<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=""><br class=""><div><blockquote type="cite" class=""><div class="">On Oct 20, 2017, at 7:36 AM, Xiaodi Wu <<a href="mailto:xiaodi.wu@gmail.com" class="">xiaodi.wu@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="auto" 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; -webkit-text-stroke-width: 0px;" class=""><br class="Apple-interchange-newline">On Fri, Oct 20, 2017 at 07:21 David Zarzycki <<a href="mailto:dave@znu.io" class="">dave@znu.io</a>> wrote:<br class=""></div><blockquote class="gmail_quote" 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; -webkit-text-stroke-width: 0px; 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 style="word-wrap: break-word;" class=""><br class=""><div class=""><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" target="_blank" class="">swift-dev@swift.org</a>> wrote:</div><br class="m_-4110680497984621742Apple-interchange-newline"><div class=""><div class="">On Fri, Oct 20, 2017 at 1:22 AM, Jonathan Hull<span class="Apple-converted-space"> </span><span class=""><<a href="mailto:jhull@gbis.com" target="_blank" class="">jhull@gbis.com</a>></span><span class="Apple-converted-space"> </span>wrote:<br class=""><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 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="m_-4110680497984621742h5"><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_-4110680497984621742m_2743765292807577601Apple-interchange-newline"></div></div><div class=""><div class=""><div class="m_-4110680497984621742h5"><div style="word-wrap: break-word;" 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_-4110680497984621742m_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><div style="word-wrap: break-word;" class=""><div class="">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 class=""><br class=""></div><div class="">In short: what IEEE requires of floating-point hardware is separable from IEEE’s opinions about language/library design.</div></div></blockquote><div dir="auto" 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; -webkit-text-stroke-width: 0px;" class=""><br class=""></div><div dir="auto" 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; -webkit-text-stroke-width: 0px;" class="">IEEE 754 requires certain behaviors of conforming implementations and is meant to allow for portable use of floating point. Swift’s floating point facilities are conformant to that standard, and breaking IEEE 754 conformance has been repeatedly stated to be a non-starter.</div><div dir="auto" 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; -webkit-text-stroke-width: 0px;" class=""><br class=""></div><div dir="auto" 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; -webkit-text-stroke-width: 0px;" class="">The essential idea behind quiet NaN is that it can be used as input in operations that take floating point values without raising errors. Since breaking IEEE 754 conformance is a non-starter, trapping on NaN is outside the realm of available solutions.</div><div dir="auto" 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; -webkit-text-stroke-width: 0px;" class=""><br class=""></div><div dir="auto" 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; -webkit-text-stroke-width: 0px;" class="">It has been pointed out, however, that IEEE does not require a specific syntax for floating point equivalence, hence the question of whether it can be spelled differently. However, trapping on NaN is emphatically not an option.</div><div dir="auto" 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; -webkit-text-stroke-width: 0px;" class=""><br class=""></div><div dir="auto" 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; -webkit-text-stroke-width: 0px;" class="">But I really don’t want to discuss this at all here. The topic of this thread is about the semantics of Equatable.</div></div></blockquote></div><div class=""><br class=""></div><div class="">I don’t know about Brent, but I was only speaking about the semantics/behavior of == with respect to NaN. Trap on NaN for ‘==‘, but restore full IEEE behavior by using ‘&==‘. For ‘Float?’ we could treat NaN as nil for ‘==‘. ‘&==‘ would still have the IEEE behavior.</div><div class=""><br class=""></div><div class="">This restores reflexivity and we can count on == being a true equivalence relation in our generic algorithms. Lack of proper handling of NaN is almost a stereotypical cause of bugs. Looking at code written for Floats, seeing &== will be a sign that the original programmer considered the possibility of NaN.</div><div class=""><br class=""></div><div class="">Thanks,</div><div class="">Jon</div><div class=""><br class=""></div><div class=""><br class=""></div><br class=""></body></html>