newtype feels like a leaky abstraction. Since the new type carries all the protocol conformance and members of the original type, in your example, I could add or divide RecordIds; but if they're database concepts, why would I want to do that?<br><br>If the original type were extended to add members or protocols, would the newtype gain those as well? It must, since types can be split into extensions like that in Swift a priori. So the new type is always effectively a subtype of the original, when what you often want is something that starts off with a smaller subset of its operations instead, and people can extend it in ways you don't expect.<br><br>I'm trying to imagine types where I automatically want *every* operation of its originating type, and having trouble. Even for a unit-like API, there are likely to be arithmetic operations that don't make sense.<br><br>I think a better model would be a general protocol forwarding mechanism for existing type constructs. Then I could define RecordId as a struct, internally it might have an underlying Int representation, but I would explicitly state which protocols I wanted the type to conform to and which of those are forwarded to that underlying Int. This allows us to maintain the semantic importance of protocols and build types in a way that fits existing Swift patterns, and without the magic/danger that newtype could provide.<br><br>The drawback is that if you want only a subset of operations in a protocol, you don't get that easily for free. But you could define your own protocol, retroactively conform the origin type to it *internally*, and then publicly conform the new type and forward... and as far as I can tell, that plugs the abstraction's leaks.<br><div class="gmail_quote"><div dir="ltr">On Thu, Mar 2, 2017 at 4:08 PM Anton Zhilin via swift-evolution <<a href="mailto:swift-evolution@swift.org">swift-evolution@swift.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr" class="gmail_msg"><div class="m_-8390134894059640746markdown-here-wrapper gmail_msg"><p style="margin:0px 0px 1.2em!important" class="gmail_msg">2017-03-03 2:13 GMT+03:00 Slava Pestov <<a href="mailto:spestov@apple.com" class="gmail_msg" target="_blank">spestov@apple.com</a>>:</p>
<p style="margin:0px 0px 1.2em!important" class="gmail_msg"></p><div class="m_-8390134894059640746markdown-here-exclude gmail_msg"><p class="gmail_msg"></p><blockquote class="gmail_quote gmail_msg" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div style="word-wrap:break-word" class="gmail_msg"><div class="gmail_msg">Does newtype add any new capability that’s not already covered by defining a struct?</div></div></blockquote><p class="gmail_msg"></p></div><p style="margin:0px 0px 1.2em!important" class="gmail_msg"></p>
</div></div><div dir="ltr" class="gmail_msg"><div class="m_-8390134894059640746markdown-here-wrapper gmail_msg"><p style="margin:0px 0px 1.2em!important" class="gmail_msg"><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" class="gmail_msg">newtype</code> would forward all members and conformances of the underlying type:</p>
<pre style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;font-size:1em;line-height:1.2em;margin:1.2em 0px" class="gmail_msg"><code class="m_-8390134894059640746hljs m_-8390134894059640746language-swift gmail_msg" 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;display:block;overflow-x:auto;padding:0.5em;color:rgb(51,51,51);background:rgb(248,248,248)">newtype <span class="m_-8390134894059640746hljs-type gmail_msg" style="color:rgb(68,85,136);font-weight:bold">RecordId</span> = <span class="m_-8390134894059640746hljs-type gmail_msg" style="color:rgb(68,85,136);font-weight:bold">Int</span>
<span class="m_-8390134894059640746hljs-keyword gmail_msg" style="color:rgb(51,51,51);font-weight:bold">let</span> x: <span class="m_-8390134894059640746hljs-type gmail_msg" style="color:rgb(68,85,136);font-weight:bold">RecordId</span> = <span class="m_-8390134894059640746hljs-number gmail_msg" style="color:rgb(0,128,128)">5</span>
<span class="m_-8390134894059640746hljs-keyword gmail_msg" style="color:rgb(51,51,51);font-weight:bold">let</span> y = x + <span class="m_-8390134894059640746hljs-number gmail_msg" style="color:rgb(0,128,128)">10</span>
<span class="m_-8390134894059640746hljs-class gmail_msg"><span class="m_-8390134894059640746hljs-keyword gmail_msg" style="color:rgb(51,51,51);font-weight:bold">extension</span> <span class="m_-8390134894059640746hljs-title gmail_msg" style="color:rgb(153,0,0);font-weight:bold;color:rgb(68,85,136);font-weight:bold">RecordId</span> </span>{
<span class="m_-8390134894059640746hljs-func gmail_msg"><span class="m_-8390134894059640746hljs-keyword gmail_msg" style="color:rgb(51,51,51);font-weight:bold">func</span> <span class="m_-8390134894059640746hljs-title gmail_msg" style="color:rgb(153,0,0);font-weight:bold">load</span><span class="m_-8390134894059640746hljs-params gmail_msg">()</span> -> <span class="m_-8390134894059640746hljs-title gmail_msg" style="color:rgb(153,0,0);font-weight:bold">String</span> </span>{ … }
}
<span class="m_-8390134894059640746hljs-keyword gmail_msg" style="color:rgb(51,51,51);font-weight:bold">let</span> a = x.load()
<span class="m_-8390134894059640746hljs-keyword gmail_msg" style="color:rgb(51,51,51);font-weight:bold">let</span> b = <span class="m_-8390134894059640746hljs-number gmail_msg" style="color:rgb(0,128,128)">42</span>.load() <span class="m_-8390134894059640746hljs-comment gmail_msg" style="color:rgb(153,153,136);font-style:italic">// error</span>
</code></pre>
<p style="margin:0px 0px 1.2em!important" class="gmail_msg"><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" class="gmail_msg">newtype</code>s aim to carry purely semantic information, in this case, that those integers can be used to fetch records from a database. We get additional members only when we are sure that semantic of current instance is appropriate.</p>
<p style="margin:0px 0px 1.2em!important" class="gmail_msg">As a side effect, it essentially allows to declare one-liner structs with pattern matching. But I agree with Jaden, just adding pattern matching to structs feels more practical. And this feature is more or less orthogonal to the core functionality 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" class="gmail_msg">newtype</code>.</p>
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