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.send { color:#77bb77; }
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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>Hello Chris, I have some questions about this passage:</p>
<blockquote>
<p>Before this proposal, the Swift language has two types that participate in call syntax: functions and metatypes (for initialization). Neither of those may conform to protocols at the moment, so this introduces no possible ambiguity into the language.</p>
</blockquote>
<ul>
<li><p>Can you shortly describe why it would be ambiguous if metatypes would conform to protocols? </p></li>
<li><p>If metatypes were allowed to conform to protocols, how would this affect your proposal?</p></li>
</ul>
<p>Last year I pitched a the idea to make metatypes conform to <code>Hashable</code> which seemed to be a welcome change.</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_1510565863677353984" class="bloop_sign"></div> <br><p class="airmail_on">Am 10. November 2017 um 18:37:26, Chris Lattner 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; -webkit-line-break: after-white-space;" class=""><div></div><div>
<title></title>
<div class="">Hello all,</div>
<div class=""><br class=""></div>
<div class="">I have a couple of proposals cooking in a quest to
make Swift interoperate with dynamically typed languages like
Python better. Instead of baking in hard coded support for
one language or the other, I’m preferring to add a few small but
general purpose capabilities to Swift. This is the first,
which allows a Swift type to become “callable”.</div>
<div class=""><br class=""></div>
<div class="">The proposal is here:</div>
<div class=""><a href="https://gist.github.com/lattner/a6257f425f55fe39fd6ac7a2354d693d" class="">https://gist.github.com/lattner/a6257f425f55fe39fd6ac7a2354d693d</a></div>
<div class=""><br class=""></div>
<div class="">I’ve also attached a snapshot below, but it will
drift out of date as the proposal is refined. Feedback and
thoughts are appreciated, thanks!</div>
<div class=""><br class=""></div>
<div class="">-Chris</div>
<div class=""><br class=""></div>
<div class=""><br class=""></div>
<div class=""><br class=""></div>
<div class=""><br class=""></div>
<div class=""><br class=""></div>
<div class="">
<h1 style="box-sizing: border-box; margin-right: 0px; margin-bottom: 16px; margin-left: 0px; line-height: 1.25; padding-bottom: 0.3em; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(234, 236, 239); color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-variant-ligatures: normal; orphans: 2; widows: 2; margin-top: 0px !important;" class="">Introduce user-defined dynamically "callable" types</h1>
<ul style="box-sizing: border-box; padding-left: 2em; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">
<li style="box-sizing: border-box;" class="">
Proposal: <a href="https://gist.github.com/lattner/NNNN-DynamicCallable.md" style="box-sizing: border-box; color: rgb(3, 102, 214); text-decoration: none;" class="">SE-NNNN</a></li>
<li style="box-sizing: border-box; margin-top: 0.25em;" class="">
Author: <a href="https://github.com/lattner" style="box-sizing: border-box; color: rgb(3, 102, 214); text-decoration: none;" class="">Chris Lattner</a></li>
<li style="box-sizing: border-box; margin-top: 0.25em;" class="">
Review Manager: TBD</li>
<li style="box-sizing: border-box; margin-top: 0.25em;" class="">
Status: <span style="box-sizing: border-box; font-weight: 600;" class="">Awaiting
implementation</span></li>
</ul>
<h2 style="box-sizing: border-box; margin-top: 24px; margin-bottom: 16px; line-height: 1.25; padding-bottom: 0.3em; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(234, 236, 239); color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-variant-ligatures: normal; orphans: 2; widows: 2;" class=""><a href="https://gist.github.com/lattner/a6257f425f55fe39fd6ac7a2354d693d#introduction" aria-hidden="true" class="anchor" id="user-content-introduction" style="box-sizing: border-box; color: rgb(3, 102, 214); text-decoration: none; float: left; padding-right: 4px; margin-left: -20px; line-height: 1;">
<svg aria-hidden="true" class="octicon octicon-link" height="16" version="1.1" viewBox="0 0 16 16" width="16">
<path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z">
</path>
</svg></a>Introduction</h2>
<p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">This proposal introduces a new <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.05); border-radius: 3px;" class="">DynamicCallable</code> protocol to the standard
library. Types that conform to it are "callable" with the function
call syntax. It is simple syntactic sugar which allows the user to
write:</p>
<div class="highlight highlight-source-swift" style="box-sizing: border-box; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;">
<pre style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; margin-top: 0px; margin-bottom: 0px; word-wrap: normal; padding: 16px; overflow: auto; line-height: 1.45; background-color: rgb(246, 248, 250); border-radius: 3px; word-break: normal;" class=""> a <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">=</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">someValue</span>(<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">keyword1</span>: <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">42</span>, <span class="pl-s" style="box-sizing: border-box; color: rgb(3, 47, 98);"><span class="pl-pds" style="box-sizing: border-box;">"</span>foo<span class="pl-pds" style="box-sizing: border-box;">"</span></span>, <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">keyword2</span>: <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">19</span>)</pre></div>
<p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">and have it be interpreted by the compiler as:</p>
<div class="highlight highlight-source-swift" style="box-sizing: border-box; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;">
<pre style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; margin-top: 0px; margin-bottom: 0px; word-wrap: normal; padding: 16px; overflow: auto; line-height: 1.45; background-color: rgb(246, 248, 250); border-radius: 3px; word-break: normal;" class=""> a <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">=</span> someValue.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">dynamicCall</span>(<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">arguments</span>: [
