<html><head></head><body>I think this design does not avoid you writing something like `private enum Foo { default ... }`, which is redudant as Jordan already pointed out in his previous post, nor does it have a consistent way of declaration:<div><br></div><div>enum Foo {</div><div> case abc</div><div> case def</div><div> default</div><div>}</div><div><br></div><div><div><span style="background-color: rgba(255, 255, 255, 0);">enum Foo {</span></div><div><span style="background-color: rgba(255, 255, 255, 0);"> case abc</span></div><div><span style="background-color: rgba(255, 255, 255, 0);"> default</span></div><div><span style="background-color: rgba(255, 255, 255, 0);"> case def</span></div><div><span style="background-color: rgba(255, 255, 255, 0);">}</span></div></div><div><span style="background-color: rgba(255, 255, 255, 0);"><br></span></div><div><span style="background-color: rgba(255, 255, 255, 0);">enum Foo {</span></div><div><span style="background-color: rgba(255, 255, 255, 0);"> default</span></div><div><span style="background-color: rgba(255, 255, 255, 0);"> case abc</span></div><div><span style="background-color: rgba(255, 255, 255, 0);"> case def</span></div><div><span style="background-color: rgba(255, 255, 255, 0);">}</span></div><div><span style="background-color: rgba(255, 255, 255, 0);"><br></span></div><div><span style="background-color: rgba(255, 255, 255, 0);">----</span></div><div><span style="background-color: rgba(255, 255, 255, 0);"><br></span></div><div><span style="background-color: rgba(255, 255, 255, 0);">On the other hand I'd be very much in favor of the design David has pitched, which makes `public` as a soft default for public API's and adds a stronger constraints with an extra keyword OR a different access modifier. In case of an access modifier it would be really interesting if someone knows other use cases where we could reuse `closed` for similar purposes.</span></div><div><span style="background-color: rgba(255, 255, 255, 0);"><br></span></div><div><span style="background-color: rgba(255, 255, 255, 0);">Long story short:</span></div><div><span style="background-color: rgba(255, 255, 255, 0);">- public is the soft default and all uses of an enum from module A in module B would require `default` in swithch statements</span></div><div><span style="background-color: rgba(255, 255, 255, 0);">- closed is the stronger implied `public` which makes the enum finite and does not require `default` if you switch on all cases</span></div><div> <div class="bloop_sign"><div style="font-family:helvetica,arial;font-size:13px">-- <br>Adrian Zubarev<br>Sent with Airmail</div></div> <p class="gmail_quote" style="color:#000;">Am 10. August 2017 um 16:55:00, Matthew Johnson via swift-evolution (<a href="mailto:swift-evolution@swift.org">swift-evolution@swift.org</a>) schrieb:</p> <blockquote type="cite" class="gmail_quote"><span><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div></div><div>
<title></title>
<br class="">
<div>
<blockquote type="cite" class="">
<div class="">On Aug 10, 2017, at 9:25 AM, Vladimir.S <<a href="mailto:svabox@gmail.com" class="">svabox@gmail.com</a>>
wrote:</div>
<br class="Apple-interchange-newline">
<div class=""><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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; float: none; display: inline !important;" class="">On 10.08.2017 16:46, Matthew Johnson via swift-evolution
wrote:</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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-size-adjust: auto; -webkit-text-stroke-width: 0px;" class="">
<blockquote type="cite" class="">On Aug 10, 2017, at 7:46 AM, James
Froggatt via swift-evolution<br class="">
<<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:<br class="">
Since it seems to have been lost in the noise, I want to second
with support for<br class="">
Xiaodi's syntax of having `default` appearing in the enum
declaration itself.<br class="">
It's much clearer in its intention, feels very ‘Swifty’, and more
importantly it<br class="">
doesn't prompt whole threads debating the semantics of `open` vs
`public`.<br class=""></blockquote>
I think Xiaodi’s syntax is very elegant if we want to avoid the
access control<br class="">
style syntax. However, it does one problem: the “error of
omission” (not thinking<br class="">
about open vs closed) leaves a library author with a closed enum,
preventing them<br class="">
from adding cases in the future without breaking compatibility.
