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}

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  padding: 3px 5px;
  font: 11px "SFMono-Regular", Consolas, "Liberation Mono", Menlo, Courier, monospace;
  line-height: 10px;
  color: #444d56;
  vertical-align: middle;
  background-color: #fafbfc;
  border: solid 1px #d1d5da;
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}

.markdown-body :checked+.radio-label {
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}

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}

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}

/*Markdown Viewer*/
.markdown-body summary:hover { cursor: pointer; }
.markdown-body ul li p { margin: 0; }

/* content.css */

i
/*global*/
html, body {
  padding: 0 !important; margin: 0 !important;
  width: auto !important; max-width: 100% !important;
}

pre#_markdown {
  word-wrap: break-word;
  white-space: pre-wrap;
}

/*github theme*/
.markdown-body {
  overflow: auto;

  min-width: 888px;
  max-width: 888px;

  background-color: #fff;
  border: 1px solid #ddd;

  padding: 45px;
  margin: 20px auto;
}
.markdown-body #_html>*:first-child {
  margin-top: 0 !important;
}
.markdown-body #_html>*:last-child {
  margin-bottom: 0 !important;
}
.markdown-body img {
  background-color: transparent;
}

/*all other themes*/
.markdown-theme {
  box-sizing: border-box;
  max-width: 100% !important;
  padding: 20px !important;
  margin: 0 auto !important;
}
@media (max-width: 767px) {
  .markdown-theme { width: auto !important; }
}
@media (min-width: 768px) and (max-width: 992px) {
  .markdown-theme { width: 713px !important; }
}
@media (min-width: 992px) and (max-width: 1200px) {
  .markdown-theme { width: 937px !important; }
}
@media (min-width: 1200px) {
  .markdown-theme { width: 1145px !important; }
}

/*toc*/
body {
  display: flex;
}
body._toc-left { padding-left: 300px !important; }
body._toc-right { padding-right: 300px !important; }
#_toc {
  position: fixed;
  top: 0; bottom: 0; left: 0;
  width: 300px;
  height: 100%;
  background: #fafafa;
  overflow-y: auto;
  overflow-x: hidden;
}
#_toc #_ul {
  padding: 0 0 0 20px !important;
  margin: 0 !important;
}
#_toc #_ul a {
  font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif !important;
  font-size: 14px !important;
  line-height: 17px !important;
  color: #364149 !important;
  font-weight: normal !important;
  font-style: normal !important;
  text-decoration: none !important;
  text-transform: none !important;
  letter-spacing: 0.2px !important;
  background: none !important;
  border: 0 !important;
  padding: 10px 15px !important;
  display: block !important;
}
#_toc #_ul a:hover {
  text-decoration: underline !important;
}

/*anchor link*/
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<table>
<tbody>
<tr>
<td>Doc. No.:</td>
<td>P1221R0</td>
</tr>
<tr>
<td>Date:</td>
<td>2018-10-03</td>
</tr>
<tr>
<td>Reply To:</td>
<td>Jason Rice 
<a href="mailto:ricejasonf@gmail.com">ricejasonf@gmail.com</a></td>
</tr>
<tr>
<td>Title:</td>
<td>Parametric Expressions</td>
</tr>
<tr>
<td>Audience:</td>
<td>Evolution Working Group</td>
</tr>
</tbody>
</table>
<h1 id="parametric-expressions"><a class="anchor" name="parametric-expressions" href="#parametric-expressions"><span class="octicon octicon-link"></span></a>Parametric Expressions</h1>
<h2 id="abstract"><a class="anchor" name="abstract" href="#abstract"><span class="octicon octicon-link"></span></a>Abstract</h2>
<p>The purpose of this paper is to propose a function-like construct that creates an expression inline that
can enable more idiomatic code with faster compile times.</p>
<p>Consider the following declaration syntax:</p>
<pre class=" language-cpp"><code class=" language-cpp"><span class="token keyword">using</span> <span class="token function">add</span><span class="token punctuation">(</span><span class="token keyword">auto</span> a<span class="token punctuation">,</span> <span class="token keyword">auto</span> b<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> a <span class="token operator">+</span> b<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre>
<p>Here is a function-like declaration that has the same behaviour as a function without creating
a function type, but the body still has its own scope. </p>
<pre class=" language-cpp"><code class=" language-cpp"><span class="token keyword">int</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">40</span><span class="token punctuation">;</span>
  <span class="token keyword">int</span> x <span class="token operator">=</span> <span class="token function">add</span><span class="token punctuation">(</span>i<span class="token operator">++</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">// the same as</span>