(<span class="pl-s" style="box-sizing: border-box; color: rgb(3, 47, 98);"><span class="pl-pds" style="box-sizing: border-box;">"</span>keyword1<span class="pl-pds" style="box-sizing: border-box;">"</span></span>, <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">42</span>), (<span class="pl-s" style="box-sizing: border-box; color: rgb(3, 47, 98);"><span class="pl-pds" style="box-sizing: border-box;">"</span><span class="pl-pds" style="box-sizing: border-box;">"</span></span>, <span class="pl-s" style="box-sizing: border-box; color: rgb(3, 47, 98);"><span class="pl-pds" style="box-sizing: border-box;">"</span>foo<span class="pl-pds" style="box-sizing: border-box;">"</span></span>), (<span class="pl-s" style="box-sizing: border-box; color: rgb(3, 47, 98);"><span class="pl-pds" style="box-sizing: border-box;">"</span>keyword2<span class="pl-pds" style="box-sizing: border-box;">"</span></span>, <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">19</span>)
])</pre></div>
<p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">Other languages have analogous features (e.g. Python
"callables"), but the primary motivation of this proposal is to
allow elegant and natural interoperation with dynamic languages in
Swift.</p>
<p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">Swift-evolution thread: <a href="https://lists.swift.org/pipermail/swift-evolution/" style="box-sizing: border-box; color: rgb(3, 102, 214); text-decoration: none;" class="">Discussion thread topic for that proposal</a></p>
<h2 style="box-sizing: border-box; margin-top: 24px; margin-bottom: 16px; line-height: 1.25; padding-bottom: 0.3em; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(234, 236, 239); color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-variant-ligatures: normal; orphans: 2; widows: 2;" class=""><a href="https://gist.github.com/lattner/a6257f425f55fe39fd6ac7a2354d693d#motivation" aria-hidden="true" class="anchor" id="user-content-motivation" style="box-sizing: border-box; color: rgb(3, 102, 214); text-decoration: none; float: left; padding-right: 4px; margin-left: -20px; line-height: 1;">
<svg aria-hidden="true" class="octicon octicon-link" height="16" version="1.1" viewBox="0 0 16 16" width="16">
<path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z">
</path>
</svg></a>Motivation</h2>
<p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">Swift is well known for being exceptional at interworking
with existing C and Objective-C APIs, but its support for calling
APIs written in scripting langauges like Python, Perl, and Ruby is
quite lacking. These languages provide an extremely dynamic
programming model where almost everything is discovered at
runtime.</p>
<p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">Through the introduction of this proposal, and the
related <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.05); border-radius: 3px;" class="">DynamicMemberLookupProtocol</code> proposal, we seek
to fix this problem. We believe we can make many common APIs feel
very natural to use directly from Swift without all the complexity
of implementing something like the Clang importer. For example,
consider this Python code:</p>
<div class="highlight highlight-source-python" style="box-sizing: border-box; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;">
<pre style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; margin-top: 0px; margin-bottom: 0px; word-wrap: normal; padding: 16px; overflow: auto; line-height: 1.45; background-color: rgb(246, 248, 250); border-radius: 3px; word-break: normal;" class=""><span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">class</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">Dog</span>:
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">def</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">__init__</span>(<span class="pl-smi" style="box-sizing: border-box;">self</span>, <span class="pl-smi" style="box-sizing: border-box;">name</span>):
<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">self</span>.name <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">=</span> name
<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">self</span>.tricks <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">=</span> [] <span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">#</span> creates a new empty list for each dog</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">def</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">add_trick</span>(<span class="pl-smi" style="box-sizing: border-box;">self</span>, <span class="pl-smi" style="box-sizing: border-box;">trick</span>):
<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">self</span>.tricks.append(trick)</pre></div>
<p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">we would like to be able to use this from Swift like this
(the comments show the corresponding syntax you would use in
Python):</p>
<div class="highlight highlight-source-swift" style="box-sizing: border-box; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;">
<pre style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; margin-top: 0px; margin-bottom: 0px; word-wrap: normal; padding: 16px; overflow: auto; line-height: 1.45; background-color: rgb(246, 248, 250); border-radius: 3px; word-break: normal;" class=""> <span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">//</span> import DogModule</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">//</span> import DogModule.Dog as Dog // an alternate</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">let</span> Dog <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">=</span> Python.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">import</span>(“DogModule.<span class="pl-smi" style="box-sizing: border-box;">Dog</span><span class="pl-s" style="box-sizing: border-box; color: rgb(3, 47, 98);"><span class="pl-pds" style="box-sizing: border-box;">"</span>)</span>
<span class="pl-s" style="box-sizing: border-box; color: rgb(3, 47, 98);"> // dog = Dog(<span class="pl-pds" style="box-sizing: border-box;">"</span></span>Brianna<span class="pl-s" style="box-sizing: border-box; color: rgb(3, 47, 98);"><span class="pl-pds" style="box-sizing: border-box;">"</span>)</span>
<span class="pl-s" style="box-sizing: border-box; color: rgb(3, 47, 98);"> let dog = Dog(<span class="pl-pds" style="box-sizing: border-box;">"</span></span>Brianna<span class="pl-s" style="box-sizing: border-box; color: rgb(3, 47, 98);"><span class="pl-pds" style="box-sizing: border-box;">"</span>)</span>
<span class="pl-s" style="box-sizing: border-box; color: rgb(3, 47, 98);"> // dog.add_trick(<span class="pl-pds" style="box-sizing: border-box;">"</span></span>Roll over<span class="pl-s" style="box-sizing: border-box; color: rgb(3, 47, 98);"><span class="pl-pds" style="box-sizing: border-box;">"</span>)</span>
