<div dir="ltr">Proposal pull request: <a href="https://github.com/apple/swift-evolution/pull/213">#213</a><div>Implementation pull request: <a href="https://github.com/apple/swift/pull/1702">#1702</a><br><div>Original pitch thread: <a href="http://thread.gmane.org/gmane.comp.lang.swift.evolution/8519">[Proposal] Change guarantee for GeneratorType.next() to always return nil past end</a></div><div><br></div><div><h1 style="font-size:2.25em;margin-right:0px;margin-bottom:16px;margin-left:0px;line-height:1.2;padding-bottom:0.3em;border-bottom-width:1px;border-bottom-style:solid;border-bottom-color:rgb(238,238,238);color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';margin-top:0px!important">Change IteratorType post-nil guarantee</h1><ul style="padding-left:2em;margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:16px"><li style="">Proposal: <a href="https://github.com/apple/swift-evolution/blob/master/proposals/NNNN-name.md" style="background-color:transparent;color:rgb(64,120,192);text-decoration:none">SE-NNNN</a></li><li style="">Author(s): <a href="https://github.com/PatrickPijnappel" style="background-color:transparent;color:rgb(64,120,192);text-decoration:none">Patrick Pijnappel</a></li><li style="">Status: <strong style="">Awaiting review</strong></li><li style="">Review manager: TBD</li></ul><h2 style="margin-top:1em;margin-bottom:16px;line-height:1.225;font-size:1.75em;padding-bottom:0.3em;border-bottom-width:1px;border-bottom-style:solid;border-bottom-color:rgb(238,238,238);color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol'"><a id="user-content-introduction" class="" href="https://github.com/PatrickPijnappel/swift-evolution/blob/patch-1/proposals/XXXX-iterator-post-nil-guarantee.md#introduction" style="background-color:transparent;color:rgb(64,120,192);text-decoration:none;display:inline-block;padding-right:2px;line-height:1"></a>Introduction</h2><p style="margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:16px">Currently, the documentation for <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px">IteratorType.next()</code> has the precondition that when calling <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px">next()</code>, no preceding call to <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px">next()</code> should have returned <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px">nil</code>, and in fact encourages implementations to raise a <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px">preconditionFailure()</code> for violations of this requirement. However, all current 27 <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px">IteratorType</code> implementations in the standard library return <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px">nil</code> indefinitely. Many users are likely unaware of the precondition, expecting all iterators to return <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px">nil</code> indefinitely and writing code that might rely on this assumption. Such code will usually run fine, until someone does in fact pass in an iterator not repeating <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px">nil</code> (it's a silent corner case).</p><p style="margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:16px">Swift-evolution thread: <a href="http://thread.gmane.org/gmane.comp.lang.swift.evolution/8519" style="background-color:transparent;color:rgb(64,120,192);text-decoration:none">[Proposal] Change guarantee for GeneratorType.next() to always return nil past end</a></p><p style="margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:16px">Pull-request: <a href="https://github.com/apple/swift/pull/1702" style="background-color:transparent;color:rgb(64,120,192);text-decoration:none">#1702</a></p><h2 style="margin-top:1em;margin-bottom:16px;line-height:1.225;font-size:1.75em;padding-bottom:0.3em;border-bottom-width:1px;border-bottom-style:solid;border-bottom-color:rgb(238,238,238);color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol'"><a id="user-content-motivation" class="" href="https://github.com/PatrickPijnappel/swift-evolution/blob/patch-1/proposals/XXXX-iterator-post-nil-guarantee.md#motivation" style="background-color:transparent;color:rgb(64,120,192);text-decoration:none;display:inline-block;padding-right:2px;line-height:1"></a>Motivation</h2><p style="margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:16px">While not overwhelmingly common, it is relatively easy to write code based on the assumption <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px">nil</code> will be returned indefinitely:</p><div class="" style="margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:16px"><pre style="overflow:auto;font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;margin-top:0px;margin-bottom:0px;line-height:1.45;padding:16px;background-color:rgb(247,247,247);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;word-wrap:normal"><span class="" style="color:rgb(150,152,150)">// Example based on standard library code (Sequence.swift)</span>
<span class="" style="color:rgb(167,29,93)">while</span> <span class="" style="color:rgb(167,29,93)">let</span> element <span class="" style="color:rgb(167,29,93)">=</span> iterator<span class="" style="color:rgb(167,29,93)">.</span><span class="" style="color:rgb(0,134,179)">next</span>() {
<span class="" style="color:rgb(167,29,93)">if</span> condition(element) {
