<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><br class=""><div><br class=""><blockquote type="cite" class=""><div class="">On Oct 20, 2017, at 07:51, Xiaodi Wu via swift-dev <<a href="mailto:swift-dev@swift.org" class="">swift-dev@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class="">On Fri, Oct 20, 2017 at 1:22 AM, Jonathan Hull <span dir="ltr" class=""><<a href="mailto:jhull@gbis.com" target="_blank" class="">jhull@gbis.com</a>></span> wrote:<br class=""><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" class="">+1 for trapping unless using &==. In the case of ‘Float?’ we could also map to nil.<div class=""><br class=""></div><div class="">This is probably a more appropriate discussion for evolution though...<br class=""><div class=""><div class=""><br class=""></div><div class=""><br class=""><div class=""><div class=""><blockquote type="cite" class=""><div class=""><div class="h5"><div class="">On Oct 19, 2017, at 9:48 PM, Brent Royal-Gordon via swift-dev <<a href="mailto:swift-dev@swift.org" target="_blank" class="">swift-dev@swift.org</a>> wrote:</div><br class="m_2743765292807577601Apple-interchange-newline"></div></div><div class=""><div class=""><div class="h5"><div style="word-wrap:break-word;line-break:after-white-space" class=""><div class=""><blockquote type="cite" class=""><div class="">On Oct 19, 2017, at 4:29 PM, Xiaodi Wu via swift-dev <<a href="mailto:swift-dev@swift.org" target="_blank" class="">swift-dev@swift.org</a>> wrote:</div><br class="m_2743765292807577601Apple-interchange-newline"><div class=""><div 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" class="">D) Must floating-point IEEE-compliant equivalence be spelled `==`?</div><div 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" class=""><br class=""></div><div 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" class="">In my view, this is something open for debate. I see no reason why it cannot be migrated to `&==` if it were felt that `==` *must* be a full equivalence relation. I believe this is controversial, however.</div></div></blockquote><br class=""></div><div class="">I actually got partway through writing up a pitch on this yesterday, but my opinion is that NaNs are so exceptional, and so prone to misuse, that we ought to treat them like integer arithmetic overflows: trap when they're detected, unless you use an `&` variant operator which indicates you know what you're doing.</div><div class=""><br class=""></div><div class="">I strongly suspect that, in practice, most float-manipulating code is not prepared to handle NaN and will not do anything sensible in its presence. For example, Apple platforms use floating-point types for geometry, color components, GPS locations, etc. Very little of this code will do anything sensible in the presence of a NaN. Arguably, it'd be better to exclude them through the type system, but I don't think that's a realistic possibility—we would need to have done that in a more source-break-friendly era. But that doesn't have to mean we're completely stuck.</div></div></div></div></div></blockquote></div></div></div></div></div></div></blockquote><div class=""><br class=""></div><div class="">Built-in floating point operators, as well as libc/libm math functions, are designed to propagate NaN correctly. This is not meant to be a thread about NaN, and we need to be cautious to define the scope of the problem to be solved from the outset. The tendency of having ever-expanding discussion where issues such as method names turn into discussions about the entire standard library go nowhere.</div><div class=""><br class=""></div><div class="">The question here is about `==` specifically and how to accommodate partial equivalence relations. For sanity, we start with the premise that NaN will forever be as it is.</div></div></div></div></div></blockquote><br class=""></div><div>I support Jonathan’s argument. If Swift wants to trap on NaN to improve self-consistency and simplicity, then the tradeoff might be worth it. The alternative, teaching the Equality protocol about NaNs, feels like “the tail wagging the dog".</div><div><br class=""></div><div>In short: what IEEE requires of floating-point hardware is separable from IEEE’s opinions about language/library design.</div><div><br class=""></div><div><br class=""></div><div>Dave</div></body></html>