I’m not sure this<br class="">
is acceptable.<br class=""></blockquote>
<br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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="">
<span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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; float: none; display: inline !important;" class="">Then, doesn't this mean that any 'usual' enum should be
'open' by default, and only enum declared with some marker (like
'final' or 'enum(sealed)') can be 'closed'?</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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 style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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="">
<span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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; float: none; display: inline !important;" class="">Otherwise we need to require an explicit marker for *each*
enum, and so break the source compatibility? (we'll have to append
that marker to each enum in your current code)</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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=""></div>
</blockquote>
<div><br class=""></div>
<div>This is a good point. A good first decision is whether
we prioritize source compatibility or the more conservative
“default”. If source compatibility is prioritized Xiaodi’s
proposed syntax is probably hard to beat.</div>
<br class="">
<blockquote type="cite" class="">
<div class=""><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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="">
<span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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; float: none; display: inline !important;" class="">Also I'd suggest this for closed enum:</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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 style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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="">
<span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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; float: none; display: inline !important;" class="">enum MyClosedEnum {</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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="">
<span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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; float: none; display: inline !important;" class=""> case a</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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="">
<span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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; float: none; display: inline !important;" class=""> case b</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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="">
<span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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; float: none; display: inline !important;" class=""> case c</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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="">
<span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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; float: none; display: inline !important;" class=""> final</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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="">
<span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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; float: none; display: inline !important;" class="">}</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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 style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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="">
<span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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; float: none; display: inline !important;" class="">So, for public closed enum it will looks
like:</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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 style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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="">
<span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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; float: none; display: inline !important;" class="">public enum MyClosedEnum {</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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="">
<span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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; float: none; display: inline !important;" class=""> case a</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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="">
<span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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; float: none; display: inline !important;" class=""> case b</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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="">
<span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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; float: none; display: inline !important;" class=""> case c</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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="">
<span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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; float: none; display: inline !important;" class=""> final</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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="">
<span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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; float: none; display: inline !important;" class="">}</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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 style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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="">
<span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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; float: none; display: inline !important;" class="">Also, if we need to explicitly mark open enum, probably we
can consider 'continue' keyword, as IMO is not clear what 'default'
is saying on declaration site('you must insert `default` in
switch'? 'there are other `default` cases'?) :</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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 style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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="">
<span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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; float: none; display: inline !important;" class="">public enum MyOpenEnum {</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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="">
<span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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; float: none; display: inline !important;" class=""> case a</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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="">
<span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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; float: none; display: inline !important;" class=""> case b</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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="">
<span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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; float: none; display: inline !important;" class=""> case c</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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="">