foo(element) <span class="" style="color:rgb(150,152,150)">// call foo on first element satisfying condition</span>
<span class="" style="color:rgb(167,29,93)">break</span>
}
}
<span class="" style="color:rgb(167,29,93)">while</span> <span class="" style="color:rgb(167,29,93)">let</span> element <span class="" style="color:rgb(167,29,93)">=</span> iterator<span class="" style="color:rgb(167,29,93)">.</span><span class="" style="color:rgb(0,134,179)">next</span>() {
bar(element) <span class="" style="color:rgb(150,152,150)">// call bar on remaining elements</span>
}
<span class="" style="color:rgb(150,152,150)">// Another example</span>
<span class="" style="color:rgb(167,29,93)">switch</span> (iterator<span class="" style="color:rgb(167,29,93)">.</span><span class="" style="color:rgb(0,134,179)">next</span>(), iterator<span class="" style="color:rgb(167,29,93)">.</span><span class="" style="color:rgb(0,134,179)">next</span>()) {
<span class="" style="color:rgb(150,152,150)">// ...</span>
}</pre></div><p style="margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:16px">Even though this can be trivially rewritten to not rely on post-<code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px">nil</code> behavior, the user won't perform this rewrite if they are unaware of the precondition. In their testing the code will work fine, and likely will in almost every case, except when passing the rare iterator that doesn't repeat <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px">nil</code>.</p><h2 style="margin-top:1em;margin-bottom:16px;line-height:1.225;font-size:1.75em;padding-bottom:0.3em;border-bottom-width:1px;border-bottom-style:solid;border-bottom-color:rgb(238,238,238);color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol'"><a id="user-content-proposed-solution" class="" href="https://github.com/PatrickPijnappel/swift-evolution/blob/patch-1/proposals/XXXX-iterator-post-nil-guarantee.md#proposed-solution" style="background-color:transparent;color:rgb(64,120,192);text-decoration:none;display:inline-block;padding-right:2px;line-height:1"></a>Proposed solution</h2><p style="margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:16px">Bring the guarantee in line with the common expectation, and require iterators to return <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px">nil</code> indefinitely.</p><p style="margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:16px">Requiring <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px">nil</code> to be returned indefinitely does require the implementors of custom <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px">IteratorType</code> conformances to respect this, but this is likely already the expectation for most users. Most iterators already get this as a natural consequence of their implementation (as is the case with all current standard library iterators), but otherwise they can simply track a <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px">done</code> flag to do so. It should be noted that this requirement would also affect closures passed to <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px">AnyIterator</code>.</p><h3 style="margin-top:1em;margin-bottom:16px;line-height:1.43;font-size:1.5em;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol'"><a id="user-content-performance-considerations" class="" href="https://github.com/PatrickPijnappel/swift-evolution/blob/patch-1/proposals/XXXX-iterator-post-nil-guarantee.md#performance-considerations" style="background-color:transparent;color:rgb(64,120,192);text-decoration:none;display:inline-block;padding-right:2px;line-height:1.2"></a>Performance considerations</h3><p style="margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:16px">The original rationale for introducing the precondition was because of concerns it might add storage and performance burden to some implementations of <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px">IteratorType</code> (see <a href="http://article.gmane.org/gmane.comp.lang.swift.evolution/8532" style="background-color:transparent;color:rgb(64,120,192);text-decoration:none">here</a>).</p><p style="margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:16px">However, in light of implementation experience, there are a few observations we can make:</p><ul style="padding-left:2em;margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:16px"><li style="">These cases are rare. The standard library currently has no iterators that require extra state or branches to return <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px">nil</code>indefinitely. The iterator for the proposed <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px">takeWhile()</code> (<a href="https://github.com/apple/swift-evolution/blob/master/proposals/0045-scan-takewhile-dropwhile.md" style="background-color:transparent;color:rgb(64,120,192);text-decoration:none">SE-0045</a>) would be the first occurance in the standard library.</li><li style="">Even in such cases, in the common case the calling code doesn't rely on post-<code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px">nil</code> behavior (e.g. <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px">for in</code>, <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px">map</code>, etc.) this extra storage and branching can usually optimized away.