<span class="token keyword">int</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">40</span><span class="token punctuation">;</span>
  <span class="token keyword">int</span> x <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token punctuation">{</span>
    <span class="token keyword">auto</span><span class="token operator">&amp;&amp;</span> a <span class="token operator">=</span> i<span class="token operator">++</span><span class="token punctuation">;</span>
    <span class="token keyword">auto</span><span class="token operator">&amp;&amp;</span> b <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">;</span>
    a <span class="token operator">+</span> b<span class="token punctuation">;</span> <span class="token comment">// result of expression</span>
  <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre>
<blockquote>
<p>Note that each input expression is bound to a variable resulting in evaluation that is consistent with
normal function call expressions. User controlled evaluation is addressed in <em>Using Parameters</em>.</p>
</blockquote>
<p>Function templates as we have them now are loaded with features such as overloading, constraints,
type deduction, SFINAE, ect.. When called not only do we get an expensive template instantiation but also
potentially large symbols and extraneous code generation all for functions we wouldn't call otherwise
unless we expected the optimizer to inline them out anyways.</p>
<p>With this tool we want to give programmers the ability to allow this inlining at an earlier stage, but
since the inlining is guaranteed and in the context of where it is called we can do things not currently
possible with normal functions without creating a special type for each invocation.</p>
<p>This ability to transform expressions is similar to the way we use type aliases, but here we can work
with constructs that have run-time effects as seen in Fusion/Hana style metaprogramming. Additionally,
since this is a language feature, we could potentially see compile-time performance that is
comparable to pure-type template metaprogramming.</p>
<h2 id="constexpr-parameters"><a class="anchor" name="constexpr-parameters" href="#constexpr-parameters"><span class="octicon octicon-link"></span></a>Constexpr Parameters</h2>
<p>Since the invocation is inlined we easily get constexpr parameters without generating a complicated
symbol for a function prototype.</p>
<pre class=" language-cpp"><code class=" language-cpp"><span class="token keyword">using</span> <span class="token function">to_int_c</span><span class="token punctuation">(</span><span class="token keyword">constexpr</span> <span class="token keyword">auto</span> x<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> std<span class="token operator">::</span>integral_constant<span class="token operator">&lt;</span><span class="token keyword">int</span><span class="token punctuation">,</span> x<span class="token operator">&gt;</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">int</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">constexpr</span> <span class="token keyword">int</span> x <span class="token operator">=</span> <span class="token number">42</span><span class="token punctuation">;</span>
  std<span class="token operator">::</span>integral_constant<span class="token operator">&lt;</span><span class="token keyword">int</span><span class="token punctuation">,</span> <span class="token number">42</span><span class="token operator">&gt;</span> foo <span class="token operator">=</span> <span class="token function">to_int_c</span><span class="token punctuation">(</span><span class="token number">42</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre>
<h2 id="using-parameters"><a class="anchor" name="using-parameters" href="#using-parameters"><span class="octicon octicon-link"></span></a>Using Parameters</h2>
<p>With <code>using</code> as a specifier, we allow the user to take control of evaluation with macro-like semantics.</p>
<pre class=" language-cpp"><code class=" language-cpp"><span class="token keyword">using</span> <span class="token function">if_</span><span class="token punctuation">(</span><span class="token keyword">using</span> <span class="token keyword">auto</span> cond<span class="token punctuation">,</span> <span class="token keyword">using</span> <span class="token keyword">auto</span> x<span class="token punctuation">,</span> <span class="token keyword">using</span> <span class="token keyword">auto</span> y<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">if</span> <span class="token punctuation">(</span>cond<span class="token punctuation">)</span>
    <span class="token keyword">return</span> x<span class="token punctuation">;</span>
  <span class="token keyword">else</span>
    <span class="token keyword">return</span> y<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">int</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token function">if_</span><span class="token punctuation">(</span>
    <span class="token boolean">true</span><span class="token punctuation">,</span>
    <span class="token function">print</span><span class="token punctuation">(</span><span class="token string">"Hello, world!"</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    <span class="token function">print</span><span class="token punctuation">(</span><span class="token string">"not evaluated so nothing gets printed"</span><span class="token punctuation">)</span>
  <span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre>
<p>This has some compelling use cases for creating idiomatic interfaces:</p>
<pre class=" language-cpp"><code class=" language-cpp"><span class="token comment">// customized conjunction and disjunction with short-circuit evaluation</span>

