<div dir="ltr">Anton,<div><br></div><div>It is my opinion that you are describing an entirely different, and somewhat orthogonal, feature. I would like the feature that you describe. Constant expressions are powerful and open up quite a few optimizations. What constexpr addresses is not purity, at the heart of it. Pure expressions that accept compile-time-known values are, by happy accident, compile-time-computable, but pure expressions that accept dynamic values are not. Conflating the two qualities of being compile-time-known and being pure within the same keyword and overloading it in this way is not desirable to me.</div><div><br></div><div><br></div><div>Thank you,</div><div>TJ</div></div><div class="gmail_extra"><br><div class="gmail_quote">On Fri, Feb 17, 2017 at 8:01 AM, Anton Zhilin via swift-evolution <span dir="ltr"><<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div class="m_5263759464356799057markdown-here-wrapper"><p style="margin:0px 0px 1.2em!important">My vision of “pure” functions was the following:</p>
<ol style="margin:1.2em 0px;padding-left:2em">
<li style="margin:0.5em 0px">Compiler automatically marks all functions and expressions as pure, wherever possible<ul style="margin:1.2em 0px;padding-left:2em;margin:0px;padding-left:1em">
<li style="margin:0.5em 0px">We should be interested not in “Haskell-ish pure” functions, but in “computable during compilation” functions</li>
<li style="margin:0.5em 0px">Therefore I prefer to use <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">@constexpr</code> or <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">const</code> instead of <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">@pure</code></li>
</ul>
</li>
<li style="margin:0.5em 0px">We can mark a function as <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">const</code> to <em>assert</em> that it is indeed pure</li>
<li style="margin:0.5em 0px">We can mark a variable as <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">const</code> to <em>ensure</em> that it’s computed at compilation time<ul style="margin:1.2em 0px;padding-left:2em;margin:0px;padding-left:1em">
<li style="margin:0.5em 0px">Compiler might compute some non-<code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">const</code> expressions, but no guarantees given</li>
</ul>
</li>
</ol>
<p style="margin:0px 0px 1.2em!important">One issue is, we don’t have or suggest any facilities to make use of pure functions, other than some optimization, which can be performed anyway as of now.</p>
<p style="margin:0px 0px 1.2em!important">One use-case would be conversion of metatypes to types:</p>
<pre style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;font-size:1em;line-height:1.2em;margin:1.2em 0px"><code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline;white-space:pre-wrap;overflow:auto;border-radius:3px;border:1px solid rgb(204,204,204);padding:0.5em 0.7em;display:block!important">const let x: Any = makeSomething()
typealias T = type(of: x)
</code></pre><p style="margin:0px 0px 1.2em!important">This feature can be powerful enough to fill the niche of macros in Swift, without unsafety of C++ or specific syntax of Rust.</p><span class="">
<p style="margin:0px 0px 1.2em!important">2017-02-17 14:14 GMT+03:00 Haravikk via swift-evolution <<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>>:</p>
<p style="margin:0px 0px 1.2em!important"></p><div class="m_5263759464356799057markdown-here-exclude"><p></p><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word"><div>I like the idea of having pure functions in Swift, but my first thought is; should we have to declare it at all? Is it not easier to just have the compiler automatically flag a function as pure or not?</div><div><br></div><div>With that in mind we don't need any new syntax, but a simple @pure attribute should be sufficient. This can be used anywhere that a function is declared, or a closure is accepted as a parameter, allowing us to be explicit that we are trying to define a pure function, or only accept pure closures.</div><div><br></div><div>The big benefit of this is that it is retroactive; all existing functions that are pure will be automatically detected as such, and can be passed into any method accepting only pure functions. The new capability will be that developers can specify that a function *must* be pure and thus produce an error if it isn't.</div></div></blockquote><p></p></div><p style="margin:0px 0px 1.2em!important"></p>
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