</li><li style="">Not having the post-<code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px">nil</code> guarantee can sometimes add storage and performance burden for the caller instead, e.g. when an iterator somehow buffers it's underlying iterator. This in contrast can usually not be optimized away. For example, the standard library's UTF8/UTF16 decoding has 4 instead of 3 branches per character for ASCII because of this.</li></ul><h2 style="margin-top:1em;margin-bottom:16px;line-height:1.225;font-size:1.75em;padding-bottom:0.3em;border-bottom-width:1px;border-bottom-style:solid;border-bottom-color:rgb(238,238,238);color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol'"><a id="user-content-detailed-design" class="" href="https://github.com/PatrickPijnappel/swift-evolution/blob/patch-1/proposals/XXXX-iterator-post-nil-guarantee.md#detailed-design" style="background-color:transparent;color:rgb(64,120,192);text-decoration:none;display:inline-block;padding-right:2px;line-height:1"></a>Detailed design</h2><p style="margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:16px">Original guarantee:</p><div class="" style="margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:16px"><pre style="overflow:auto;font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;margin-top:0px;margin-bottom:0px;line-height:1.45;padding:16px;background-color:rgb(247,247,247);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;word-wrap:normal"><span class="" style="color:rgb(150,152,150)">/// Advance to the next element and return it, or `nil` if no next</span>
<span class="" style="color:rgb(150,152,150)">/// element exists.</span>
<span class="" style="color:rgb(150,152,150)">///</span>
<span class="" style="color:rgb(150,152,150)">/// - Precondition: `next()` has not been applied to a copy of `self`</span>
<span class="" style="color:rgb(150,152,150)">/// since the copy was made, and no preceding call to `self.next()`</span>
<span class="" style="color:rgb(150,152,150)">/// has returned `nil`. Specific implementations of this protocol </span>
<span class="" style="color:rgb(150,152,150)">/// are encouraged to respond to violations of this requirement by </span>
<span class="" style="color:rgb(150,152,150)">/// calling `preconditionFailure("...")`.</span></pre></div><p style="margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:16px">Proposed guarantee:</p><div class="" style="margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:16px"><pre style="overflow:auto;font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;margin-top:0px;margin-bottom:0px;line-height:1.45;padding:16px;background-color:rgb(247,247,247);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;word-wrap:normal"><span class="" style="color:rgb(150,152,150)">/// Advance to the next element and return it, or `nil` if no next element</span>
<span class="" style="color:rgb(150,152,150)">/// exists. Once `nil` has been returned, all subsequent calls return `nil`.</span>
<span class="" style="color:rgb(150,152,150)">///</span>
<span class="" style="color:rgb(150,152,150)">/// - Precondition: `next()` has not been applied to a copy of `self`</span>
<span class="" style="color:rgb(150,152,150)">/// since the copy was made.</span></pre></div><h2 style="margin-top:1em;margin-bottom:16px;line-height:1.225;font-size:1.75em;padding-bottom:0.3em;border-bottom-width:1px;border-bottom-style:solid;border-bottom-color:rgb(238,238,238);color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol'"><a id="user-content-impact-on-existing-code" class="" href="https://github.com/PatrickPijnappel/swift-evolution/blob/patch-1/proposals/XXXX-iterator-post-nil-guarantee.md#impact-on-existing-code" style="background-color:transparent;color:rgb(64,120,192);text-decoration:none;display:inline-block;padding-right:2px;line-height:1"></a>Impact on existing code</h2><p style="margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:16px">All <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px">IteratorType</code> implementations in the standard library already comply with the new guarantee. It is likely most existing custom iterators will as well, however some might be rendered in violation of their guarantee by the change.</p><h2 style="margin-top:1em;margin-bottom:16px;line-height:1.225;font-size:1.75em;padding-bottom:0.3em;border-bottom-width:1px;border-bottom-style:solid;border-bottom-color:rgb(238,238,238);color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol'"><a id="user-content-alternatives-considered" class="" href="https://github.com/PatrickPijnappel/swift-evolution/blob/patch-1/proposals/XXXX-iterator-post-nil-guarantee.md#alternatives-considered" style="background-color:transparent;color:rgb(64,120,192);text-decoration:none;display:inline-block;padding-right:2px;line-height:1"></a>Alternatives considered</h2><ul style="padding-left:2em;margin-top:0px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:16px;margin-bottom:0px!important"><li style="">Require <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px">IteratorType</code> to not crash but keep the return value up to specific implementations. This allows them to use it for other behavior e.g. repeating the sequence after <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px">nil</code> is returned. This however retains most of the problems of the original guaranteee described in this proposal.</li></ul><div><font color="#333333" face="Helvetica Neue, Helvetica, Segoe UI, Arial, freesans, sans-serif, Apple Color Emoji, Segoe UI Emoji, Segoe UI Symbol" size="3"><br></font></div></div></div></div>