Foo x <span class="token operator">=</span> x <span class="token operator">||</span> <span class="token keyword">new</span> <span class="token function">Foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token keyword">auto</span> y <span class="token operator">=</span> <span class="token function">do_something</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">&amp;&amp;</span> <span class="token function">maybe_do_something_else</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre>
<pre class=" language-cpp"><code class=" language-cpp"><span class="token comment">// constexpr ternary expression</span>

<span class="token keyword">auto</span> x <span class="token operator">=</span> <span class="token function">constexpr_if</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">,</span> <span class="token keyword">int</span><span class="token punctuation">{</span><span class="token number">42</span><span class="token punctuation">}</span><span class="token punctuation">,</span> std<span class="token operator">::</span>make_unique<span class="token operator">&lt;</span><span class="token keyword">int</span><span class="token operator">&gt;</span><span class="token punctuation">(</span><span class="token number">42</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre>
<pre class=" language-cpp"><code class=" language-cpp"><span class="token comment">// debug logging that eliminates evaluation in production (like macros)</span>

<span class="token keyword">namespace</span> log <span class="token punctuation">{</span>
  <span class="token keyword">struct</span> null_stream <span class="token punctuation">{</span>
    <span class="token keyword">using</span> <span class="token keyword">operator</span><span class="token operator">&lt;&lt;</span><span class="token punctuation">(</span><span class="token keyword">using</span> <span class="token keyword">auto</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">return</span> null_stream<span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span><span class="token punctuation">;</span>

  <span class="token keyword">using</span> <span class="token function">warn</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">if</span> <span class="token keyword">constexpr</span> <span class="token punctuation">(</span>log_level <span class="token operator">==</span> Warning<span class="token punctuation">)</span>
      <span class="token keyword">return</span> std<span class="token operator">::</span>wcerr<span class="token punctuation">;</span>
    <span class="token keyword">else</span>
      <span class="token keyword">return</span> null_stream<span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

foo <span class="token operator">&gt;</span> <span class="token number">42</span> <span class="token operator">&amp;&amp;</span> log<span class="token operator">::</span>warn <span class="token operator">&lt;&lt;</span> <span class="token string">"Foo is too large: "</span> <span class="token operator">&lt;&lt;</span> foo <span class="token operator">&lt;&lt;</span> <span class="token string">'\n'</span><span class="token punctuation">;</span>
              <span class="token comment">// ^ implicitly invokes nullary call</span>
</code></pre>
<h2 id="concise-forwarding"><a class="anchor" name="concise-forwarding" href="#concise-forwarding"><span class="octicon octicon-link"></span></a>Concise Forwarding</h2>
<p>With this feature we can implement a forwarding operator that is more concise than the existing
<code>std::forward</code>.</p>
<pre class=" language-cpp"><code class=" language-cpp"><span class="token keyword">using</span> <span class="token function">fwd</span><span class="token punctuation">(</span><span class="token keyword">using</span> <span class="token keyword">auto</span> x<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> <span class="token keyword">static_cast</span><span class="token operator">&lt;</span><span class="token keyword">decltype</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token operator">&amp;&amp;</span><span class="token operator">&gt;</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">auto</span> old_f <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token keyword">auto</span><span class="token operator">&amp;&amp;</span> x<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> std<span class="token operator">::</span>forward<span class="token operator">&lt;</span><span class="token keyword">decltype</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token operator">&gt;</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">auto</span> new_f <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token keyword">auto</span><span class="token operator">&amp;&amp;</span> x<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> <span class="token function">fwd</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
</code></pre>
<h2 id="operations-on-parameter-packs"><a class="anchor" name="operations-on-parameter-packs" href="#operations-on-parameter-packs"><span class="octicon octicon-link"></span></a>Operations on Parameter Packs</h2>
<p>We can allow unexpanded parameter packs as inputs and avoid creating tuples in many cases.</p>
<pre class=" language-cpp"><code class=" language-cpp"><span class="token keyword">template</span> <span class="token operator">&lt;</span><span class="token keyword">typename</span> <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span>Xs<span class="token operator">&gt;</span>
<span class="token keyword">auto</span> <span class="token function">foo</span><span class="token punctuation">(</span>Xs<span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span> xs<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">auto</span> fifth_element <span class="token operator">=</span> pack<span class="token operator">::</span>get<span class="token operator">&lt;</span><span class="token number">5</span><span class="token operator">&gt;</span><span class="token punctuation">(</span>xs<span class="token punctuation">)</span><span class="token punctuation">;</span>

