<div dir="ltr">> <span style="font-size:12.8px"> T() as a placeholder is just as valid/invalid as any other value in T's space.</span><div><span style="font-size:12.8px"><br></span></div><div><span style="font-size:12.8px">Correct but you how do you get any other value?</span></div><div><br></div></div><div class="gmail_extra"><br><div class="gmail_quote">On Mon, Dec 26, 2016 at 1:39 PM, Tony Allevato via swift-evolution <span dir="ltr"><<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><br><br><div class="gmail_quote"><div dir="ltr">On Mon, Dec 26, 2016 at 1:19 PM David Sweeris <<a href="mailto:davesweeris@mac.com" target="_blank">davesweeris@mac.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word" class="m_329265698061725855gmail_msg"><div class="m_329265698061725855gmail_msg"><br class="m_329265698061725855gmail_msg"></div><div class="m_329265698061725855gmail_msg">On Dec 26, 2016, at 12:10, Tony Allevato <<a href="mailto:tony.allevato@gmail.com" class="m_329265698061725855gmail_msg" target="_blank">tony.allevato@gmail.com</a>> wrote:<br class="m_329265698061725855gmail_msg"><br class="m_329265698061725855gmail_msg"></div><blockquote type="cite" class="m_329265698061725855gmail_msg"><div dir="ltr" class="m_329265698061725855gmail_msg">On Mon, Dec 26, 2016 at 11:57 AM David Sweeris via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="m_329265698061725855gmail_msg" target="_blank">swift-evolution@swift.org</a>> wrote:<br class="m_329265698061725855gmail_msg"><div class="gmail_quote m_329265698061725855gmail_msg"><blockquote class="gmail_quote m_329265698061725855gmail_msg" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><span class=""><div dir="auto" class="m_329265698061725855gmail_msg"><div class="m_329265698061725855gmail_msg"><br class="m_329265698061725855gmail_msg"></div><div class="m_329265698061725855gmail_msg">On Dec 26, 2016, at 11:35, Tony Allevato <<a href="mailto:allevato@google.com" class="m_329265698061725855gmail_msg" target="_blank">allevato@google.com</a>> wrote:<br class="m_329265698061725855gmail_msg"><br class="m_329265698061725855gmail_msg"></div><blockquote type="cite" class="m_329265698061725855gmail_msg"><div class="m_329265698061725855gmail_msg">Mathematically, identities are associated with (type, operation) pairs, not types alone.<br class="m_329265698061725855gmail_msg"><br class="m_329265698061725855gmail_msg">This conversation has put me in the column of "numeric types shouldn't have default initializers at all", personally.
</div></blockquote><br class="m_329265698061725855gmail_msg"></div></span><div dir="auto" class="m_329265698061725855gmail_msg"><div class="m_329265698061725855gmail_msg">I'd agree, except sometimes you need a T, <i class="m_329265698061725855gmail_msg">any</i> T, for when you want to create a "pre-sized" array for stuffing results into by index:</div><div class="m_329265698061725855gmail_msg">for i in ... {</div><div class="m_329265698061725855gmail_msg"> a[i] = ...</div><div class="m_329265698061725855gmail_msg">}</div><div class="m_329265698061725855gmail_msg">Simply saying "var a =[T](); a.reserveCapacity()" doesn't cut it because it'll still crash if you try to store anything in a[i] without somehow putting at least i+1 elements in the array first.</div></div></blockquote><div class="m_329265698061725855gmail_msg"><br class="m_329265698061725855gmail_msg"></div><div class="m_329265698061725855gmail_msg">Array already has init(repeating:count:) that puts the responsibility of choosing the default value at the call site. If someone were writing a generic algorithm around this, then why not just propagate that responsibility out to its call site as well? That way, the algorithm isn't making any assumptions about what the "default" value is or even if one exists, and it doesn't impose additional requirements on the element type. For example, the user could get the default from a static factory method, an instance method on another object, or something else entirely.</div></div></div>
</blockquote><br class="m_329265698061725855gmail_msg"></div><div style="word-wrap:break-word" class="m_329265698061725855gmail_msg"><div class="m_329265698061725855gmail_msg">Yeah, that's what I would use… The "filled out" example would be:</div><blockquote style="margin:0 0 0 40px;border:none;padding:0px" class="m_329265698061725855gmail_msg"><div class="m_329265698061725855gmail_msg"><div style="margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><div style="margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><div style="font-family:Menlo;color:rgb(187,44,162);margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><span style="font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">extension</span><span style="font-variant-ligatures:no-common-ligatures;color:#000000" class="m_329265698061725855gmail_msg"> </span><span style="font-variant-ligatures:no-common-ligatures;color:#703daa" class="m_329265698061725855gmail_msg">Array</span><span style="font-variant-ligatures:no-common-ligatures;color:#000000" class="m_329265698061725855gmail_msg"> {</span></div></div></div></div><div class="m_329265698061725855gmail_msg"><div style="margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><div style="margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><div style="font-family:Menlo;margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><span style="color:rgb(187,44,162);font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg"> </span><span style="color:rgb(187,44,162);font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">public</span><span style="color:rgb(187,44,162);font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg"> </span><span