<html><head><style>body{font-family:Helvetica,Arial;font-size:13px}</style></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div id="bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;">Yes, `where` clause is welcome to typealises (including generic ones) and existentials in general. I would love to help on such proposal. I think David Hart is also interested in this one. (cc)</div> <br> <div id="bloop_sign_1503308348105416960" class="bloop_sign"></div> <br><p class="airmail_on">Am 21. August 2017 um 11:38:14, Gor Gyolchanyan 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>
Hello, Swift community!
<div class=""><br class=""></div>
<div class="">I'd like to start a discussion about a possibility of
constrained protocol aliases. The declaration would look like
this:</div>
<div class=""><br class=""></div>
<div class="">
<div class=""><font face="Menlo" class=""><font color="#941751" class="">typealias</font> <font color="#008F00" class="">BinaryProtocol</font> = <font color="#008F00" class="">RandomAccessCollection</font> & <font color="#008F00" class="">MutablCollection</font> & <font color="#008F00" class="">RangeReplaceableCollection</font> <font color="#941751" class="">where</font> <font color="#008F00" class="">Binary</font>.<font color="#008F00" class="">Index</font> ==
<font color="#008F00" class="">Int</font>, <font color="#008F00" class="">Binary</font>.<font color="#008F00" class="">Element</font> == <font color="#008F00" class="">Bool</font></font></div>
</div>
<div class=""><br class=""></div>
<div class="">The syntax and semantics of this declaration are
exactly the same as an analogous <font color="#941751" face="Menlo" class="">associatedtype</font> declaration inside a protocol.</div>
<div class="">In the example above, the type <font face="Menlo" color="#008F00" class="">BinaryProtocol</font> represents a
logical array of bits and is a generic-only protocol that is usable
in any context where an integer-indexed mutable range-replaceable
random-access collection is expected.</div>
<div class="">Now, it can be used in a very concise and elegant
way:</div>
<div class=""><br class=""></div>
<div class=""><font face="Menlo" class=""><font color="#941751" class="">public</font> <font color="#941751" class="">protocol</font> <font color="#008F00" class="">BinaryInitializable</font> {</font></div>
<div class=""><font face="Menlo" class=""><font color="#941751" class="">init</font><<font color="#008F00" class="">Binary</font>>(binary: <font color="#008F00" class="">Binary</font>) <font color="#941751" class="">where</font>
<font color="#008F00" class="">Binary</font>: <font color="#008F00" class="">BinaryProtocol</font></font></div>
<div class=""><font face="Menlo" class="">}</font></div>
<div class=""><br class=""></div>
<div class="">which would otherwise look very verbose and
inelegant:</div>
<div class=""><br class=""></div>
<div class=""><font face="Menlo" class=""><font color="#941751" class="">public</font> <font color="#941751" class="">protocol</font> <font color="#008F00" class="">BinaryInitializable</font> {</font></div>
<div class=""><font face="Menlo" class=""><font color="#941751" class="">init</font><<font color="#008F00" class="">Binary</font>>(binary: <font color="#008F00" class="">Binary</font>) <font color="#941751" class="">where</font>
Binary: <font color="#008F00" class="">RandomAccessCollection</font> & <font color="#008F00" class="">MutablCollection</font> & <font color="#008F00" class="">RangeReplaceableCollection,</font> <font color="#008F00" class="">Binary</font>.<font color="#008F00" class="">Index</font> == <font color="#008F00" class="">Int</font>, <font color="#008F00" class="">Binary</font>.<font color="#008F00" class="">Element</font> == <font color="#008F00" class="">Bool</font></font></div>
<div class=""><font face="Menlo" class="">}</font></div>
<div class="">
<div class=""><br class=""></div>
<div class="">Considering that smaller sets of constraints could be