  <span class="token keyword">auto</span> bar <span class="token operator">=</span> pack<span class="token operator">::</span><span class="token function">apply</span><span class="token punctuation">(</span>f<span class="token punctuation">,</span> pack<span class="token operator">::</span><span class="token function">drop_front</span><span class="token punctuation">(</span>xs<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre>
<h2 id="composing-operations"><a class="anchor" name="composing-operations" href="#composing-operations"><span class="octicon octicon-link"></span></a>Composing Operations</h2>
<p>Function calls add overhead, and while they are usually inlined by the optimizer, it is still generally
avoided as it can add significant compile-time overhead.</p>
<p>Consider the pattern of having functions return the result of other functions:</p>
<pre class=" language-cpp"><code class=" language-cpp"><span class="token keyword">template</span> <span class="token operator">&lt;</span><span class="token keyword">typename</span> T<span class="token operator">&gt;</span>
<span class="token keyword">auto</span> <span class="token function">calc_a</span><span class="token punctuation">(</span>T input<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">int</span> on_the stack <span class="token operator">=</span> input <span class="token operator">+</span> <span class="token number">42</span><span class="token punctuation">;</span>
  <span class="token keyword">return</span> <span class="token function">calc_b</span><span class="token punctuation">(</span>on_the_stack<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">template</span> <span class="token operator">&lt;</span><span class="token keyword">typename</span> T<span class="token operator">&gt;</span>
<span class="token keyword">auto</span> <span class="token function">calc_b</span><span class="token punctuation">(</span>T<span class="token operator">&amp;</span> on_the_stack<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> <span class="token function">calc_c</span><span class="token punctuation">(</span>input <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre>
<p>We can decouple these functions by using a generic continuation interface.</p>
<pre class=" language-cpp"><code class=" language-cpp"><span class="token function">do_</span><span class="token punctuation">(</span>
  calc_a<span class="token punctuation">,</span>
  calc_b<span class="token punctuation">,</span>
  calc_c
<span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre>
<p>This is currently possible with normal functions, but it adds a bunch of noise to the call stack which
makes debugging difficult, and the additional function calls make more work for the compiler which adds
up.</p>
<h2 id="as-a-member-of-a-class"><a class="anchor" name="as-a-member-of-a-class" href="#as-a-member-of-a-class"><span class="octicon octicon-link"></span></a>As a Member of a Class</h2>
<p>Parametric expressions can be used as a member in conjunction with operator overloading to create an
invocable object:</p>
<pre class=" language-cpp"><code class=" language-cpp">  <span class="token keyword">struct</span> id_fn <span class="token punctuation">{</span>
    <span class="token keyword">using</span> <span class="token keyword">operator</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token keyword">auto</span> x<span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">return</span> x<span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span><span class="token punctuation">;</span>

  id_fn<span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">(</span><span class="token number">42</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