<span class="pl-s" style="box-sizing: border-box; color: rgb(3, 47, 98);"> dog.add_trick(<span class="pl-pds" style="box-sizing: border-box;">"</span></span>Roll over<span class="pl-s" style="box-sizing: border-box; color: rgb(3, 47, 98);"><span class="pl-pds" style="box-sizing: border-box;">"</span>)</span>
<span class="pl-s" style="box-sizing: border-box; color: rgb(3, 47, 98);"> // dog2 = Dog(<span class="pl-pds" style="box-sizing: border-box;">"</span></span>Kaylee<span class="pl-s" style="box-sizing: border-box; color: rgb(3, 47, 98);"><span class="pl-pds" style="box-sizing: border-box;">"</span>).add_trick(<span class="pl-pds" style="box-sizing: border-box;">"</span></span>snore<span class="pl-s" style="box-sizing: border-box; color: rgb(3, 47, 98);"><span class="pl-pds" style="box-sizing: border-box;">"</span>)</span>
<span class="pl-s" style="box-sizing: border-box; color: rgb(3, 47, 98);"> let dog2 = Dog(<span class="pl-pds" style="box-sizing: border-box;">"</span></span>Kaylee<span class="pl-s" style="box-sizing: border-box; color: rgb(3, 47, 98);"><span class="pl-pds" style="box-sizing: border-box;">"</span>).add_trick(<span class="pl-pds" style="box-sizing: border-box;">"</span></span>snore<span class="pl-s" style="box-sizing: border-box; color: rgb(3, 47, 98);"><span class="pl-pds" style="box-sizing: border-box;">"</span>)</span></pre></div>
<p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">Of course, this would also apply to standard Python APIs
as well. Here is an example working with the
Python <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.05); border-radius: 3px;" class="">pickle</code>API and the builtin Python
function <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.05); border-radius: 3px;" class="">open</code>:</p>
<div class="highlight highlight-source-swift" style="box-sizing: border-box; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;">
<pre style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; margin-top: 0px; margin-bottom: 0px; word-wrap: normal; padding: 16px; overflow: auto; line-height: 1.45; background-color: rgb(246, 248, 250); border-radius: 3px; word-break: normal;" class=""> <span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">//</span> import pickle</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">let</span> pickle <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">=</span> Python.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">import</span>(<span class="pl-s" style="box-sizing: border-box; color: rgb(3, 47, 98);"><span class="pl-pds" style="box-sizing: border-box;">"</span>pickle<span class="pl-pds" style="box-sizing: border-box;">"</span></span>)
<span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">//</span> file = open(filename)</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">let</span> file <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">=</span> Python.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">open</span>(filename)
<span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">//</span> blob = file.read()</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">let</span> blob <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">=</span> file.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">read</span>()
<span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">//</span> result = pickle.loads(blob)</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">let</span> result <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">=</span> pickle.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">loads</span>(blob)</pre></div>
<p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">This can all be expressed today as library functionality
written in Swift, but without this proposal, the code required is
unnecessarily verbose and gross. Without it (but with the related
dynamic member lookup proposal) the code would have a method name
(like <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.05); border-radius: 3px;" class="">call</code>) all over the code:</p>
<div class="highlight highlight-source-swift" style="box-sizing: border-box; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;">
<pre style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; margin-top: 0px; margin-bottom: 0px; word-wrap: normal; padding: 16px; overflow: auto; line-height: 1.45; background-color: rgb(246, 248, 250); border-radius: 3px; word-break: normal;" class=""> <span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">//</span> import pickle</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">let</span> pickle <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">=</span> Python.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">import</span>(<span class="pl-s" style="box-sizing: border-box; color: rgb(3, 47, 98);"><span class="pl-pds" style="box-sizing: border-box;">"</span>pickle<span class="pl-pds" style="box-sizing: border-box;">"</span></span>) <span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">//</span> normal method in Swift, no change.</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">//</span> file = open(filename)</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">let</span> file <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">=</span> Python.<span class="pl-smi" style="box-sizing: border-box;">open</span>.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">call</span>(filename)
<span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">//</span> blob = file.read()</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">let</span> blob <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">=</span> file.<span class="pl-smi" style="box-sizing: border-box;">read</span>.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">call</span>()
<span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">//</span> result = pickle.loads(blob)</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">let</span> result <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">=</span> pickle.<span class="pl-smi" style="box-sizing: border-box;">loads</span>.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">call</span>(blob)
