<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=""><div class="">tl;dr The fallthrough keyword, as far as I am aware, isn’t costing us anything; and has at least minimal utility, as I try to demonstrate.</div></div><div class=""><br class=""></div>Apologies for jumping into this thread at an awkward point, but I’ve only just now subscribed to this list.<div class=""><br class=""></div><div class="">I think the fallthrough keyword is useful in certain circumstances. I’ve also yet to see an example of where it creates a negative impact, either in code, optimization, or what have you. Other than “It’s like something in C, and C is old and busted” I’m unsure of the rationale for removing it. (Feel free to point me in the right direction.)</div><div class=""><br class=""></div><div class="">Consider the Planet enum from the documentation. One of the simplest way to define the number of a planet (i.e. its 1-based index in the ordering of planets wrt. distance from the sun) is using a switch and fall-through:</div><div class=""><br class=""></div><div class=""><a href="https://gist.github.com/cbarrett/23b24a9fe76efdf006df" class="">https://gist.github.com/cbarrett/23b24a9fe76efdf006df</a></div><div class=""><br class=""></div><div class="">This technique is very extensible — for instance imagine computing the force induced by the gravity of the other planets on a particular planet. All that would need to change is the case statements.</div><div class=""><br class=""></div><div class="">Yes, you could write this by putting the planets into a list and mapping or folding (or looping) over that, but unless the compiler can “unroll” that construct, you’re paying for an allocation simply bc of your choice of control flow. But in fact, you could imagine generalizing this construct into the implementation of fold for the Planet type — low-overhead folds for monomorphic types seems like a pretty compelling an natural use case for fallthrough to me.</div><div class=""><br class=""></div><div class="">Thanks,</div><div class="">-Colin</div><div class=""><br class=""><div><blockquote type="cite" class=""><div class="">On Dec 6, 2015, at 4:52 AM, Jacopo Andrea Giola 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 dir="auto" class=""><div class="">+1 for this idea, but I will prefer the reswitch keyword instead of overloading continue with a new syntax.</div><div class=""><br class=""></div><div class="">If this proposal is accepted, it must be coupled with a compiler check that the reswitch statements don't introduce an infinite "switch" loop.</div><div class=""><br class="">Sent from my iPad</div><div class=""><br class="">On 06 Dec 2015, at 00:23, Steve Canon via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:<br class=""><br class=""></div><blockquote type="cite" class=""><div class=""><meta http-equiv="content-type" content="text/html; charset=utf-8" class=""><div class="">Very much thinking out loud and not really the implications, I wonder if we might just use "continue" instead of "reswitch".</div><div class=""><br class=""></div><div class="">I very much like specifying what case to fall through into, no matter how we spell it.<br class=""><br class="">- Steve</div><div class=""><br class="">On Dec 5, 2015, at 4:45 PM, John McCall via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:<br class=""><br class=""></div><blockquote type="cite" class=""><div class=""><meta http-equiv="Content-Type" content="text/html charset=utf-8" class=""><div class=""><blockquote type="cite" class=""><div class="">On Dec 5, 2015, at 1:31 PM, John McCall via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:</div><div class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class=""><blockquote type="cite" class=""><div class="">On Dec 4, 2015, at 11:37 PM, John Calsbeek <<a href="mailto:john.calsbeek+lists@gmail.com" class="">john.calsbeek+lists@gmail.com</a>> wrote:</div><div class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class=""><div class="">`fallthrough` is conceptually similar to `goto` in that both allow natural expression of concepts that exist at the instruction level but are otherwise difficult to express with nested control structures. `fallthrough` is perhaps slightly less objectionable because control flow remains local, but it has a similar role.</div><div class=""><br class=""></div><div class="">It is not particularly natural to write `switch` statements with `fallthrough` in the reverse order that can be seen in Duff’s Device and similar constructs (case 7 falls through to 6 which falls through to 5, etc.). It’s just because you know for certain that all the code in case 6 would be duplicated in case 7, so 7 can transfer into 6 without a jump instruction. Communicating that to the compiler without `fallthrough` requires deeply nested `if`s.