aliased to their own protocol and then composited into more complex
aliases, this feature would dramatically improve readability and
maintainability of code that uses complex constraints, that
currently leads to arcane mess:</div>
<div class=""><br class=""></div>
<div class=""><font face="Menlo" class=""><font color="#941751" class="">struct</font> <font color="#008F00" class="">Mirror</font> {</font></div>
<div class="">
<ul class="container" style="box-sizing: border-box; margin: 0px; padding: 0px; width: auto; list-style: none;">
<li class=""><font face="Menlo" class=""><font color="#797979" class="">/// ...</font></font></li>
<li class=""><font face="Menlo" class=""><font color="#941751" class="">init</font><<font color="#008F00" class="">Subject</font>, <font color="#008F00" class="">C</font> <font color="#941751" class="">where</font> <font color="#008F00" class="">C</font> : <font color="#008F00" class="">Collection</font>, <font color="#008F00" class="">C</font>.<font color="#008F00" class="">Indices</font> : <font color="#008F00" class="">Collection</font>, <font color="#008F00" class="">C</font>.<font color="#008F00" class="">SubSequence</font> : <font color="#008F00" class="">Collection</font>, <font color="#008F00" class="">C</font>.<font color="#008F00" class="">Indices</font>.<font color="#008F00" class="">Index</font> == <font color="#008F00" class="">C</font>.<font color="#008F00" class="">Index</font>, <font color="#008F00" class="">C</font>.<font color="#008F00" class="">Indices</font>.<font color="#008F00" class="">SubSequence</font> == <font color="#008F00" class="">C</font>.<font color="#008F00" class="">Indices</font>, <font color="#008F00" class="">C</font>.<font color="#008F00" class="">Iterator</font>.<font color="#008F00" class="">Element</font> == <font color="#008F00" class="">Mirror</font>.<font color="#008F00" class="">Child</font>, <font color="#008F00" class="">C</font>.<font color="#008F00" class="">SubSequence</font>.<font color="#008F00" class="">Index</font> == <font color="#008F00" class="">C</font>.<font color="#008F00" class="">Index</font>, <font color="#008F00" class="">C</font>.<font color="#008F00" class="">SubSequence</font>.<font color="#008F00" class="">Indices</font> : <font color="#008F00" class="">Collection</font>, <font color="#008F00" class="">C</font>.<font color="#008F00" class="">SubSequence</font>.<font color="#008F00" class="">SubSequence</font> == <font color="#008F00" class="">C</font>.<font color="#008F00" class="">SubSequence</font>, <font color="#008F00" class="">C</font>.<font color="#008F00" class="">Indices</font>.<font color="#008F00" class="">Iterator</font>.<font color="#008F00" class="">Element</font> == <font color="#008F00" class="">C</font>.<font color="#008F00" class="">Index</font>, <font color="#008F00" class="">C</font>.<font color="#008F00" class="">SubSequence</font>.<font color="#008F00" class="">Indices</font>.<font color="#008F00" class="">Index</font> == <font color="#008F00" class="">C</font>.<font color="#008F00" class="">Index</font>, <font color="#008F00" class="">C</font>.<font color="#008F00" class="">SubSequence</font>.<font color="#008F00" class="">Indices</font>.<font color="#008F00" class="">SubSequence</font> == <font color="#008F00" class="">C</font>.<font color="#008F00" class="">SubSequence</font>.<font color="#008F00" class="">Indices</font>, <font color="#008F00" class="">C</font>.<font color="#008F00" class="">SubSequence</font>.<font color="#008F00" class="">Iterator</font>.<font color="#008F00" class="">Element</font> == <font color="#008F00" class="">Mirror</font>.<font color="#008F00" class="">Child</font>, <font color="#008F00" class="">C</font>.<font color="#008F00" class="">SubSequence</font>.<font color="#008F00" class="">Indices</font>.<font color="#008F00" class="">Iterator</font>.<font color="#008F00" class="">Element</font> == <font color="#008F00" class="">C</font>.<font color="#008F00" class="">Index</font>>(_