<span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">//</span> dog2 = Dog("Kaylee").add_trick("snore")</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">let</span> dog2 <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">=</span> Dog.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">call</span>(<span class="pl-s" style="box-sizing: border-box; color: rgb(3, 47, 98);"><span class="pl-pds" style="box-sizing: border-box;">"</span>Kaylee<span class="pl-pds" style="box-sizing: border-box;">"</span></span>).<span class="pl-smi" style="box-sizing: border-box;">add_trick</span>.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">call</span>(<span class="pl-s" style="box-sizing: border-box; color: rgb(3, 47, 98);"><span class="pl-pds" style="box-sizing: border-box;">"</span>snore<span class="pl-pds" style="box-sizing: border-box;">"</span></span>)</pre></div>
<p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">While this is a syntactic sugar proposal, we believe that
this expands Swift to be usable in important new domains. This sort
of capability is also highly precedented in other languages, and is
a generally useful language feature that could be used for other
purposes as well.</p>
<h2 style="box-sizing: border-box; margin-top: 24px; margin-bottom: 16px; line-height: 1.25; padding-bottom: 0.3em; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(234, 236, 239); color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-variant-ligatures: normal; orphans: 2; widows: 2;" class=""><a href="https://gist.github.com/lattner/a6257f425f55fe39fd6ac7a2354d693d#proposed-solution" aria-hidden="true" class="anchor" id="user-content-proposed-solution" style="box-sizing: border-box; color: rgb(3, 102, 214); text-decoration: none; float: left; padding-right: 4px; margin-left: -20px; line-height: 1;">
<svg aria-hidden="true" class="octicon octicon-link" height="16" version="1.1" viewBox="0 0 16 16" width="16">
<path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z">
</path>
</svg></a>Proposed solution</h2>
<p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">We propose introducing this protocol to the standard
library:</p>
<div class="highlight highlight-source-swift" style="box-sizing: border-box; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;">
<pre style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; margin-top: 0px; margin-bottom: 0px; word-wrap: normal; padding: 16px; overflow: auto; line-height: 1.45; background-color: rgb(246, 248, 250); border-radius: 3px; word-break: normal;" class=""><span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">protocol</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">DynamicCallable</span> {
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">associatedtype</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">DynamicCallableArgument</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">associatedtype</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">DynamicCallableResult</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">func</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">dynamicCall</span>(<span class="pl-smi" style="box-sizing: border-box;"><span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">arguments</span></span>: [(<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">String</span>, DynamicCallableArgument)]) <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">throws</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">-></span> DynamicCallableResult
}</pre></div>
<p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">It also extends the language such that function call
syntax - when applied to a value of <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.05); border-radius: 3px;" class="">DynamicCallable</code> type - is accepted and
transformed into a call to the <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.05); border-radius: 3px;" class="">dynamicCall</code> member. The <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.05); border-radius: 3px;" class="">dynamicCall</code> method takes a list of tuples: the
first element is the keyword label (or an empty string if absent)
and the second value is the formal parameter specified at the call
site.</p>
<p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">Before this proposal, the Swift language has two types
that participate in call syntax: functions and metatypes (for
initialization). Neither of those may conform to protocols at the
moment, so this introduces no possible ambiguity into the
language.</p>
<p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">It is worth noting that this does not introduce the
ability to provide dynamicly callable <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.05); border-radius: 3px;" class="">static</code>/<code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.05); border-radius: 3px;" class="">class</code> members. We don't believe that this is
important given the goal of supporting dynamic languages like
Python, but if there is a usecase discovered in the future, it
could be explored as future work. Such future work should keep in
mind that call syntax on metatypes is already meaningful, and that
ambiguity would have to be resolved somehow.</p>
<h2 style="box-sizing: border-box; margin-top: 24px; margin-bottom: 16px; line-height: 1.25; padding-bottom: 0.3em; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(234, 236, 239); color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-variant-ligatures: normal; orphans: 2; widows: 2;" class=""><a href="https://gist.github.com/lattner/a6257f425f55fe39fd6ac7a2354d693d#discussion" aria-hidden="true" class="anchor" id="user-content-discussion" style="box-sizing: border-box; color: rgb(3, 102, 214); text-decoration: none; float: left; padding-right: 4px; margin-left: -20px; line-height: 1;">
<svg aria-hidden="true" class="octicon octicon-link" height="16" version="1.1" viewBox="0 0 16 16" width="16">
<path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z">
</path>
</svg></a>Discussion</h2>
<p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">While the signature for <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.05); border-radius: 3px;" class="">dynamicCall</code> is highly general we expect the
most common use will be clients who are programming against
concrete types that implement this proposal. One very nice aspect
of this is that, as a result of Swift's existing subtyping
mechanics, implementations of this type can choose whether they can
actually throw an error or not. For example, consider this silly
implementation:</p>
<div class="highlight highlight-source-swift" style="box-sizing: border-box; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;">