style="color:rgb(187,44,162);font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">func</span><span style="font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg"> pmap<T: </span><span style="color:rgb(79,129,135);font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">DefaultInitable</span><span style="font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">> (transform: (</span><span style="color:rgb(112,61,170);font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">Element</span><span style="font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">) -> </span><span style="color:rgb(79,129,135);font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">T</span><span style="font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">) -> [</span><span style="color:rgb(79,129,135);font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">T</span><span style="font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">] {</span></div></div></div></div><div class="m_329265698061725855gmail_msg"><div style="margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><div style="margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><div style="font-family:Menlo;margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><span style="color:rgb(187,44,162);font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg"> </span><span style="color:rgb(187,44,162);font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">var</span><span style="font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg"> result = </span><span style="color:rgb(112,61,170);font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">Array</span><span style="font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg"><</span><span style="color:rgb(79,129,135);font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">T</span><span style="font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">>(repeating:</span><span style="color:rgb(187,44,162);font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg"> </span><span style="color:rgb(79,129,135);font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">T</span><span style="font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">(), count: </span><span style="color:rgb(187,44,162);font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">self</span><span style="color:rgb(187,44,162);font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">.</span><span style="color:rgb(112,61,170);font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">count</span><span style="font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">)</span><span style="font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg"> </span><span style="color:rgb(0,132,0);font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">//Pick a T... any T...</span></div></div></div></div><div class="m_329265698061725855gmail_msg"><div style="margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><div style="margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><div style="font-family:Menlo;margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><span style="color:rgb(187,44,162);font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg"> </span><span style="color:rgb(187,44,162);font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">for</span><span style="font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg"> i </span><span style="color:rgb(187,44,162);font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">in</span><span style="color:rgb(187,44,162);font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg"> </span><span style="color:rgb(187,44,162);font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">self</span><span style="color:rgb(187,44,162);font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">.</span><span style="color:rgb(112,61,170);font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">indices</span><span style="font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg"> {</span></div></div></div></div><div class="m_329265698061725855gmail_msg"><div style="margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><div style="margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><div style="font-family:Menlo;margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><span style="font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg"> result[i] =<font color="#bb2ca2" class="m_329265698061725855gmail_msg"> </font></span><span style="color:rgb(49,89,93);font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">whateverTheConcurrentExectutio<wbr>nSyntaxIs</span><span style="font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">(</span><span style="color:rgb(187,44,162);font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">self</span><span style="font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">[i</span><span style="font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">], </span><span style="font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">transform</span><span style="font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">)</span></div></div></div></div><div class="m_329265698061725855gmail_msg"><div style="margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><div style="margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><div style="font-family:Menlo;margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><span style="font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg"><font color="#bb2ca2" class="m_329265698061725855gmail_msg"> </font>}</span></div></div></div></div><div class="m_329265698061725855gmail_msg"><div style="margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><div style="margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><div