<div><div dir="auto">You’re welcome to bikeshed the entire API surface area of sequences and collections, but you won’t be the first to explore this area. A number of us looked into this area in the past few years and did not reach a measurable improved result.</div><div dir="auto"><br></div><div dir="auto">Sequences can be ordered or unordered, single-pass or multi-pass, finite or infinite, lazy or eager. Not all the combinations of these attributes make sense, but many do. For each combination, a different subset of the sequence algorithms are “useful.” As an example, “last” is not great for an infinite sequence. It’s possibly also not what you want for a single-pass sequence.</div></div><div dir="auto"><br></div><div dir="auto">Now, as to the problem discussed here. It’s an orthogonal problem to what you are discussing because, whether or not you reorganize the protocols entirely, there is still going to be confusion about how exactly “elementsEqual” differs from “==“ even for an ordered sequence. The name is clearly problematic in that respect. However, I would argue that the behavior of the method isn’t “improper” and the behavior is not “badly defined.”</div><div dir="auto"><br></div><div dir="auto"><br></div><div><div><div class="gmail_quote"><div>On Fri, Oct 13, 2017 at 07:09 Benjamin G <<a href="mailto:benjamin.garrigues@gmail.com" target="_blank">benjamin.garrigues@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div>+1 on both points. As for your solutions, i see 1/ as the best solution. Breaking source code that rely on badly defined, or improper behavior isn't "breaking". You don't break something that's already half broken.<div><div>As an app developer relying on swift on my day to day job and making a living of it, i want to emphasis this: I really don't mind if a language version change is making me look more carefully on some parts of my code that i probably had overlooked. </div><div>Sure i may pester a bit when the code doesn't compile, but it sure is better than discovering the weird behavior of a badly defined protocol hierarchy in customer support.</div><div><br></div></div></div><div class="gmail_extra"><br><div class="gmail_quote"></div></div><div class="gmail_extra"><div class="gmail_quote">On Fri, Oct 13, 2017 at 6:57 AM, Kevin Nattinger via swift-evolution <span><<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>></span> wrote:<br></div></div><div class="gmail_extra"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word"><div>–∞</div><div><br></div>1. I strongly object to the proposed name. It doesn't make it more clear to me what the method does, and is misleading at best. Among other issues, "lexicographical" is defined as alphabet order, and (1) this method applies to objects that are not Strings, and (2) this method's behavior isn't any more well-defined for Strings, so that name is even more of a lie than the original.<div><br><div>2. This is really just a symptom of a bigger problem. The fact that two Sets can compare equal and yet return different results for that method (among too many others) is logically inconsistent and points to a much deeper issue with Set and Sequence. It is probably about 3 releases too late to get this straightened out properly, but I'll outline the real issue in case someone has an idea for fixing it.</div><div><br></div><div><b>The root of the problem is that Set conforms to Sequence, but Sequence doesn't require a well-defined order.</b> Since Set doesn't have a well-defined order, a significant portion of its interface is unspecified. The methods are implemented because they have to be, but they doesn't have well-defined or necessarily consistent results.<br><br>A sequence is, by definition, ordered. That is reflected in the fact that over half the methods in the main Sequence definition* make no sense and are not well-defined unless there is a well-defined order to the sequence itself. What does it even mean to `dropFirst()` in a Set? The fact that two objects that compare equal can give different results for a 100% deterministic function is illogical, nonsensical, and dangerous.</div><div><br></div><div>* 7/12 by my count, ignoring `_*` funcs but including the `var`<br><br>The current contents of Sequence can be cleanly divided into two groups; those that return SubSequence imply a specific ordering, and the rest do not.