" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Proposed solution</h2><p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">We should take this opportunity to clean up the interface of default <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.0470588); border-radius: 3px;" class="">Sequence</code> implementation to present Swift users with homogenous interface of eager methods that directly return <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.0470588); border-radius: 3px;" class="">Array</code> as <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.0470588); border-radius: 3px;" class="">SubSequence</code>. We'll move the lazy implementation for <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.0470588); border-radius: 3px;" class="">prefix</code> and <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.0470588); border-radius: 3px;" class="">dropFirst</code> to <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.0470588); border-radius: 3px;" class="">LazySequence</code>.</p><h2 style="box-sizing: border-box; margin-top: 24px; margin-bottom: 16px; line-height: 1.25; padding-bottom: 0.3em; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(234, 236, 239); color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-variant-ligatures: normal; orphans: 2; widows: 2;" class=""><a id="user-content-detailed-design" class="anchor" href="https://github.com/palimondo/swift-evolution/blob/sequence-cleanup/proposals/NNNN-SequenceCleanup.md#detailed-design" aria-hidden="true" style="box-sizing: border-box; color: rgb(3, 102, 214); text-decoration: none; float: left; padding-right: 4px; margin-left: -20px; line-height: 1;"><svg aria-hidden="true" class="octicon octicon-link" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Detailed design</h2><p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">The following changes will be made to the standard library:</p><div class="highlight highlight-source-swift" style="box-sizing: border-box; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;"><pre style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 13.6px; margin-top: 0px; margin-bottom: 0px; font-variant-ligatures: normal; font-variant-east-asian: normal; font-variant-position: normal; font-stretch: normal; line-height: 1.45; word-wrap: normal; padding: 16px; overflow: auto; background-color: rgb(246, 248, 250); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; word-break: normal;" class=""><span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">extension</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);"><span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Sequence</span></span> {
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">public</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">func</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">suffix</span>(<span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">_</span> <span class="pl-smi" style="box-sizing: border-box; color: rgb(51, 51, 51);">maxLength</span>: <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Int</span>) <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">-></span> [<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Iterator</span>.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Element</span>] { <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">...</span> }
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">public</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">func</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">split</span>(
<span class="pl-smi" style="box-sizing: border-box; color: rgb(51, 51, 51);"><span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">maxSplits</span></span>: <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Int</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">=</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Int</span>.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">max</span>,
<span class="pl-smi" style="box-sizing: border-box; color: rgb(51, 51, 51);"><span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">omittingEmptySubsequences</span></span>: <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Bool</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">=</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">true</span>,
<span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">whereSeparator</span> <span class="pl-smi" style="box-sizing: border-box; color: rgb(51, 51, 51);">isSeparator</span>: (<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Iterator</span>.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Element</span>) <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">throws</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">-></span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Bool</span>
) <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">rethrows</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">-></span> [[<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Iterator</span>.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Element</span>]] { <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">...</span> }
}
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">extension</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);"><span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Sequence</span></span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">where</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Iterator</span>.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Element</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">:</span> <span class="pl-e" style="box-sizing: border-box; color: rgb(121, 93, 163);"><span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Equatable</span></span> {
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">public</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">func</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">split</span>(
<span class="pl-smi" style="box-sizing: border-box; color: rgb(51, 51, 51);"><span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">separator</span></span>: <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Iterator</span>.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Element</span>,
<span class="pl-smi" style="box-sizing: border-box; color: rgb(51, 51, 51);"><span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">maxSplits</span></span>: <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Int</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">=</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Int</span>.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">max</span>,
<span class="pl-smi" style="box-sizing: border-box; color: rgb(51, 51, 51);"><span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">omittingEmptySubsequences</span></span>: <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Bool</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">=</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">true</span>
) <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">-></span> [[<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Iterator</span>.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Element</span>]] { <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">...</span> }
}
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">extension</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);"><span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Sequence</span></span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">where</span>
<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">SubSequence</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">:</span> <span class="pl-e" style="box-sizing: border-box; color: rgb(121, 93, 163);"><span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Sequence</span></span>,
<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">SubSequence</span>.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Iterator</span>.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Element</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">==</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Iterator</span>.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Element</span>,
<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">SubSequence</span>.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">SubSequence</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">==</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">SubSequence</span> {