<pre style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; margin-top: 0px; margin-bottom: 0px; word-wrap: normal; padding: 16px; overflow: auto; line-height: 1.45; background-color: rgb(246, 248, 250); border-radius: 3px; word-break: normal;" class=""><span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">struct</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">ParameterSummer</span> : <span class="pl-e" style="box-sizing: border-box; color: rgb(111, 66, 193);">DynamicCallable </span>{
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">func</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">dynamicCall</span>(<span class="pl-smi" style="box-sizing: border-box;"><span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">arguments</span></span>: [(<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">String</span>, <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">Int</span>)]) <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">-></span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">Int</span> {
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">return</span> arguments.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">reduce</span>(<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">0</span>) { <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">$0</span><span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">+</span><span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">$1</span>.1 }
}
}
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">let</span> x <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">=</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">ParameterSummer</span>()
<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">print</span>(<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">x</span>(<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">1</span>, <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">7</span>, <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">12</span>)) <span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">//</span> prints 20</span></pre></div>
<p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">Because <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.05); border-radius: 3px;" class="">ParameterSummer</code>'s implementation
of <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.05); border-radius: 3px;" class="">dynamicCall</code> does not throw, the call site is
known not to throw either, so the print doesn't need to be marked
with <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.05); border-radius: 3px;" class="">try</code>.</p>
<h2 style="box-sizing: border-box; margin-top: 24px; margin-bottom: 16px; line-height: 1.25; padding-bottom: 0.3em; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(234, 236, 239); color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-variant-ligatures: normal; orphans: 2; widows: 2;" class=""><a href="https://gist.github.com/lattner/a6257f425f55fe39fd6ac7a2354d693d#example-usage" aria-hidden="true" class="anchor" id="user-content-example-usage" style="box-sizing: border-box; color: rgb(3, 102, 214); text-decoration: none; float: left; padding-right: 4px; margin-left: -20px; line-height: 1;">
<svg aria-hidden="true" class="octicon octicon-link" height="16" version="1.1" viewBox="0 0 16 16" width="16">
<path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z">
</path>
</svg></a>Example Usage</h2>
<p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">A more realistic (and motivating) example comes from a
prototype Python interop layer. While the concrete details of this
use case are subject to change and not important for this proposal,
it is perhaps useful to have a concrete example to see how this
comes together.</p>
<p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">That prototype currently has two types which model Python
values, one of which handles Python exceptions and one of which
does not. Their conformances would look like this, enabling the use
cases described in the Motivation section above:</p>
<div class="highlight highlight-source-swift" style="box-sizing: border-box; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;">
<pre style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; margin-top: 0px; margin-bottom: 0px; word-wrap: normal; padding: 16px; overflow: auto; line-height: 1.45; background-color: rgb(246, 248, 250); border-radius: 3px; word-break: normal;" class=""><span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">extension</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">ThrowingPyRef</span>: <span class="pl-e" style="box-sizing: border-box; color: rgb(111, 66, 193);">DynamicCallable </span>{
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">func</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">dynamicCall</span>(<span class="pl-smi" style="box-sizing: border-box;"><span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">arguments</span></span>: [(<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">String</span>, PythonConvertible)]) <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">throws</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">-></span> PythonConvertible {
<span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">//</span> Make sure state errors are not around.</span>
<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">assert</span>(<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">PyErr_Occurred</span>() <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">==</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">nil</span>, <span class="pl-s" style="box-sizing: border-box; color: rgb(3, 47, 98);"><span class="pl-pds" style="box-sizing: border-box;">"</span>Python threw an error but wasn't handled<span class="pl-pds" style="box-sizing: border-box;">"</span></span>)
<span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">//</span> Count how many keyword arguments are in the list.</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">let</span> numKeywords <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">=</span> arguments.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">reduce</span>(<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">0</span>) {
<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">$0</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">+</span> (<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">$1</span>.0.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">isEmpty</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">?</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">0</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">:</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">1</span>)
}