</div><div><br></div><div> I think those should be/should have been two separate protocols:</div><div><br></div><div><div style="margin:0px;line-height:normal;font-family:'Andale Mono';color:rgb(233,175,205);background-color:rgba(0,0,0,0.901961)"><span style="font-variant-ligatures:no-common-ligatures">public</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> </span><span style="font-variant-ligatures:no-common-ligatures">protocol</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#34bbc8">Iterable</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> {</span></div><div style="margin:0px;line-height:normal;font-family:'Andale Mono';color:rgb(52,188,38);background-color:rgba(0,0,0,0.901961)"><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#cd7923">associatedtype</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> Iterator</span><span style="font-variant-ligatures:no-common-ligatures;color:#e9afcd">:</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> </span><span style="font-variant-ligatures:no-common-ligatures">IteratorProtocol</span></div><div style="margin:0px;line-height:normal;font-family:'Andale Mono';color:rgb(130,185,255);background-color:rgba(0,0,0,0.901961)"><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#e9afcd">func</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#34bbc8">map</span><span style="font-variant-ligatures:no-common-ligatures;color:#e9afcd"><</span><span style="font-variant-ligatures:no-common-ligatures;color:#34bbc8">T</span><span style="font-variant-ligatures:no-common-ligatures;color:#e9afcd">></span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6">(</span><span style="font-variant-ligatures:no-common-ligatures;color:#34bbc8">...</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6">) </span><span style="font-variant-ligatures:no-common-ligatures;color:#e9afcd">-></span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#e9afcd">[</span><span style="font-variant-ligatures:no-common-ligatures;color:#34bc26">T</span><span style="font-variant-ligatures:no-common-ligatures;color:#e9afcd">]</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> </span><span style="font-variant-ligatures:no-common-ligatures">// Iterable where .Iterator.Element == T</span></div><div style="margin:0px;line-height:normal;font-family:'Andale Mono';color:rgb(130,185,255);background-color:rgba(0,0,0,0.901961)"><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#e9afcd">func</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#34bbc8">filter</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6">(</span><span style="font-variant-ligatures:no-common-ligatures;color:#34bbc8">...</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6">) </span><span style="font-variant-ligatures:no-common-ligatures;color:#e9afcd">-></span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#e9afcd">[</span><span style="font-variant-ligatures:no-common-ligatures;color:#34bc26">Iterator.Element</span><span style="font-variant-ligatures:no-common-ligatures;color:#e9afcd">]</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> </span><span style="font-variant-ligatures:no-common-ligatures">// Iterable where .Iterator.Element == Self.Iterator.Element</span></div><div style="margin:0px;line-height:normal;font-family:'Andale Mono';color:rgb(52,187,200);background-color:rgba(0,0,0,0.901961)"><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#e9afcd">func</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> </span><span style="font-variant-ligatures:no-common-ligatures">forEach</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6">(</span><span style="font-variant-ligatures:no-common-ligatures">...</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6">)</span></div><div style="margin:0px;line-height:normal;font-family:'Andale Mono';color:rgb(52,187,200);background-color:rgba(0,0,0,0.901961)"><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#e9afcd">func</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> </span><span style="font-variant-ligatures:no-common-ligatures">makeIterator</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6">() </span><span style="font-variant-ligatures:no-common-ligatures;color:#e9afcd">-></span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#34bc26">Iterator</span></div><div style="margin:0px;line-height:normal;font-family:'Andale Mono';color:rgb(52,187,200);background-color:rgba(0,0,0,0.901961)"><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#e9afcd">var</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> </span><span style="font-variant-ligatures:no-common-ligatures">underestimatedCount</span><span style="font-variant-ligatures:no-common-ligatures;color:#e9afcd">:</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#34bc26">Int</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> { </span><span style="font-variant-ligatures:no-common-ligatures;color:#cd7923">get</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> }</span></div><div style="margin:0px;line-height:normal;font-family:'Andale Mono';color:rgb(214,214,214);background-color:rgba(0,0,0,0.901961)"><span style="font-variant-ligatures:no-common-ligatures">}</span></div><div style="margin:0px;line-height:normal;font-family:'Andale Mono';color:rgb(214,214,214);background-color:rgba(0,0,0,0.901961);min-height:14px"><span style="font-variant-ligatures:no-common-ligatures"></span><br></div><div