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.send { color:#77bb77; }
.server { color:#7799bb; }
.error { color:#AA0000; }</style></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class="bloop_markdown"><p>I’m not sure about the design itself, but the idea is great. I couldn’t even foresee that there might be a need for <code>switch!</code>. This is definitely better than my suggestion of a new attribute. I most cases I’d need <code>switch?</code> to replace the ugly-looking <code>if case … { … } else if case … { … }</code>.</p>
<p>Was there a rejected proposal on this that I missed?</p>
<p></p></div><div class="bloop_original_html"><style>body{font-family:Helvetica,Arial;font-size:13px}</style><div id="bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;"><br></div> <br> <div id="bloop_sign_1475493801888514048" class="bloop_sign"><div style="font-family:helvetica,arial;font-size:13px">-- <br>Adrian Zubarev<br>Sent with Airmail</div></div> <br><p class="airmail_on">Am 3. Oktober 2016 um 13:16:54, Ross O'Brien (<a href="mailto:narrativium+swift@gmail.com">narrativium+swift@gmail.com</a>) schrieb:</p> <blockquote type="cite" class="clean_bq"><span><div><div></div><div>
<title></title>
<div dir="ltr">There has been a little discussion on this before,
and I think there's a need for something along these lines - I've
written code where I've used 'guard' to ensure that an Int was
within a certain range, and then performed a switch on the Int,
requiring an ugly-looking 'default: fatalError()' at the end to
dismiss the warning.
<div><br></div>
<div>But exhaustive switches are also useful.</div>
<div><br></div>
<div>There was an elegant suggestion that we apply '?' and '!' to
the switch keyword. Basically:</div>
<div>- 'switch <expression>' is exhaustive across values and
enum states and the compiler will warn you if you omit an enum
state or default case.</div>
<div>- 'switch? <expression>' is not exhaustive but the
compiler should still check the flow (to ensure all paths return
values, that kind of thing).</div>
<div>- 'switch! <expression>' is not exhaustive but it
assumes one of the cases will match, and crashes otherwise.</div>
<div><br></div>
<div>Basically, switch wouldn't change, but appending the '?' is
equivalent to typing 'default: break' as your final case, and
appending '!' is equivalent to typing 'default: fatalError()' as
your final case. The meanings are roughly analogous to their
meanings for Optionals, so hopefully there wouldn't be much
confusion.</div>
<div><br></div>
</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">On Mon, Oct 3, 2016 at 11:55 AM, Adrian
Zubarev via swift-evolution <span dir="ltr"><<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>></span> wrote:<br>
<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 class="m_2037290431548325344bloop_markdown">
<p>This is clearly not a huge issue to solve, but a pitch is a
pitch.</p>
<p>From Swift book we know this:</p>
<blockquote>
<p><strong>Switch Statements Must Be Exhaustive</strong></p>
<p>In Swift, every possible value of the control expression’s type
must match the value of at least one pattern of a case. When this
simply isn’t feasible (for instance, when the control expression’s
type is Int), you can include a default case to satisfy the
requirement.</p>
</blockquote>
<p>Even for enum values an optional <code>default</code> would mean
that you don’t care about the other cases in your current switch,
which basically again would be another useless nop.</p>
<pre><code class="m_2037290431548325344swift">enum A {
case a, b, c
}
var value = A.a
switch value {
case .a:
value = A.b
default:
() // I don't care
}
// Do something else
switch value {
case .b:
value = A.c
default:
() // I don't care
}
</code></pre>
<p>Sure the error message is there to notify you that you might
forget to handle some case, but how we handle that specific case is
still up to us.</p>
<p>I’d really like to know what could be dangerous here when
<code>default</code> would be optional.</p>
<p>I can’t tell if this would have some impact on the ABI or not.
I’d say it’s additive because it doesn’t break any existing code
but removes an existing restriction.</p>
<hr>
<p>The next thought might be an overkill (or maybe not):</p>
<p>How about making <code>default</code> optional everywhere +
introducing a new attribute that allows the optional
<code>default</code> on that particular enum, but by default every
current existing enum should be handled exhaustively.</p>
<p>Bikeshedding:</p>
<pre><code class="m_2037290431548325344swift">enum A {
case a, b, c
}
var value = A.a
switch value {
case .a:
value = A.b
} // Error: switch must be exhaustive, consider adding a default clause
// VS:
@discardableCase enum B {
case d, e, f
}
let anotherValue = B.d
switch anotherValue {
case .d:
// Do something
case .e:
// Do something else
} // Just fine; We don't care here about `.f`
</code></pre></div>
<div class="m_2037290431548325344bloop_original_html">
<div id="m_2037290431548325344bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px;color:rgba(0,0,0,1.0);margin:0px;line-height:auto">
<span class=""><br></span></div>
<span class=""><br></span>
<div id="m_2037290431548325344bloop_sign_1475490682107675136" class="m_2037290431548325344bloop_sign">
<div style="font-family:helvetica,arial;font-size:13px">
<span class="">-- <br>
Adrian Zubarev<br>
Sent with Airmail</span></div>
</div>
<span class=""><br></span>
<div>
<div class="h5">
<p class="m_2037290431548325344airmail_on">Am 3. Oktober 2016 um
12:28:58, Rien (<a href="mailto:rien@balancingrock.nl" target="_blank">rien@balancingrock.nl</a>) schrieb:</p>
<blockquote type="cite" class="m_2037290431548325344clean_bq">
<div>
<div><span>On non-enum values, yes I could support this. However I
do not see this as a big enough issue.<br>
On enum values? no way….<br>
<br>
Btw this would get rid of:<br>
<br>
let bytesSend = send(…) // returns an Int<br>
<br>
switch bytesSend {<br>
case Int.min ... -1: {…}<br>
case 0: {…}<br>
case 1 ... Int.max: {…}<br>
default: break // <<<<<< Imposible<br>
}<br>
<br>
<br>
<br>
<br>
> On 03 Oct 2016, at 12:14, Adrian Zubarev via swift-evolution
<<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>> wrote:<br>
><br>
> I know that there is this note in Commonly Rejected
Changes:<br>
><br>
> Remove support for default: in switch and just use case _::
default is widely used, case _ is too magical, and default is
widely precedented in many C family languages.<br>
> I really like to use the switch instead of if case for pattern
matching, just because it’s neat block design. I do not want to
remove default from switches because it’s a must have and powerful
feature.<br>
><br>
> I’d like to know why switches must be exhaustive.<br>
><br>
> switch someValue {<br>
><br>
> case …:<br>
> // Do something<br>
><br>
> case …:<br>
> // Do something else<br>
><br>
> default:<br>
> () // useless nop; do nothing when no pattern matched<br>
> }<br>
><br>
> // VS:<br>
><br>
> if case … {<br>
><br>
> } else if case … {<br>
><br>
> } else if case … {<br>
><br>
> } // No need for `else`<br>
><br>
> Can’t we make default optional, or at least on non-enum
values?<br>
><br>
><br>
><br>
><br>
> --<br>
> Adrian Zubarev<br>
> Sent with Airmail<br>
><br>
> ______________________________<wbr>_________________<br>
> swift-evolution mailing list<br>
> <a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a><br>
> <a href="https://lists.swift.org/mailman/listinfo/swift-evolution" target="_blank">https://lists.swift.org/<wbr>mailman/listinfo/swift-<wbr>evolution</a><br>
<br></span></div>
</div>
</blockquote>
</div>
</div>
</div>
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</div>
<br>
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<br></blockquote>
</div>
<br></div>
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