</div></div></div></div></blockquote><div class=""><br class=""></div>Right. One idea that I’ve always had for “fallthrough” is that we might parameterize it in the future; parameterized it would mean “repeat the switch with this new value”, so that unparameterized fallthrough would mean “repeat the switch with a notional value that ends up in the next case”. There’s a very common pattern in switches of deferring to another case that I’ve always found very awkward to write in C, and while sometimes there’s no choice but to extract a helper function, there’s a still-fairly-structural code pattern here that I think we can sensibly support.</div><div class=""><br class=""></div><div class="">On the other hand, there’s an argument that this is an inappropriate extension for “fallthrough” specifically, which is one reason we’ve never pursued it.</div></div></div></blockquote><div class=""><br class=""></div>Oh, I see that Joe already brought this up, spelled “reswitch”.</div><div class=""><br class=""></div><div class="">John.</div><div class=""><br class=""><blockquote type="cite" class=""><div class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class=""><br class=""></div><div class="">John.</div><div class=""><br class=""><blockquote type="cite" class=""><div class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class=""><div class=""><br class=""></div><div class="">One defense comes to mind: there is talk of Swift aiming at systems programming. Is writing a threaded interpreter loop within the potential scope of Swift? That’s a use case that could make use of both `fallthrough` and `goto` (computed goto, really).</div><div class=""><br class=""></div><div class="">switch op {</div><div class="">case LOAD_INDIRECT:</div><div class=""> in0 = memory[in1]</div><div class=""> fallthrough</div><div class="">case LOAD:</div><div class=""> out0 = memory[in0]</div><div class="">//...</div><div class="">}</div><div class=""><br class=""></div><div class="">I am personally interested in the prospect of a language that can scale up to high-level concepts and down to “portable assembler,” but I don’t know if that is the right direction for Swift’s evolution.</div><div class=""><br class=""></div><div class="">Cheers,</div><div class="">John</div></div><br class=""><div class=""><blockquote type="cite" class=""><div class="">On Dec 4, 2015, at 2:42 PM, John McCall <<a href="mailto:rjmccall@apple.com" class="">rjmccall@apple.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><blockquote type="cite" class=""><div class="">On Dec 4, 2015, at 2:33 PM, Kevin Ballard <<a href="mailto:kevin@sb.org" class="">kevin@sb.org</a>> wrote:</div><div class=""><div class=""><div class="">It's not actually Duff's Device. Duff's Device relies on the fact that C switch statements don't actually introduce a new scope, and so it overlaps a switch with a do-while loop. This lets it only test the number of bytes once, to jump into the middle of the loop, and then it switches over to a while loop that decrements a counter every 8 instructions. Basically, it's a trick for manual loop unrolling that deals with non-multiple-of-8 counts efficiently.<br class=""></div></div></div></blockquote><div class=""><br class=""></div>To be pedantic, C switch statements do introduce a new scope. What Duff’s Device exploits is that switch is allowed to jump into (almost) arbitrary scopes, and cases can appear anywhere recursively inside a switch.</div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><br class=""></div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class="">But your point that Swift’s switch requires cases to be at the top level within a switch and thus prevents the use of Duff’s Device is 100% correct.</div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><br class=""></div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class="">John.</div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><br class=""><blockquote type="cite" class=""><div class=""><div class=""><div class=""> </div><div class="">Steve's code is also an example of manual loop unrolling that deals with non-multiple-of-8 counts, but it has calculate the number of bytes on every iteration instead of once. It's a good example of one of the uses of `fallthrough`, it's just not Duff's Device. It's impossible to use Duff's Device in Swift.<br class=""></div><div class=""> </div><div class="">-Kevin Ballard</div><div class=""> </div><div class="">On Fri, Dec 4, 2015, at 02:16 PM, Greg Titus wrote:<br class=""></div><blockquote type="cite" class=""><div class="">Streza’s source code is an example of Duff’s Device, which is a big place where switch fallthrough is arguably the cleanest way to do things and the reason why I’d personally prefer to keep it as part of the language.<br class=""></div><div class=""> </div><div class=""><blockquote type="cite" class=""><div class="">On Dec 4, 2015, at 2:12 PM, Erica Sadun <<a href="mailto:erica@ericasadun.com" class="">erica@ericasadun.com</a>> wrote:<br class=""></div><div class=""> </div><div class=""><div class="" style="word-wrap: break-word; -webkit-line-break: after-white-space;"><div class="">Oh let it die, let it die. Any time I use fallthrough I find myself re-factoring to stop using it. <br class=""></div><div class=""> </div><div class=""><b class="">True fact</b>: On all of<span class="Apple-converted-space"> </span><a href="http://gist.github.com/" class="">gist.github.com</a>, there are only 22 gist results for "fallthrough language:swift".