style="margin:0px;line-height:normal;font-family:'Andale Mono';color:rgb(130,185,255);background-color:rgba(0,0,0,0.901961)"><span style="font-variant-ligatures:no-common-ligatures;color:#e9afcd">public</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#e9afcd">protocol</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#34bbc8">Sequence</span><span style="font-variant-ligatures:no-common-ligatures;color:#e9afcd">:</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#34bc26">Iterable</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> { </span><span style="font-variant-ligatures:no-common-ligatures">// Maybe OrderedSequence just to make the well-defined-order requirement explicit</span></div><div style="margin:0px;line-height:normal;font-family:'Andale Mono';color:rgb(214,214,214);background-color:rgba(0,0,0,0.901961)"><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#cd7923">associatedtype</span><span style="font-variant-ligatures:no-common-ligatures"> SubSequence</span></div><div style="margin:0px;line-height:normal;font-family:'Andale Mono';color:rgb(130,185,255);background-color:rgba(0,0,0,0.901961)"><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#e9afcd">func</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#34bbc8">dropFirst</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6">(</span><span style="font-variant-ligatures:no-common-ligatures;color:#34bbc8">...</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6">) </span><span style="font-variant-ligatures:no-common-ligatures;color:#e9afcd">-></span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#34bc26">SubSequence</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> </span><span style="font-variant-ligatures:no-common-ligatures">// Sequence where .Iterator.Element == Self.Iterator.Element</span></div><div style="margin:0px;line-height:normal;font-family:'Andale Mono';color:rgb(214,214,214);background-color:rgba(0,0,0,0.901961)"><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#e9afcd">func</span><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#34bbc8">dropLast</span><span style="font-variant-ligatures:no-common-ligatures">(</span><span style="font-variant-ligatures:no-common-ligatures;color:#34bbc8">...</span><span style="font-variant-ligatures:no-common-ligatures">) </span><span style="font-variant-ligatures:no-common-ligatures;color:#e9afcd">-></span><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#34bc26">SubSequence</span><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#82b9ff">// " "</span></div><div style="margin:0px;line-height:normal;font-family:'Andale Mono';color:rgb(214,214,214);background-color:rgba(0,0,0,0.901961)"><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#e9afcd">func</span><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#34bbc8">drop</span><span style="font-variant-ligatures:no-common-ligatures">(</span><span style="font-variant-ligatures:no-common-ligatures;color:#cd7923">while</span><span style="font-variant-ligatures:no-common-ligatures;color:#34bbc8">...</span><span style="font-variant-ligatures:no-common-ligatures">) </span><span style="font-variant-ligatures:no-common-ligatures;color:#e9afcd">-></span><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#34bc26">SubSequence</span><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#82b9ff">// " "</span></div><div style="margin:0px;line-height:normal;font-family:'Andale Mono';color:rgb(214,214,214);background-color:rgba(0,0,0,0.901961)"><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#e9afcd">func</span><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#34bbc8">prefix</span><span style="font-variant-ligatures:no-common-ligatures">(</span><span style="font-variant-ligatures:no-common-ligatures;color:#34bbc8">...</span><span style="font-variant-ligatures:no-common-ligatures">) </span><span style="font-variant-ligatures:no-common-ligatures;color:#e9afcd">-></span><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#34bc26">SubSequence</span><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#82b9ff">// " "</span></div><div style="margin:0px;line-height:normal;font-family:'Andale Mono';color:rgb(52,188,38);background-color:rgba(0,0,0,0.901961)"><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#e9afcd">func</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#34bbc8">prefix</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6">(</span><span style="font-variant-ligatures:no-common-ligatures;color:#cd7923">while</span><span style="font-variant-ligatures:no-common-ligatures;color:#34bbc8">...