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">let</span> kwdict <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">=</span> numKeywords <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">!=</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">0</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">?</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">PyDict_New</span>() <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">:</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">nil</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">//</span> Non-keyword arguments are passed as a tuple of values.</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">let</span> argTuple <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">=</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">PyTuple_New</span>(arguments.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">count</span><span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">-</span>numKeywords)<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">!</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">var</span> nonKeywordIndex <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">=</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">0</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">for</span> (keyword, argValue) <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">in</span> arguments {
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">if</span> keyword.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">isEmpty</span> {
<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">PyTuple_SetItem</span>(argTuple, nonKeywordIndex, argValue.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">toPython</span>())
nonKeywordIndex <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">+=</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">1</span>
} <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">else</span> {
<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">PyDict_SetItem</span>(kwdict<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">!</span>, keyword.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">toPython</span>(), argValue.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">toPython</span>())
}
}
<span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">//</span> Python calls always return a non-null value when successful. If the</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">//</span> Python function produces the equivalent of C "void", it returns the None</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">//</span> value. A null result of PyObjectCall happens when there is an error,</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">//</span> like 'self' not being a Python callable.</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">guard</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">let</span> resultPtr <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">=</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">PyObject_Call</span>(state, argTuple, kwdict) <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">else</span> {
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">throw</span> PythonError.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">invalidCall</span>(<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">self</span>)
}
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">let</span> result <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">=</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">PyRef</span>(<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">owned</span>: resultPtr)
<span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">//</span> Translate a Python exception into a Swift error if one was thrown.</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">if</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">let</span> exception <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">=</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">PyErr_Occurred</span>() {
<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">PyErr_Clear</span>()
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">throw</span> PythonError.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">exception</span>(<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">PyRef</span>(<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">borrowed</span>: exception))
}
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">return</span> result
}
}
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">extension</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">PyRef</span>: <span class="pl-e" style="box-sizing: border-box; color: rgb(111, 66, 193);">DynamicCallable </span>{
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">func</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">dynamicCall</span>(<span class="pl-smi" style="box-sizing: border-box;"><span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">arguments</span></span>: [(<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">String</span>, PythonConvertible)])
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">-></span> PythonConvertible {
<span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">//</span> Same as above, but internally aborts instead of throwing Swift</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">//</span> errors.</span>
}
}</pre></div>
<h2 style="box-sizing: border-box; margin-top: 24px; margin-bottom: 16px; line-height: 1.25; padding-bottom: 0.3em; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(234, 236, 239); color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-variant-ligatures: normal; orphans: 2; widows: 2;" class=""><a href="https://gist.github.com/lattner/a6257f425f55fe39fd6ac7a2354d693d#source-compatibility" aria-hidden="true" class="anchor" id="user-content-source-compatibility" style="box-sizing: border-box; color: rgb(3, 102, 214); text-decoration: none; float: left; padding-right: 4px; margin-left: -20px; line-height: 1;">
<svg aria-hidden="true" class="octicon octicon-link" height="16" version="1.1" viewBox="0 0 16 16" width="16">
<path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z">
</path>
</svg></a>Source compatibility</h2>
<p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">This is a strictly additive proposal with no source
breaking changes.</p>