  <span class="token keyword">static_assert</span><span class="token punctuation">(</span>std<span class="token operator">::</span>is_invocable<span class="token operator">&lt;</span>id_fn<span class="token punctuation">,</span> <span class="token keyword">int</span><span class="token operator">&gt;</span><span class="token operator">::</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre>
<h2 id="raii-scope-elision"><a class="anchor" name="raii-scope-elision" href="#raii-scope-elision"><span class="octicon octicon-link"></span></a>RAII Scope Elision</h2>
<p>Specific conditions allow us to generate an expression that does not require RAII and therefore allows
the compiler to substitute the call expression directly which can yield a prvalue and can also be used in
SFINAE and other contexts where variable declarations and statements are not welcome.</p>
<pre class=" language-cpp"><code class=" language-cpp"><span class="token keyword">using</span> <span class="token function">funcify</span><span class="token punctuation">(</span><span class="token keyword">using</span> <span class="token keyword">auto</span> f<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token keyword">auto</span><span class="token operator">&amp;&amp;</span> <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span>x<span class="token punctuation">)</span>
    <span class="token keyword">noexcept</span><span class="token punctuation">(</span><span class="token keyword">noexcept</span><span class="token punctuation">(</span><span class="token keyword">decltype</span><span class="token punctuation">(</span><span class="token function">f</span><span class="token punctuation">(</span>std<span class="token operator">::</span>forward<span class="token operator">&lt;</span><span class="token keyword">decltype</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token operator">&gt;</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
    <span class="token operator">-</span><span class="token operator">&gt;</span> <span class="token keyword">decltype</span><span class="token punctuation">(</span><span class="token function">f</span><span class="token punctuation">(</span>std<span class="token operator">::</span>forward<span class="token operator">&lt;</span><span class="token keyword">decltype</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token operator">&gt;</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
    <span class="token punctuation">{</span> <span class="token keyword">return</span> <span class="token function">f</span><span class="token punctuation">(</span>std<span class="token operator">::</span>forward<span class="token operator">&lt;</span><span class="token keyword">decltype</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token operator">&gt;</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">struct</span> make_array_impl <span class="token punctuation">{</span>
  <span class="token keyword">using</span> <span class="token keyword">operator</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token keyword">using</span> <span class="token keyword">auto</span> <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span>x<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> std<span class="token operator">::</span>array<span class="token operator">&lt;</span>std<span class="token operator">::</span>common_type<span class="token operator">&lt;</span><span class="token keyword">decltype</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token operator">&gt;</span><span class="token punctuation">,</span> <span class="token keyword">sizeof</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token operator">&gt;</span><span class="token punctuation">{</span>x<span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token keyword">constexpr</span> <span class="token keyword">auto</span> make_array <span class="token operator">=</span> <span class="token function">funcify</span><span class="token punctuation">(</span>make_array_impl<span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre>
<h2 id="comparison-with-other-proposed-features"><a class="anchor" name="comparison-with-other-proposed-features" href="#comparison-with-other-proposed-features"><span class="octicon octicon-link"></span></a>Comparison with Other Proposed Features</h2>
<p>One of the primary benefits of this proposed feature is the compile-time performance and
the fact that it has zero impact on the type system. That with operations on packs, there are no other
proposals addressing this. However this proposed feature does have overlap with other proposed features
concerning constexpr parameters and conditional evaluation.</p>
<p>The paper <em><code>constexpr</code> Function Parameters</em> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1045r0.html">(P1045R1)</a> by David Stone proposes the <code>constexpr</code>
specifier be allowed in normal function parameter declarations effectively making the parameter
<code>constexpr</code>. The advantage that it has over this Parametric Expressions proposal is that it allows
function overloading. While this is still highly desirable, it comes with a few costs which will affect