<span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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; float: none; display: inline !important;" class=""> continue // to be continue...</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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="">
<span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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; float: none; display: inline !important;" class="">}</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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 style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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 style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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-size-adjust: auto; -webkit-text-stroke-width: 0px;" class="">
<blockquote type="cite" class="">------------ Begin Message
------------ Group: gmane.comp.lang.swift.evolution MsgID:
<<a href="mailto:CAGY80u=kVQA1q=5TMxXxFgM4tLGFUQh61EN1daepEMAA_FoE9Q@mail.gmail.com" class="">CAGY80u=kVQA1q=5TMxXxFgM4tLGFUQh61EN1daepEMAA_FoE9Q@mail.gmail.com</a>><br class="">
On Tue, Aug 8, 2017 at 5:27 PM, Jordan Rose via swift-evolution
< <a href="mailto:swift-evolution-m3FHrko0VLzYtjvyW6yDsg@public.gmane.org" class="">swift-evolution-m3FHrko0VLzYtjvyW6yDsg@public.gmane.org</a>>
wrote:<br class="">
<blockquote type="cite" class="">Hi, everyone. Now that Swift 5 is
starting up, I'd like to circle back to an<br class="">
issue that's been around for a while: the source compatibility of
enums. Today, it's an error to switch over an enum without handling
all the cases, but this breaks down in a number of ways:<br class="">
- A C enum may have "private cases" that aren't defined inside the
original<br class="">
enum declaration, and there's no way to detect these in a switch
without<br class="">
dropping down to the rawValue. - For the same reason, the
compiler-synthesized<br class="">
'init(rawValue:)' on an imported enum never produces 'nil', because
who knows<br class="">
how anyone's using C enums anyway? - Adding a new case to a *Swift*
enum in a<br class="">
library breaks any client code that was trying to switch over
it.<br class="">
(This list might sound familiar, and that's because it's from a
message of mine on a thread started by Matthew Johnson back in
February called "[Pitch]<br class="">
consistent public access modifiers". Most of the rest of this email
is going<br class="">
to go the same way, because we still need to make progress
here.)<br class="">
At the same time, we really like our exhaustive switches,
especially over enums we define ourselves. And there's a
performance side to this whole thing<br class="">
too; if all cases of an enum are known, it can be passed around
much more<br class="">
efficiently than if it might suddenly grow a new case containing a
struct with<br class="">
5000 Strings in it.<br class="">
*Behavior*<br class="">
I think there's certain behavior that is probably not *terribly*
controversial:<br class="">
- When enums are imported from Apple frameworks, they should always
require a<br class="">
default case, except for a few exceptions like NSRectEdge. (It's
Apple's job<br class="">
to handle this and get it right, but if we get it wrong with an
imported enum<br class="">
there's still the workaround of dropping down to the raw value.) -
When I<br class="">
define Swift enums in the current framework, there's obviously no
compatibility issues; we should allow exhaustive
switches.<br class="">
Everything else falls somewhere in the middle, both for enums
defined in Objective-C:<br class="">
- If I define an Objective-C enum in the current framework, should
it allow<br class="">
exhaustive switching, because there are no compatibility issues, or
not,<br class="">
because there could still be private cases defined in a .m file? -
If there's<br class="">
an Objective-C enum in *another* framework (that I built locally
with Xcode,<br class="">
Carthage, CocoaPods, SwiftPM, etc.), should it allow exhaustive
switching,<br class="">
because there are no *binary* compatibility issues, or not, because
there may<br class="">
be *source* compatibility issues? We'd really like adding a new
enum case to<br class="">
*not* be a breaking change even at the source level. - If there's
an<br class="">
Objective-C enum coming in through a bridging header, should it
allow<br class="">
exhaustive switching, because I might have defined it myself, or
not, because<br class="">
it might be non-modular content I've used the bridging header to
import?<br class="">
And in Swift:<br class="">
- If there's a Swift enum in another framework I built locally,
should it allow exhaustive switching, because there are no binary
compatibility issues,<br class="">
or not, because there may be source compatibility issues? Again,
we'd really<br class="">
like adding a new enum case to *not* be a breaking change even at
the source<br class="">
level.<br class="">
Let's now flip this to the other side of the equation. I've been
talking about<br class="">
us disallowing exhaustive switching, i.e. "if the enum might grow
new cases<br class="">
you must have a 'default' in a switch". In previous (in-person)
discussions<br class="">
about this feature, it's been pointed out that the code in an
otherwise-fully-covered switch is, by definition, unreachable, and
therefore<br class="">
untestable. This also isn't a desirable situation to be in, but
it's mitigated<br class="">
somewhat by the fact that there probably aren't many framework
enums you<br class="">
should exhaustively switch over anyway. (Think about Apple's
frameworks<br class="">
again.) I don't have a great answer, though.<br class="">
For people who like exhaustive switches, we thought about adding a
new kind of<br class="">
'default'—let's call it 'unknownCase' just to be able to talk about
it. This<br class="">
lets you get warnings when you update to a new SDK, but is even
more likely to<br class="">
be untested code. We didn't think this was worth the
complexity.<br class="">
*Terminology*<br class="">
The "Library Evolution <<a href="http://jrose-apple.github.io/swift-library-evolution/" class="">http://jrose-apple.github.io/swift-library-evolution/</a>>"
doc (mostly written<br class="">
by me) originally called these "open" and "closed" enums ("requires
a default"<br class="">
and "allows exhaustive switching", respectively), but this predated
the use of<br class="">
'open' to describe classes and class members. Matthew's original
thread did<br class="">
suggest using 'open' for enums as well, but I argued against that,
for a few<br class="">
reasons:<br class="">
- For classes, "open" and "non-open" restrict what the *client* can
do. For<br class="">
enums, it's more about providing the client with additional
guarantees—and<br class="">
"non-open" is the one with more guarantees. - The "safe" default is
backwards:<br class="">
a merely-public class can be made 'open', while an 'open' class
cannot be made<br class="">
non-open. Conversely, an "open" enum can be made "closed" (making
default<br class="">
cases unnecessary), but a "closed" enum cannot be made
"open".<br class="">
That said, Clang now has an 'enum_extensibility' attribute that
does take 'open' or 'closed' as an argument.<br class="">
On Matthew's thread, a few other possible names came up, though
mostly only<br class="">
for the "closed" case:<br class="">
- 'final': has the right meaning abstractly, but again it behaves
differently<br class="</body">
</blockquote></blockquote></blockquote></div></blockquote></div></div></div></span></blockquote></div></body></html>