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<META name="GENERATOR" content="MSHTML 11.00.9600.18036"></HEAD> 
<BODY>
<TABLE>
  <THEAD>
  <TR>
    <TH>Document Number:</TH>
    <TH>P0071R00</TH></TR></THEAD>
  <TBODY>
  <TR>
    <TD>Date:</TD>
    <TD>2015-09-12</TD></TR>
  <TR>
    <TD>Project:</TD>
    <TD>Programming Language C++, Evolution</TD></TR>
  <TR>
    <TD>Revises:</TD>
    <TD>none</TD></TR>
  <TR>
    <TD>Reply to:</TD>
    <TD>gorn@microsoft.com</TD></TR></TBODY></TABLE>
<H1 id="p0071r00-coroutines-keyword-alternatives">P0071R00: Coroutines: Keyword 
alternatives</H1>
<H2 id="introduction">Introduction</H2>
<P>One of the question raised in Lenexa was: "Can we take a <CODE>yield</CODE> 
identifier and make it a C++ keyword?". The answer was resounding no. There was 
some ad-hoc brainstorming during the session on possible alternatives. This 
paper examines the alternatives and makes recommendations.</P>
<H2 id="ideal">Ideal</H2>
<P>We believe that in the world where we could have any keywords we would like, 
our choice would be <CODE>await</CODE> and <CODE>yield</CODE>, as they most 
clearly capture the notion of <CODE>await</CODE>-ing on something or 
<CODE>yield</CODE>-ing a value to the consumer.</P>
<PRE><CODE>  future&lt;int&gt; deep_thought() {        generator&lt;char&gt; hello() {
    await 5ms;                          for(auto ch: "Hello, world")
    return 42;                            yield ch;
  }                                   }
</CODE></PRE>
<P>But, we don't live in an ideal world. Developers use nice concise words as 
names of functions, variables, classes, namespaces, etc. <!--Even the `await` keyword that was not considered controversial during Lenexa discussion, is used a member function on a latch and future classes in libprocess, latch classes in mondodb and fedoraproject.--> 
Can we keep nice keywords and don't break existing code?</P>
<H2 id="soft-keywords">Soft Keywords</H2>
<P>Soft Keywords proposal (<A href="http://wg21.link/p0056r0">p0056r0</A>) 
introduces a new named entity into the language: soft keyword. Soft keywords are 
predefined in <CODE>std</CODE> namespace and participate in name lookup and name 
hiding according to existing language rules with some exceptions listed in the 
paper.</P>
<P>Making <CODE>await</CODE> and <CODE>yield</CODE> soft keywords, not only 
allows to use <CODE>yield</CODE> and <CODE>await</CODE> in coroutines and don't 
break existing code, it will also allow use of coroutines in software where 
<CODE>await</CODE> and <CODE>yield</CODE> are names of existing functions or 
classes, since soft keywords allow qualification, as in <CODE>std::yield</CODE>. 
</P>
<H2 id="magic-functions">Magic functions</H2>
<P>What if <CODE>yield</CODE> and <CODE>await</CODE> are magic functions defined 
in the <CODE>std</CODE> namespace?<BR>It will make the syntax a little bit 
noisier, since we now always have to put parenthesis around the expression being 
yielded or awaited upon. </P>
<PRE><CODE>    auto conn = await Tcp::Connect("127.0.0.1", 1337);
    auto bytesRead = await conn.Read(buf, sizeof(buf));

will become

    auto conn = await(Tcp::Connect("127.0.0.1", 1337));
    auto bytesRead = await(conn.Read(buf, sizeof(buf)));
</CODE></PRE>
<P>If we go one step further and increase the magic by giving magic functions 
precedence and unary operator call syntax, we just reinvented the soft keywords 
described in the previous section.</P>
<H2 id="find-a-new-name-for-yield">Find a new name for yield</H2>
<P>Lowercase <CODE>await</CODE> is relative rarely used identifier (as compared 
to yield) and it has a suitable replacement, namely <CODE>wait</CODE>, which 
carries roughly the same meaning and spelling. Thus, while taking 
<CODE>await</CODE> as a keyword will break some software, having a closely 
related replacement <CODE>wait</CODE> will lessen the pain.</P>
<P><CODE>yield</CODE> is used more frequently and there is no obvious term to 
replace it with. </P>
<H3 id="export">export</H3>
<P>For  coroutines <CODE>yield</CODE> means both <EM>yield a value</EM> as in 
output the value, produce the value as well <EM>yield execution back to the 
caller</EM>. Out of existing set of C++ keywords only one carries a meaning that 
is at least somewhat close to output, that is <CODE>export</CODE>. However this 
usage of <CODE>export</CODE> will be confusing as the export is used in the 
modules (<A href="http://wg21.link/n4465">N4465</A>) with different meaning.</P>
<PRE><CODE>module greeting;