<h2 style="box-sizing: border-box; margin-top: 24px; margin-bottom: 16px; line-height: 1.25; padding-bottom: 0.3em; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(234, 236, 239); color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-variant-ligatures: normal; orphans: 2; widows: 2;" class=""><a href="https://gist.github.com/lattner/a6257f425f55fe39fd6ac7a2354d693d#effect-on-abi-stability" aria-hidden="true" class="anchor" id="user-content-effect-on-abi-stability" style="box-sizing: border-box; color: rgb(3, 102, 214); text-decoration: none; float: left; padding-right: 4px; margin-left: -20px; line-height: 1;">
<svg aria-hidden="true" class="octicon octicon-link" height="16" version="1.1" viewBox="0 0 16 16" width="16">
<path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z">
</path>
</svg></a>Effect on ABI stability</h2>
<p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">This is a strictly additive proposal with no ABI breaking
changes.</p>
<h2 style="box-sizing: border-box; margin-top: 24px; margin-bottom: 16px; line-height: 1.25; padding-bottom: 0.3em; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(234, 236, 239); color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-variant-ligatures: normal; orphans: 2; widows: 2;" class=""><a href="https://gist.github.com/lattner/a6257f425f55fe39fd6ac7a2354d693d#effect-on-api-resilience" aria-hidden="true" class="anchor" id="user-content-effect-on-api-resilience" style="box-sizing: border-box; color: rgb(3, 102, 214); text-decoration: none; float: left; padding-right: 4px; margin-left: -20px; line-height: 1;">
<svg aria-hidden="true" class="octicon octicon-link" height="16" version="1.1" viewBox="0 0 16 16" width="16">
<path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z">
</path>
</svg></a>Effect on API resilience</h2>
<p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">This has no impact on API resilience which is not already
captured by other language features.</p>
<h2 style="box-sizing: border-box; margin-top: 24px; margin-bottom: 16px; line-height: 1.25; padding-bottom: 0.3em; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(234, 236, 239); color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-variant-ligatures: normal; orphans: 2; widows: 2;" class=""><a href="https://gist.github.com/lattner/a6257f425f55fe39fd6ac7a2354d693d#alternatives-considered" aria-hidden="true" class="anchor" id="user-content-alternatives-considered" style="box-sizing: border-box; color: rgb(3, 102, 214); text-decoration: none; float: left; padding-right: 4px; margin-left: -20px; line-height: 1;">
<svg aria-hidden="true" class="octicon octicon-link" height="16" version="1.1" viewBox="0 0 16 16" width="16">
<path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z">
</path>
</svg></a>Alternatives considered</h2>
<p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">A few alternatives were considered:</p>
<h3 style="box-sizing: border-box; margin-top: 24px; margin-bottom: 16px; font-size: 1.25em; line-height: 1.25; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-variant-ligatures: normal; orphans: 2; widows: 2;" class=""><a href="https://gist.github.com/lattner/a6257f425f55fe39fd6ac7a2354d693d#add-ability-to-reject-parameter-labels" aria-hidden="true" class="anchor" id="user-content-add-ability-to-reject-parameter-labels" style="box-sizing: border-box; color: rgb(3, 102, 214); text-decoration: none; float: left; padding-right: 4px; margin-left: -20px; line-height: 1;">
<svg aria-hidden="true" class="octicon octicon-link" height="16" version="1.1" viewBox="0 0 16 16" width="16">
<path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z">
</path>
</svg></a>Add ability to reject parameter labels</h3>
<p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">The implementation above does not allow an implementation
to staticly reject argument labels. If this was important to add,
we could add another protocol to model this, along the lines
of:</p>
<div class="highlight highlight-source-swift" style="box-sizing: border-box; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;">
<pre style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; margin-top: 0px; margin-bottom: 0px; word-wrap: normal; padding: 16px; overflow: auto; line-height: 1.45; background-color: rgb(246, 248, 250); border-radius: 3px; word-break: normal;" class=""><span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">///</span> A type conforming just to this protocol would not accept parameter</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">///</span> labels in its calls.</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">protocol</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">DynamicCallable</span> {
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">associatedtype</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">DynamicCallableArgument</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">associatedtype</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">DynamicCallableResult</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">func</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">dynamicCall</span>(<span class="pl-smi" style="box-sizing: border-box;"><span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">arguments</span></span>: [DynamicCallableArgument]) <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">throws</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">-></span> DynamicCallableResult
}
<span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">///</span> A type conforming to this protocol does allow optional parameter</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">///</span> labels.</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">protocol</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">DynamicCallableWithKeywordsToo</span> : <span class="pl-e" style="box-sizing: border-box; color: rgb(111, 66, 193);">DynamicCallable </span>{
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">func</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">dynamicCall</span>(<span class="pl-smi" style="box-sizing: border-box;"><span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">arguments</span></span>: [(<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">String</span>, DynamicCallableArgument)]) <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">throws</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">-></span> DynamicCallableResult
}</pre></div>
<p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">This would allow a type to implement one or the other
based on their capabilities. This proposal is going with a very
simple design, but if there is demand for this, the author is happy
to switch.</p>
