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.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 think it might be a good idea to reuse optional function syntax in this case. I don’t think it would even break existing code with the rule I described in my previous post. I mean, in the end the result of the whole function becomes optional (only if it was non-optional before, or it simply stays optional otherwise).</p>
<pre><code class="swift">foo?(bar?(x)) +? y
</code></pre>
<p>Problematic are the operator functions because there is no way to really disambiguate those - at least right now, nor can we reference them like <code>Int.+</code> and in the worse case it would require us to burn all operators with a trailing <code>?</code>. Even more complicated is the <code>??</code> operator.</p>
<p>On the other hand we could simply say that an operator function is a special case, where we cannot make the function optional with this syntax. That would mean that operator functions (and subscripts?) still would require the old fashion way:</p>
<pre><code class="swift">let result: FooResult?
if lhs = foo?(bar?(x)), let rhs = y {
result = lhs + rhs
} else {
result = nil
}
</code></pre>
<p>However here is my thinking in a different direction for operator functions and subscripts:</p>
<p>If we could explicitly reference operator functions like <code>Int.+</code> or <code>Swift.??</code>, would it be possible to disambiguate the above case like this?</p>
<pre><code class="swift">(Int.+)?(lhs: foo?(bar?(x)), rhs: y)
</code></pre>
<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;"><p>Here is the syntax as pitched that works today:</p><pre style="color: rgb(0, 0, 0);"><code class="swift" style="-webkit-margin-before: 0px;">@objc protocol P : AnyObject {
@objc optional static func functionName(label: Int)
@objc optional func otherFunctionName(label: Int)
}
@objc class A : NSObject, P {}
let type: P.Type = A.self
(type.functionName)?(label: 42)
let instance: P = A()
(instance.otherFunctionName)?(label: 42)</code></pre></div> <div class="bloop_markdown"><p>We may need to fix optional subscripts first because the implicit behavior is really weird IMHO:</p><pre><code class="swift">@objc protocol P : AnyObject {
@objc optional subscript (label value: Int) -> Int { get }
}
@objc class A : NSObject, P {}
let instance: P = A()
instance[label: 42] // => compiles and returns nil
</code></pre><p>We definitely should require an infix <code>?</code> here - <code>instance?[label: 42]</code>.</p><p>Then even subscripts would work with the same syntax I pitched before. ;)</p><p></p></div><div class="bloop_original_html"></div> <div id="bloop_sign_1513061619662343936" class="bloop_sign"></div> <br><p class="airmail_on">Am 11. Dezember 2017 um 23:42:58, Slava Pestov via swift-evolution (<a href="mailto:swift-evolution@swift.org">swift-evolution@swift.org</a>) schrieb:</p> <blockquote type="cite" class="clean_bq"><span><div style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><div></div><div>
<title></title>
<br class="">
<div><br class="">
<blockquote type="cite" class="">
<div class="">On Dec 11, 2017, at 2:41 PM, Jared Khan via
swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:</div>
<br class="Apple-interchange-newline">
<div class="">
<div style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class="">I missed the previous threads! I’ve found one of the
relevant threads here:
<div class=""><a href="https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160711/024201.html" class="">https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160711/024201.html</a></div>
<div class=""><br class=""></div>
<div class="">Thanks for this important point.</div>
<div class=""><br class=""></div>
<div class="">If you were to write this logic out by hand then you
would short-circuit it and this is analogous to current chaining
behaviour so to me evaluating left to right (as Swift usually does)
and stopping at the first failed unwrap would make sense. I
wouldn’t necessarily say it’s intuitive but I don’t think it’s
really less intuitive than the current chaining
behaviour. </div>
</div>
</div>
</blockquote>
<div><br class=""></div>
<div>I think it gets confusing when you have multiple levels of
nested expressions, eg</div>
<div><br class=""></div>
<div>foo(bar(x?)) + y?</div>
<div><br class=""></div>
<div>Slava</div>
<br class="">
<blockquote type="cite" class="">
<div class="">
<div style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class="">
<div class=""><br class=""></div>
<div class="">Jared</div>
<div class=""><br class="">
<blockquote type="cite" class="">
<div class="">On 11 Dec 2017, at 19:28, Xiaodi Wu via
swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:</div>
<br class="Apple-interchange-newline">
<div class="">This topic has been discussed at least two and maybe
more times in the past.. It’s hard for me to post links at the
moment, but it should be possible to find on Google.<br class="">
<br class="">
One major challenge to this idea, for which no satisfactory answer
has been achieved after all these years, is the following
issue:<br class="">
<br class="">
f(g()?, h()?, i(), j()?)?<br class="">
<br class="">
If g() evaluates to nil, is h() called or not? How about i(), which
is not failable? Since any function or property access can have
side effects, in what order are the arguments evaluated, and how
does one reason about this code flow?<br class="">
<br class="">
To my mind, in the absence of an intuitive answer to the
above—which does not appear to be possible—this idea is not
feasible.<br class="">
<div class="gmail_quote">
<div dir="ltr" class="">On Mon, Dec 11, 2017 at 12:34 Magnus
Ahltorp via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:<br class=""></div>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<br class="">
> 12 Dec. 2017 02:58 Jared Khan <<a href="mailto:jaredkhan@me.com" class="">jaredkhan@me.com</a>> wrote:<br class="">
><br class="">
> 2. It felt natural to me. It’s analogous to the existing
optional chaining scenarios and composes nicely. I think it’s about
as understandable as existing chaining, a newbie would have to look
it up to discover its meaning. What are your thoughts on this
particular syntax (ignoring 3. momentarily)? Hopefully others in
this thread can share their views too.<br class="">
<br class="">
Chaining methods is linear, while nesting fills a similar purpose
when we use function calls. This of course affects the way existing
Swift code is written, but that is something we have to live with
if we want to use familiar syntax patterns. However, I think we
have to consider this difference in this case, since the syntax
becomes more convoluted. Your suggestion is definitely not as easy
to read as the optional chaining syntax, and maybe it can't
be.<br class="">
<br class="">
> As for how common I’d expect it to be, it’s something I’ve run
into myself a few times. Again, I hope members of this list can
give their view on if this would be useful to them.<br class="">
<br class="">
I don't have any real examples, but I certainly think that I have
run into it, so I'm quite open to solving the problem. For me, it
is probably only a matter of finding a syntax that is
acceptable.<br class="">
<br class="">
> 3. I’m not entirely sure what the grammar situation is yet but
afaik ‘?’ has never been available as a postfix operator. Perhaps
I’m missing your point, could you demonstrate where it is
allowed?<br class="">
<br class="">
I did not expect that you would be able to answer that, it was more
a question directed to people who are more connected to the inner
workings of the parsing of Swift than I am. It is not allowed, but
the error message is not the one I expect, something that gives me
a hint that it does have some meaning early in the
parsing.<br class="">
<br class="">
/Magnus<br class="">
<br class="">
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</blockquote>
</div>
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