export generator&lt;char&gt; hello() { // exporting function hello
  for(auto ch: "Hello, world")
     export ch;                  // yielding a character
}
</CODE></PRE>
<P>On the plus side, it is short and it is already a keyword.</P>
<H3 id="yieldexpr-or-yield_expr">yieldexpr or yield_expr</H3>
<P><CODE>yieldexpr</CODE> and <CODE>yield_expr</CODE> are sufficiently odd 
identifiers and score no hits on github or codesearch.</P>
<PRE><CODE>generator&lt;char&gt; hello() {
  for(auto ch: "Hello, world") {
     yieldexpr ch;  // similar to constexpr
     yield_expr ch; // similar to const_cast
  }
}
</CODE></PRE><!--If we select `yield_expr` or `yieldexpr`, we need to decide if we need to mangle `await` into `await_expr` or `awaitexpr` for symmetry.
--> 
<H3 id="coyield-or-co_yield">coyield or co_yield</H3>
<P>Instead of using a suffix to oddify <CODE>await</CODE> and <CODE>yield</CODE> 
we can look at having an oddification prefix, such as <CODE>co_</CODE> or 
<CODE>co</CODE> as was suggested during Lenexa coroutine discussion.</P>
<P>Without the underscore, <CODE>co</CODE> prefix leads to wrong visual parsing 
as in <CODE>coy-ield</CODE> and thus inferior to <CODE>co_</CODE>. We also need 
to make a choice if we want to mangle <CODE>await</CODE> for symmetry.</P>
<PRE><CODE>  future&lt;int&gt; deep_thought() {    generator&lt;char&gt; hello() {        auto xform(Stream&amp; s) {
    await 5ms;                      for(auto ch: "Hello, world")     for await(auto x: s)
    return 42;                        co_yield ch;                     co_yield make_pair(x,time());
  }                               }                                }
  auto operator await(future&lt;void&gt;&amp;) { ... }

or

  future&lt;int&gt; deep_thought() {    generator&lt;char&gt; hello() {        auto xform(Stream&amp; s) {
    co_await 5ms;                   for(auto ch: "Hello, world")     for co_await(auto x: s)
    return 42;                        co_yield ch;                     co_yield make_pair(x,time());
  }                               }                                }
  auto operator co_await(future&lt;void&gt;&amp;) { ... }
</CODE></PRE>
<P>Given that <CODE>await</CODE> is used in more contexts, such as name of the 
operator and an option for a range-based-for statement, keeping a short version 
<CODE>await</CODE> as opposed to oddification <CODE>co_await</CODE> is 
preferrable.</P>
<H3 id="yield_placeholder">yield_placeholder</H3>
<P>To save committee time on bikeshed, use some obviously bad name, such as 
<CODE>yield_placeholder</CODE> and defer decision aboug <EM>yield-keyword</EM> 
until a more suitable alternative is discovered / invented.   </P>
<H2 id="conclusion">Conclusion</H2>
<P>Soft keyword or its less general version magic functions leads to the best 
coroutine syntax. None of the other alternatives considered come close to the 
beauty of the clean <CODE>await</CODE> and <CODE>yield</CODE> keywords. If at 
all possible, our overwhelming preference is for selecting one of the first two 
alternatives or defer the bikeshed using <CODE>yield_placeholder</CODE> until 
some better alternatives are invented or we hit a deadline that a keyword must 
be picked or else the feature will not make the standard.</P>
<H2 id="references">References</H2>
<P><A href="http://wg21.link/p0056r0">p0056r0</A>: Soft Keywords (<A href="http://wg21.link/p0056r0">http://wg21.link/p0056r0</A>)</P>
<P><A href="http://wg21.link/n4465">N4465</A>:  A Module System for C++ 
(Revision 3) (<A href="http://wg21.link/n4465">http://wg21.link/n4465</A>)  
</P></BODY></HTML><!-- This document was created with MarkdownPad, the Markdown editor for Windows (http://markdownpad.com) -->