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"><span class="pl-c" style="box-sizing: border-box;">///</span> Returns a subsequence containing all but the given number of initial</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"></span> <span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"><span class="pl-c" style="box-sizing: border-box;">///</span> elements.</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"></span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">...</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"><span class="pl-c" style="box-sizing: border-box;">///</span> - Complexity: O(*n*), where *n* is the length of the sequence.</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"></span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">@_inlineable</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">public</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">func</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">dropFirst</span>(<span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">_</span> <span class="pl-smi" style="box-sizing: border-box; color: rgb(51, 51, 51);">n</span>: <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Int</span>) <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">-></span> [<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Iterator</span>.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Element</span>] { <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">...</span> }
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"><span class="pl-c" style="box-sizing: border-box;">///</span> Returns a subsequence containing all but the given number of final</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"></span> <span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"><span class="pl-c" style="box-sizing: border-box;">///</span> elements.</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"></span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">...</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"><span class="pl-c" style="box-sizing: border-box;">///</span> - Complexity: O(*n*), where *n* is the length of the sequence.</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"></span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">@_inlineable</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">public</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">func</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">dropLast</span>(<span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">_</span> <span class="pl-smi" style="box-sizing: border-box; color: rgb(51, 51, 51);">n</span>: <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Int</span>) <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">-></span> [<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Iterator</span>.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Element</span>] { <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">...</span> }
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"><span class="pl-c" style="box-sizing: border-box;">///</span> Returns a subsequence by skipping the initial, consecutive elements that</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"></span> <span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"><span class="pl-c" style="box-sizing: border-box;">///</span> satisfy the given predicate.</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"></span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">...</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"><span class="pl-c" style="box-sizing: border-box;">///</span> - Complexity: O(*n*), where *n* is the length of the collection.</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"></span> <span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"><span class="pl-c" style="box-sizing: border-box;">///</span> - SeeAlso: `prefix(while:)`</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"></span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">@_inlineable</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">public</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">func</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">drop</span>(
<span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">while</span> <span class="pl-smi" style="box-sizing: border-box; color: rgb(51, 51, 51);">predicate</span>: (<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Iterator</span>.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Element</span>) <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">throws</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">-></span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Bool</span>
) <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">rethrows</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">-></span> [<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Iterator</span>.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Element</span>] { <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">...</span> }
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"><span class="pl-c" style="box-sizing: border-box;">///</span> Returns a subsequence, up to the specified maximum length, containing the</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"></span> <span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"><span class="pl-c" style="box-sizing: border-box;">///</span> initial elements of the sequence.</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"></span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">...</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"><span class="pl-c" style="box-sizing: border-box;">///</span> - Complexity: O(*n*), where *n* is the length of the sequence.</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"></span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">@_inlineable</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">public</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">func</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">prefix</span>(<span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">_</span> <span class="pl-smi" style="box-sizing: border-box; color: rgb(51, 51, 51);">maxLength</span>: <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Int</span>) <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">-></span> [<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Iterator</span>.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Element</span>] { <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">...</span> }
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"><span class="pl-c" style="box-sizing: border-box;">///</span> Returns a subsequence containing the initial, consecutive elements that</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"></span> <span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"><span class="pl-c" style="box-sizing: border-box;">///</span> satisfy the given predicate.</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"></span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">...</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"><span class="pl-c" style="box-sizing: border-box;">///</span> - Complexity: O(*n*), where *n* is the length of the collection.</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"></span> <span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"><span class="pl-c" style="box-sizing: border-box;">///</span> - SeeAlso: `drop(while:)`</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"></span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">@_inlineable</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">public</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">func</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">prefix</span>(
<span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">while</span> <span class="pl-smi" style="box-sizing: border-box; color: rgb(51, 51, 51);">predicate</span>: (<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Iterator</span>.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Element</span>) <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">throws</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">-></span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Bool</span>
) <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">rethrows</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">-></span> [<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Iterator</span>.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Element</span>] { <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">...</span> }
}
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"><span class="pl-c" style="box-sizing: border-box;">//</span> TODO describe lazy views for prefix and dropFirst</span></pre><div class=""><span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"><br class=""></span></div></div><h2 style="box-sizing: border-box; margin-top: 24px; margin-bottom: 16px; line-height: 1.25; padding-bottom: 0.3em; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(234, 236, 239); color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-variant-ligatures: normal; orphans: 2; widows: 2;" class=""><a id="user-content-source-compatibility" class="anchor" href="https://github.com/palimondo/swift-evolution/blob/sequence-cleanup/proposals/NNNN-SequenceCleanup.md#source-compatibility" aria-hidden="true" style="box-sizing: border-box; color: rgb(3, 102, 214); text-decoration: none; float: left; padding-right: 4px; margin-left: -20px; line-height: 1;"><svg aria-hidden="true" class="octicon octicon-link" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a></h2></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>