compiler throughput. First, it requires overload resolution to check if an argument expression can be
used in a constant expression. Second, each constexpr parameter would affect the type creating a new
function type with symbols that would have to be similar to anonymous classes for each permutation of
input arguments. If this is deemed feasible, there is no conflict and the <code>constexpr</code> parameters proposed
by this paper are consistent with David Stone's proposal and the feature proposed in this paper would be
a useful augment for when overloading is not needed.</p>
<p>The paper <em>Towards A (Lazy) Forwarding Mechanism for C++</em> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0927r0.pdf">(P0927R0)</a> by James Dennet and Geoff Romer
specifically sets out to add a feature to enable conditional evaluation of function parameters. The paper
suggests the use of syntax similar to a lambda expression for the parameter declaration to specify
laziness of evaluation which is a major shift in convention as far as variable declarations are
concerned. This paper is in agreement with the motiviations stated, but the solutions are in contrast.
Having the same issues with creating a function prototype, the paper addresses the implications on the
type system suggesting that lazy parameters be added to the type system which is high impact, complex, and
in the opinion of this paper, unnecessary. This paper, however, is proposing the use of an existing
keyword in the declaration specifier which is more in keeping with existing conventions with parameter
declarations, and there is no impact on the type system which makes it a better solution.</p>
<h2 id="notable-concerns"><a class="anchor" name="notable-concerns" href="#notable-concerns"><span class="octicon octicon-link"></span></a>Notable Concerns</h2>
<h3 id="the-case-for-user-controlled-inlining"><a class="anchor" name="the-case-for-user-controlled-inlining" href="#the-case-for-user-controlled-inlining"><span class="octicon octicon-link"></span></a>The Case for User Controlled Inlining</h3>
<p>As far as function call inlining is concerned, optimizers bear a monopoly on this ability as far as what
can be done with standard C++. Abuse of this proposed feature could lead to creating code that might be
suboptimal versus having cases where the optimizer deduces that inlining should not occur.</p>
<p>However, there are cases where it would benefit the user to be able to prevent symbol bloat in the ABI,
and while it is probably a compiler QOI issue, optimizing generic or template heavy code with <code>-Os</code>
usually results in very large targets because the optimizer doesn't inline as agressively. Allowing the
user to surgically inline functions where it makes sense can mitigate this.</p>
<p>Additionally inlining code on the compiler front end can bypass massive amounts of symbol generation and
work for the optimizer for simple cases such as accessing the members of a large tuple. Compiler
performance is no small issue and many go to great lengths to avoid libraries that, while providing
interfaces that are more idiomatic and make applications more maintainable, they add significant overhead
to build times effectively killing productivity. Adding parametric expressions would solve this.</p>
<h3 id="why-not-go-for-broke-and-jump-straight-into-granular-ast-node-manipulation"><a class="anchor" name="why-not-go-for-broke-and-jump-straight-into-granular-ast-node-manipulation" href="#why-not-go-for-broke-and-jump-straight-into-granular-ast-node-manipulation"><span class="octicon octicon-link"></span></a>Why not go for broke and jump straight into granular AST node manipulation?</h3>
<p>Some have suggested that we add something more akin to raw AST generation as is found in Rust macros.
While this would be a powerful superset of the feature this paper is proposing, it comes with some
issues.  One, this would make the interface <strong>significantly more complicated</strong> and inaccessible to
typical users. It also comes with the same problems with evaluation that preprocessor macros have in that
it easily allows mistakes that result in duplicate evaluation or double move operations.</p>
<p>The interface proposed with <em>Parametric Expressions</em> is such that it defaults to consistent evaluation of
input expressions by binding them to an actual variable declaration in the parameter list. If the author
of a function wants to take control of evaluation they can do so via the <code>using</code> parameter declaration
specifier and thus opt in explictly and without a lot of crazy AST syntax.</p>
<p>That said, the advent of Reflection and Metaclasses may open the door to allowing more refined control
over the AST, and this paper's stance is that this feature is <strong>forwards compatible</strong> with the possibilty
of adding <code>reflexpr</code> parameters or some similar construct to enable this superset.</p>
<h3 id="invocable-and-the-dangers-of-using-parameters"><a class="anchor" name="invocable-and-the-dangers-of-using-parameters" href="#invocable-and-the-dangers-of-using-parameters"><span class="octicon octicon-link"></span></a>Invocable and the Dangers of <code>using</code> Parameters</h3>