<h3 style="box-sizing: border-box; margin-top: 24px; margin-bottom: 16px; font-size: 1.25em; line-height: 1.25; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-variant-ligatures: normal; orphans: 2; widows: 2;" class=""><a href="https://gist.github.com/lattner/a6257f425f55fe39fd6ac7a2354d693d#staticly-checking-for-exact-signatures" aria-hidden="true" class="anchor" id="user-content-staticly-checking-for-exact-signatures" style="box-sizing: border-box; color: rgb(3, 102, 214); text-decoration: none; float: left; padding-right: 4px; margin-left: -20px; line-height: 1;">
<svg aria-hidden="true" class="octicon octicon-link" height="16" version="1.1" viewBox="0 0 16 16" width="16">
<path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z">
</path>
</svg></a>Staticly checking for exact signatures</h3>
<p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">This protocol does not allow a type to specify an exact
signature for the callable - a specific number of parameters with
specific types. If we went down that route, the best approach would
be to introduce a new declaration kind (which would end up being
very similar to get-only subscripts) since, in general, a type
could want multiple concrete callable signatures, and those
signatures should participate in overload resolution.</p>
<p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">While such a feature could be interesting for some use
cases, it is almost entirely orthogonal from this proposal: it
addresses different use cases and does not solve the needs of this
proposal. It does not address our needs because even a variadic
callable declaration would not provide access to the keyword
argument labels we need.</p>
<h3 style="box-sizing: border-box; margin-top: 24px; margin-bottom: 16px; font-size: 1.25em; line-height: 1.25; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-variant-ligatures: normal; orphans: 2; widows: 2;" class=""><a href="https://gist.github.com/lattner/a6257f425f55fe39fd6ac7a2354d693d#direct-language-support-for-python" aria-hidden="true" class="anchor" id="user-content-direct-language-support-for-python" style="box-sizing: border-box; color: rgb(3, 102, 214); text-decoration: none; float: left; padding-right: 4px; margin-left: -20px; line-height: 1;">
<svg aria-hidden="true" class="octicon octicon-link" height="16" version="1.1" viewBox="0 0 16 16" width="16">
<path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z">
</path>
</svg></a>Direct language support for Python</h3>
<p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">We considered implementing something analogous to the
Clang importer for Python, which would add a first class Python
specific type(s) to Swift language or standard library. We rejected
this option because it would be significantly more invasive in the
compiler, would set the precedent for all other dynamic languages
to get first class language support, and because that first class
support doesn't substantially improve the experience of working
with Python over existing Swift with a couple small "generally
useful" extensions like this one.</p>
<h3 style="box-sizing: border-box; margin-top: 24px; margin-bottom: 16px; font-size: 1.25em; line-height: 1.25; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-variant-ligatures: normal; orphans: 2; widows: 2;" class=""><a href="https://gist.github.com/lattner/a6257f425f55fe39fd6ac7a2354d693d#naming" aria-hidden="true" class="anchor" id="user-content-naming" style="box-sizing: border-box; color: rgb(3, 102, 214); text-decoration: none; float: left; padding-right: 4px; margin-left: -20px; line-height: 1;">
<svg aria-hidden="true" class="octicon octicon-link" height="16" version="1.1" viewBox="0 0 16 16" width="16">
<path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z">
</path>
</svg></a>Naming</h3>
<p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">The most fertile ground for bikeshedding is the naming of
the protocol and the members. We welcome other ideas and
suggestions for naming, but here are some thoughts on obvious
options to consider:</p>
<p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">We considered but rejected the name <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.05); border-radius: 3px;" class="">CustomCallable</code>, because the
existing <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.05); border-radius: 3px;" class="">Custom*</code> protocols in the standard library
(<code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.05); border-radius: 3px;" class="">CustomStringConvertible</code>, <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.05); border-radius: 3px;" class="">CustomReflectable</code>, etc) provide a way to override
and custom existing builtin abilities of Swift. In contrast, this
feature grants a new capability to a type.</p>
<p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2;" class="">We considered but rejected a name like <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.05); border-radius: 3px;" class="">ExpressibleByCalling</code> to fit with
the <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.05); border-radius: 3px;" class="">ExpressibleBy*</code> family of protocols
(like <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.05); border-radius: 3px;" class="">ExpressibleByFloatLiteral</code>, <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.05); border-radius: 3px;" class="">ExpressibleByStringLiteral</code>, etc). This name family
is specifically used by literal syntax, and calls are not literals.
Additionally the type itself is not "expressible by calling" -
instead, instances of the type may be called.</p>
<div style="box-sizing: border-box; margin-top: 0px; color: rgb(36, 41, 46); font-family: -apple-system, system-ui, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; font-variant-ligatures: normal; orphans: 2; widows: 2; margin-bottom: 0px !important;" class="">On member and associated type naming, we intentionally
gave these long and verbose names so they stay out of the way of
user code completion. The members of this protocol are really just
compiler interoperability glue. If there was a Swift attribute to
disable the members from showing up in code completion, we would
use it (such an attribute would also be useful for
the <code style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 13.6px; padding: 0.2em 0px; margin: 0px; background-color: rgba(27, 31, 35, 0.05); border-radius: 3px;" class="">LiteralConvertible</code> and other compiler magic
protocols).</div>
</div>
<div class=""><br class=""></div>
<div class=""><br class=""></div>
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