</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6">) </span><span style="font-variant-ligatures:no-common-ligatures;color:#e9afcd">-></span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> </span><span style="font-variant-ligatures:no-common-ligatures">SubSequence</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#82b9ff">// " "</span></div><div style="margin:0px;line-height:normal;font-family:'Andale Mono';color:rgb(214,214,214);background-color:rgba(0,0,0,0.901961)"><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#e9afcd">func</span><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#34bbc8">suffix</span><span style="font-variant-ligatures:no-common-ligatures">(</span><span style="font-variant-ligatures:no-common-ligatures;color:#34bbc8">...</span><span style="font-variant-ligatures:no-common-ligatures">) </span><span style="font-variant-ligatures:no-common-ligatures;color:#e9afcd">-></span><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#34bc26">SubSequence</span><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#82b9ff">// " "</span></div><div style="margin:0px;line-height:normal;font-family:'Andale Mono';color:rgb(130,185,255);background-color:rgba(0,0,0,0.901961)"><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#e9afcd">func</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#34bbc8">split</span><span style="font-variant-ligatures:no-common-ligatures;color:#d6d6d6">(</span><span style="color:rgb(52,187,200)">...</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(205,121,35)">where</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(52,187,200)">...</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(214,214,214)">) </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(233,175,205)">-></span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(214,214,214)"> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(233,175,205)">[</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(52,188,38)">SubSequence</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(233,175,205)">]</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(214,214,214)"> </span><span style="font-variant-ligatures:no-common-ligatures">// Iterable where .Iterator.Element == (Sequence where .Iterator.Element == Self.Iterator.Element)</span></div><div style="margin:0px;line-height:normal;font-family:'Andale Mono';color:rgb(214,214,214);background-color:rgba(0,0,0,0.901961)"><span style="font-variant-ligatures:no-common-ligatures">}</span></div><div><span style="font-variant-ligatures:no-common-ligatures"><br></span></div><div><span style="font-variant-ligatures:no-common-ligatures">(The comments, of course, would be more sensible types once the ideas can actually be expressed in Swift)<br><br>Then unordered collections (Set and Dictionary) would just conform to Iterable and not Sequence, so ALL the methods on those classes would make logical sense and have well-defined behavior; no change would be needed for ordered collections.<br><br>Now, the practical matter. If this were Swift 1->2 or 2->3, I doubt there would be a significant issue with actually making this change. Unfortunately, we're well beyond that and making a change this deep is an enormous deal. So I see two ways forward.<br><br>1. We could go ahead and make this separation. Although it's a potentially large breaking change, I would argue that because the methods are ill-defined anyway, the breakage is justified and a net benefit.<br><br>2. We could try and think of a way to make the distinction between ordered and unordered "sequences" in a less-breaking manner. Unfortunately, I don't have a good suggestion for this, but if anyone has ideas, I'm all ears. Or eyes, as the case may be.<br><br></span></div><br></div><div><div><blockquote type="cite"><span><div>On Oct 12, 2017, at 4:24 PM, Xiaodi Wu via swift-evolution <<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>> wrote:</div><br class="m_-8318953893838690629m_-6746564519493216477m_7432166097604328613Apple-interchange-newline"></span><div><div><span><h1>Rename <code>Sequence.elementsEqual</code></h1>
<ul><li>Proposal: <a href="https://gist.github.com/xwu/NNNN-rename-elements-equal.md" target="_blank">SE-NNNN</a></li><li>Authors: <a href="https://github.com/xwu" target="_blank">Xiaodi Wu</a></li><li>Review Manager: TBD</li><li>Status: <strong>Awaiting review</strong></li></ul><h2><a href="https://gist.github.com/xwu/1f0ef4e18a7f321f22ca65a2f56772f6#introduction" class="m_-8318953893838690629m_-6746564519493216477m_7432166097604328613gmail-anchor" id="m_-8318953893838690629m_-6746564519493216477m_7432166097604328613gmail-user-content-introduction" target="_blank"></a>Introduction</h2><p>The current behavior of <code>Sequence.elementsEqual</code> is
potentially confusing to users given its name. Having surveyed the
alternative solutions to this problem, it is proposed that the method be
renamed to <code>Sequence.lexicographicallyEquals</code>.</p></span><div>[...]</div></div></div></blockquote></div></div></div></div><br></blockquote></div></div><div class="gmail_extra"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">_______________________________________________<br>
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