<p>As previously stated, <code>using</code> parameters add macro-like semantics. While the pitfalls of macros are well
understood, here is a particularly onerous edge case that is worth mentioning:</p>
<pre class=" language-cpp"><code class=" language-cpp"><span class="token keyword">struct</span> eval_twice_fn <span class="token punctuation">{</span>
  <span class="token keyword">using</span> <span class="token keyword">operator</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token keyword">using</span> <span class="token keyword">auto</span> x<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> std<span class="token operator">::</span><span class="token function">pair</span><span class="token punctuation">(</span>x<span class="token punctuation">,</span> x<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span> <span class="token keyword">inline</span> <span class="token keyword">constexpr</span> eval_twice<span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token keyword">auto</span> result1 <span class="token operator">=</span> <span class="token function">eval_twice</span><span class="token punctuation">(</span>my<span class="token operator">::</span><span class="token function">string</span><span class="token punctuation">(</span><span class="token string">"foo"</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">auto</span> result2 <span class="token operator">=</span> std<span class="token operator">::</span><span class="token function">invoke</span><span class="token punctuation">(</span>eval_twice<span class="token punctuation">,</span> my<span class="token operator">::</span><span class="token function">string</span><span class="token punctuation">(</span><span class="token string">"foo"</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre>
<p>Here the semantics are changed when used with <code>std::invoke</code>. Since <code>std::invoke</code> is a normal function, it
evaluates it and binds it to a parameter that is perfectly forwarded. The forwarding expression would
then be passed to <code>eval_twice</code> and the result of the input would get moved twice. With the call
expression the result is a pair of two distinct strings containing a pointer to their own "foo".</p>
<p><code>std::Invokable</code> does not require that the call to <code>std::invoke</code> be equality preserving on its arguments
so this is likely permissible. Regardless, this paper's stance is to allow these types of edge cases, and
leave it to the discretion of the user as the user is opting in explicitly via the <code>using</code> specifier.
Attempts to prevent these would involve complex rules and could inadvertantly rule out desirable use
cases.</p>
<h2 id="impact-on-the-existing-standard"><a class="anchor" name="impact-on-the-existing-standard" href="#impact-on-the-existing-standard"><span class="octicon octicon-link"></span></a>Impact on the Existing Standard</h2>
<p>While there is zero impact on existing features in C++, the following additions are required:</p>
<ol>
<li>
<p>Parsing the <code>using</code> keyword followed by an identifier or operator name followed by a left
parentheses would indicate the declaration of a parametric expression disambiguating from
other valid uses of the <code>using</code> keyword for other types of declarations.</p>
</li>
<li>
<p>Allow the <code>constexpr</code> specifier in parameter declarations in the context of a parametric
expression parameter list declaration.</p>
</li>
<li>
<p>The <code>using</code> keyword is used as a declaration specifier for parameter declarations in the
context a parametric expression parameter list declaration.</p>
</li>
<li>
<p>The <code>auto</code> keyword is used as placeholder for a constraint for parametric expression parameters and it
implictly promotes parameters as <code>auto&amp;&amp;</code>. This is more concise but a slight deviation from
what C++ programmers might expect.</p>
</li>
</ol>
<h2 id="the-rules"><a class="anchor" name="the-rules" href="#the-rules"><span class="octicon octicon-link"></span></a>The Rules</h2>
<p>The following rules have been refined based on experience from the <a href="https://github.com/ricejasonf/clang/tree/parmexpr">reference implementation</a>:</p>
<ol>
<li>
<p>Declaration or invocation does not create a type. Overloading or ADL is not supported.</p>
</li>
<li>
<p>The input parameter list may contain only one parameter pack in any position.</p>
</li>
<li>
<p>Input parameter type specifiers must be completely unconstrained and not specify a type.</p>
<ul>
<li>Only <code>auto</code> is allowed as the type specifier. The idea here is to be forwards compatible with
a concise syntax for specifying a constraint via the concepts language feature.</li>
<li>The other option is to allow specifying a type, but this is ugly and not useful as overloading is
not supported.</li>
</ul>
</li>
<li>
<p>Parameter declarations can have <code>constexpr</code> or <code>using</code> specifiers which change semantics:</p>
<ul>
<li>With none of these specifiers the parameters is an rvalue reference initialized with the input