style="font-family:Menlo;margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><span style="color:rgb(187,44,162);font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg"> </span><span style="color:rgb(187,44,162);font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">return</span><span style="font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg"> result</span></div></div></div></div><div class="m_329265698061725855gmail_msg"><div style="margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><div style="margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><div style="font-family:Menlo;margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><span style="font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg"> }</span></div></div></div></div><div class="m_329265698061725855gmail_msg"><div style="margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><div style="margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><div style="font-family:Menlo;margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><span style="font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">}</span></div></div></div></div><div class="m_329265698061725855gmail_msg"><div style="margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><div style="margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><div style="font-family:Menlo;margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><span style="color:rgb(187,44,162);font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">var</span><span style="font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg"> thisCouldTakeAWhile = </span><span style="color:rgb(112,61,170);font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">Array</span><span style="font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">((</span><span style="color:rgb(39,42,216);font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">0</span><span style="font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">...</span><span style="color:rgb(39,42,216);font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">10000</span><span style="font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">)).</span><span style="color:rgb(49,89,93);font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">pmap</span><span style="font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg"> {</span></div></div></div></div><div class="m_329265698061725855gmail_msg"><div style="margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><div style="margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><div style="font-family:Menlo;color:rgb(49,89,93);margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><span style="font-variant-ligatures:no-common-ligatures;color:#000000" class="m_329265698061725855gmail_msg"> </span><span style="font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">someReallySlowFunction</span><span style="font-variant-ligatures:no-common-ligatures;color:#000000" class="m_329265698061725855gmail_msg">($0)</span></div></div></div></div><div class="m_329265698061725855gmail_msg"><div style="margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><div style="margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><div style="font-family:Menlo;margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><span style="font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">}</span></div></div></div></div></blockquote><div class="m_329265698061725855gmail_msg"><div style="margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><div style="margin:0px;line-height:normal" class="m_329265698061725855gmail_msg"><div class="m_329265698061725855gmail_msg"><span style="font-variant-ligatures:no-common-ligatures" class="m_329265698061725855gmail_msg">At least I <i class="m_329265698061725855gmail_msg">think</i> that’d work... I haven’t tried it yet... Anyway, without some way (<i class="m_329265698061725855gmail_msg">any</i> way) of getting an instance of T to fill in the `result` array, it becomes much trickier to keep track of all the concurrently-calculated transformed values. In this case, the semantics of `T()` are fairly irrelevant because the semantics of the <i class="m_329265698061725855gmail_msg">overall statement</i> is just to work around</span> a language limitation (Swift not having separate allocation and initialization phases), which doesn’t have anything to do with the semantics of the initial value that got passed as the `repeating` argument.</div></div></div></div></div></blockquote><div><br></div><div>This looks like it's abusing T() to stand in for nil, though. Before the result[i] assignment executes, result[i] shouldn't conceptually have a value—you're putting a default in there because the implementation requires it. T() as a placeholder is just as valid/invalid as any other value in T's space.</div><div><br></div><div>It's a square-peg-in-a-round-hole problem—design-wise, an array of optionals would be a better fit, but you rightly don't want to return that from the function, and you don't want to bear the cost of converting the array of optionals to an array of non-optionals after the computation is complete. That makes complete sense, but perhaps *that* is the problem that should be addressed, instead of trying to use T() to avoid it? Or alternatively, a parallel map operation should return an array of futures (or a collection type that itself is a future).</div><div><br></div><div><br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word" class="m_329265698061725855gmail_msg"><div class="m_329265698061725855gmail_msg"><br class="m_329265698061725855gmail_msg"></div><div class="m_329265698061725855gmail_msg">- Dave Sweeris</div></div></blockquote></div></div>
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