<br class=""></div><div class="">Half of those are people just testing out the feature. Most of the remaining ones are just complex cases:<br class=""></div><div class=""><i class="">case .Enum1, .Enum2:</i><br class=""></div><div class="">expressed as <br class=""></div><div class=""><i class="">case .Enum1: fallthrough</i><br class=""></div><div class=""><i class="">case .Enum2:</i><br class=""></div><div class=""> </div><div class="">And then there's streza: <a href="https://gist.github.com/stevestreza/2557dc5ec9e7c694d7ea" class="">https://gist.github.com/stevestreza/2557dc5ec9e7c694d7ea</a><span class="Apple-converted-space"> </span> I'm pretty sure that ponies were harmed in the production of whatever that last bit is.<br class=""></div><div class=""> </div><div class=""> </div><div class=""> </div><div class=""> </div><div class=""><blockquote type="cite" class=""><div class="">On Dec 4, 2015, at 3:05 PM,<span class="Apple-converted-space"> </span><a href="mailto:jalkut@red-sweater.com" class="">jalkut@red-sweater.com</a><span class="Apple-converted-space"> </span>wrote:<br class=""></div><div class=""> </div><div class=""><div class=""><div class="">In the spirit of some other proposals that remove C or C++ style artifacts, what do folks think about the possibility of removing the "fallthrough" keyword from the language?<br class=""></div><div class=""> </div><div class="">My understanding is this keyword is only used for the archaic seeming purpose of perpetuating C-style fallthrough from one switch statement to the subsequent one. The documentation hedges the use of this keyword in forbidding terms that make it clear its use is not encouraged. The presence of the keyword, while an improvement over C’s implicit fallthrough, is a mark of inelegance on an otherwise well-designed, opinionated implementation of swtich statements.<br class=""></div><div class=""> </div><div class="">The ugliness of fallthrough’s C-style behavior even demands a caveat in the documentation:<br class=""></div><div class=""> </div><div class="">"The fallthrough keyword does not check the case conditions for the switch case that it causes execution to fall into. The fallthrough keyword simply causes code execution to move directly to the statements inside the next case (or default case) block, as in C’s standard switch statement behavior."<br class=""></div><div class=""> </div><div class="">To my mind, the caveat explains just what is wrong with fallthrough, both in C or Swift: coded that is clearly labeled with deliberate conditions can nonetheless be reached.<br class=""></div><div class=""> </div><div class="">I quipped about this on Twitter, and the most common pushback I got seemed to be from people who either did not know about Swift’s support for comma-separated case statements, or harbored an aesthetic preference for clustering such cases together with fallthrough statements.<br class=""></div><div class=""> </div><div class="">In my opinion, unless somebody can think of a strong defense for supporting intentional fallthrough in Swift, removing the keyword would be a move in the direction of minimizing the language’s complexity while also discouraging poor coding style in switch statements.<br class=""></div><div class=""> </div><div class="">Thoughts?<br class=""></div><div class=""> </div><div class="">Daniel<br class=""></div><div class=""> </div><div class="">_______________________________________________<br class=""></div><div class="">swift-evolution mailing list<br class=""></div><div class=""><a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a><br class=""></div><div class=""><a href="https://lists.swift.org/mailman/listinfo/swift-evolution" class="">https://lists.swift.org/mailman/listinfo/swift-evolution</a><br class=""></div></div></div></blockquote></div><div class=""> </div><div class=""><img border="0" height="1" width="1" alt="" src="https://www.fastmailusercontent.com/proxy/989cb4366ad41b4455cc3f50f9b71777dbe9035cdabeb439a2cbe058c39f9d97/8647470737a3f2f25723030323431303e23647e23756e64676279646e2e65647f27766f2f60756e6f35707e6d33757732745b446d22364f4368776b4e4749665f6b636e4447513e6a7469514948537938614931365f49435d2236437264485d4d223647664c6a505a58416a4a7b623341545d6165725b6750524145507a695767683b626c6235326430743252446c61313a46477d4154424c4357513454793d2232475366637a774349466a76603d40545d223248415142686e4f63723a745a747d22364a6a647850515f4d4859696f684b64796346526a6a697336616a79667a6e626a73545a47664a76323979434d22364d4d496f6a7251407779685557624c6867425c435563707c603e697868366969587d22364a705341677473783d22364444794a666d22324a64337d23344/open" class="" style="height: 1px !important; 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