<div dir="ltr">On Tue, Jan 2, 2018 at 8:25 PM, Matthew Johnson <span dir="ltr"><<a href="mailto:matthew@anandabits.com" target="_blank">matthew@anandabits.com</a>></span> wrote:<br><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"><br><div><span class=""><blockquote type="cite"><div>On Jan 2, 2018, at 3:41 PM, Xiaodi Wu <<a href="mailto:xiaodi.wu@gmail.com" target="_blank">xiaodi.wu@gmail.com</a>> wrote:</div><br class="m_-3382138953498341251Apple-interchange-newline"><div><div dir="ltr">On Tue, Jan 2, 2018 at 3:27 PM, Kevin Nattinger <span dir="ltr"><<a href="mailto:swift@nattinger.net" target="_blank">swift@nattinger.net</a>></span> wrote:<br><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><span><br><blockquote type="cite"><div><div dir="ltr" style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px"><div class="gmail_extra"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex"><div dir="auto"><div><div><span><blockquote type="cite"><div><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div>in what other circumstances do we insist that the compiler inform the end user about future additions to the API at compile time?</div></div></div></div></div></blockquote><div><br></div></span><div>This isn’t a request for the compiler to inform the user about future additions to an API. It is a request to validate the compiler’s knowledge of the<span class="m_-3382138953498341251m_1210698447792194873Apple-converted-space"> </span><b>current</b> state of an API with the<span class="m_-3382138953498341251m_1210698447792194873Apple-converted-space"> </span><b>current</b> state of the source code. </div></div></div></div></blockquote><div><br></div><div>Well, it's of course impossible to inform the user about future additions, so that's poorly phrased on my part. It's about the compiler informing the end user about *new* additions, part of the *current* state of the API, that have cropped up since the user last revised the code when the API was in a *previous* state (or, indistinguishably, members of which a user is unaware regardless of the temporal sequence of when such members were added). In what other circumstances do we insist that the compiler perform this service?</div></div></div></div></div></blockquote><div><br></div></span><div>Enums. That's literally how they work today. You are arguing in favor of actively removing compiler-aided correctness.</div><div><br></div><div>There's also protocol requirements</div></div></div></blockquote><div><br></div><div>No, that's now how enums work today, and it's not how protocol requirements work today. Enums today are all semantically exhaustive; if a case is added in a later version of a library, it's semantically a *different* enum type that happens to share the same name. Not considering all the cases of an exhaustive enum is an _error_, not a _warning_, because there is no basis on which to proceed. This will not change with the proposal. Likewise, adding a protocol requirement without a default implementation is source-breaking. The result is a compiler _error_.</div><div><br></div><div>The question is, what non-source breaking API additions today cause the compiler to inform the end user of such additions? </div></div></div></div></div></blockquote><div><br></div></span><div>Posing the question this way takes it as a given that adding a case to a resilient enum is non-source breaking with a full stop.</div></div></div></blockquote><div><br></div><div>Ah, yes. As I wrote in an earlier email in reply to Jon and others, it seems we have a fundamental difference here. I take it as a given, full stop, that this is what we are setting out to do. If it is not, then we are trying to design with different end goals in mind.</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"><div><div>The position of everyone asking for something like `future` / `unknown` as an alternative to `default` is exactly that this should not be the case. Instead, adding a case should always be binary compatible and should be source compatible by default, but authors should have the ability to opt-in to making case additions be source-breaking for individual switch statements. </div><div><br></div><div>When you view it this way we are not asking the compiler to inform us of a non-source breaking addition. We are asking the compiler to treat an addition as source breaking in a specific context.</div><span class=""><br><blockquote type="cite"><div><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div>The answer is: none whatsoever. Not new methods or properties on a type, not even new protocol requirements that have a default implementation.</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"><div><div>and, arguably, deprecated methods with a proper message ("use foo instead").</div></div></div></blockquote><div><br></div></div><br></div></div>
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