expression. Its name is treated as an lvalue. (just like auto&amp;&amp;)</li>
<li>The <code>constexpr</code> specifier is a value initialized with an expression that must be a
constant-expression</li>
<li>The <code>using</code> specifier does not create a variable and the input expression is substituted anywhere
the name is used. (like a macro)</li>
</ul>
</li>
<li>
<p>The input parameters may be unexpanded parameter packs.</p>
<ul>
<li>This adds a bit more power for working with lists.</li>
</ul>
</li>
<li>
<p>The definition is a compound statement that may have RAII scope when invoked.</p>
<ul>
<li>A RAII scope is not created if there is only one statement that is a return statement, and all
parameters (if any) are declared with the <code>using</code> specifier.</li>
<li>A RAII scope is created if there are any statements other than a single return statement or if
there are any variable declarations without the <code>using</code> specifier.</li>
<li>Implicit this does not count as a variable declaration for the purposes of detecting if there
should be a RAII scope.</li>
</ul>
</li>
<li>
<p>If a RAII scope is created then invocation executes the compound statement as an expression with the
result being determined by its return statements with the same RVO/NRVO guaranteed copy elision rules
that apply to normal functions.</p>
</li>
<li>
<p>If no RAII scope is created then invocation results in substituting the expression from the return
statement.</p>
</li>
<li>
<p>As with deduced return types in functions, the return statements must all result in the same type not
considering statements omitted after static selection.</p>
</li>
<li>
<p>Recursion is not allowed</p>
<ul>
<li>Recursion is not feasible, but we could at least use Odin Holmes' recursive
alias pattern</li>
</ul>
</li>
<li>
<p>Operator overloading is supported. (only as a member since ADL is not supported)</p>
</li>
<li>
<p>When defined as a member of a class, invocations have implicit <code>this</code>. An optional const qualifier
after the parameter list can be used restrict <code>this</code> to const.</p>
<ul>
<li>This is consistent with the way member functions behave.</li>
<li>An object with a defined <code>using operator()</code> should be Invocable.</li>
</ul>
</li>
<li>
<p>As a member, the static keyword can be used to simply make it a free entity that is local to
the owning class.</p>
<ul>
<li>This is consistent with the way member functions behave.</li>
</ul>
</li>
<li>
<p>The use of an identifier that refers the name of a parametric expression, when used in an expression
without a call expression (ie no parens), is implicitly promoted to a nullary call.</p>
<ul>
<li>This is a nice-to-have as it can reduce syntatic noise and make nullary parametric expressions
appear as a variable.</li>
</ul>
</li>
</ol>
<h2 id="summary"><a class="anchor" name="summary" href="#summary"><span class="octicon octicon-link"></span></a>Summary</h2>
<p>With this we have a facility for writing more idiomatic code and with a significant improvement in
compile-time performance having an impact that would rival what was seen with the advent of variadic
templates.</p>
<p>This feature has the potential to have a huge impact on the ability to implement ranges, expression
templates, metaprogramming libraries, and functional and generic code as well as replace the use of
macros in a lot of use cases. It would be really cool to get this in C++20 if it is not too late.</p>
<p>Thanks for looking at this!</p>
<p>Jason Rice</p>
<h2 id="acknowledgements"><a class="anchor" name="acknowledgements" href="#acknowledgements"><span class="octicon octicon-link"></span></a>Acknowledgements</h2>
<p>Special thanks to Louis Dionne and Miguel Ojeda who provided feedback and suggestions as well as others
who provided feedback on the mailing list.</p>
</div><div id="_toc"><div id="_ul"><a href="#parametric-expressions">Parametric Expressions</a><div id="_ul"><a href="#abstract">Abstract</a><a href="#constexpr-parameters">Constexpr Parameters</a><a href="#using-parameters">Using Parameters</a><a href="#concise-forwarding">Concise Forwarding</a><a href="#operations-on-parameter-packs">Operations on Parameter Packs</a><a href="#composing-operations">Composing Operations</a><a href="#as-a-member-of-a-class">As a Member of a Class</a><a href="#raii-scope-elision">RAII Scope Elision</a><a href="#comparison-with-other-proposed-features">Comparison with Other Proposed Features</a><a href="#notable-concerns">Notable Concerns</a><div id="_ul"><a href="#the-case-for-user-controlled-inlining">The Case for User Controlled Inlining</a><a href="#why-not-go-for-broke-and-jump-straight-into-granular-ast-node-manipulation">Why not go for broke and jump straight into granular AST node manipulation?</a><a href="#invocable-and-the-dangers-of-using-parameters">Invocable and the Dangers of using Parameters</a></div><a href="#impact-on-the-existing-standard">Impact on the Existing Standard</a><a href="#the-rules">The Rules</a><a href="#summary">Summary</a><a href="#acknowledgements">Acknowledgements</a></div></div></div></body></html>