subject: <font color="#008F00" class="">Subject</font>,
children: <font color="#008F00" class="">C</font>,
displayStyle: <font color="#008F00" class="">Mirror</font>.<font color="#008F00" class="">DisplayStyle</font>? = <font color="#941751" class="">default</font>, ancestorRepresentation: <font color="#008F00" class="">Mirror</font>.<font color="#008F00" class="">AncestorRepresentation</font> = <font color="#941751" class="">default</font>)</font></li>
</ul>
<div class=""><font face="Menlo" color="#797979" class="">///
...</font></div>
</div>
<div class=""><font face="Menlo" class="">}</font></div>
</div>
<div class=""><font face="Menlo" class=""><br class=""></font></div>
<div class=""><span style="font-family: Menlo;" class=""><br class=""></span></div>
<div class=""><font color="#797979" face="Menlo" class="">/// A
collection that is its own sub-sequence</font></div>
<div class=""><font face="Menlo" class=""><font color="#941751" class="">typealias</font> <font color="#008F00" class="">RecursivelySliceableCollection</font> = <font color="#008F00" class="">Collection</font> <font color="#941751" class="">where</font></font></div>
<div class=""><font color="#008F00" face="Menlo" class="">RecursivelySliceableCollection</font><span style="font-family: Menlo;" class="">.</span><font color="#008F00" face="Menlo" class="">SubSequence</font><font face="Menlo" class="">:
<font color="#008F00" class="">Collection</font>,</font></div>
<div class=""><font color="#008F00" face="Menlo" class="">RecursivelySliceableCollection</font><span style="font-family: Menlo;" class="">.</span><font color="#008F00" face="Menlo" class="">SubSequence</font><span style="font-family: Menlo;" class="">.</span><font color="#008F00" face="Menlo" class="">Element</font> <span style="font-family: Menlo;" class="">== </span><font color="#008F00" face="Menlo" class="">RecursivelySliceableCollection</font><span style="font-family: Menlo;" class="">.</span><font color="#008F00" face="Menlo" class="">Element</font></div>
<div class=""><font color="#008F00" face="Menlo" class="">RecursivelySliceableCollection</font><span style="font-family: Menlo;" class="">.</span><font color="#008F00" face="Menlo" class="">SubSequence</font><span style="font-family: Menlo;" class="">.</span><font color="#008F00" face="Menlo" class="">Indices</font> <span style="font-family: Menlo;" class="">== </span><font color="#008F00" face="Menlo" class="">RecursivelySliceableCollection</font><span style="font-family: Menlo;" class="">.</span><font color="#008F00" face="Menlo" class="">Indices</font><span style="font-family: Menlo;" class="">,</span></div>
<div class=""><font color="#008F00" face="Menlo" class="">RecursivelySliceableCollection</font><span style="font-family: Menlo;" class="">.</span><font color="#008F00" face="Menlo" class="">SubSequence</font><span style="font-family: Menlo;" class="">.</span><font color="#008F00" face="Menlo" class="">SubSequence</font> <span style="font-family: Menlo;" class="">== </span><font color="#008F00" face="Menlo" class="">RecursivelySliceableCollection</font><span style="font-family: Menlo;" class="">.</span><font color="#008F00" face="Menlo" class="">SubSequence</font></div>
<div class=""><font face="Menlo" class=""><br class=""></font></div>
<div class=""><font color="#797979" face="Menlo" class="">/// A
collection that is its own index collection</font></div>
<div class=""><font color="#941751" face="Menlo" class="">typealias</font> <font color="#008F00" face="Menlo" class="">RecursivelyIndexableCollection</font> <span style="font-family: Menlo;" class="">=</span> <font color="#008F00" face="Menlo" class="">Collection</font> <font color="#941751" face="Menlo" class="">where</font></div>
<div class=""><font color="#008F00" face="Menlo" class="">RecursivelyIndexableCollection</font><span style="font-family: Menlo;" class="">.</span><font color="#008F00" face="Menlo" class="">Indices</font> <span style="font-family: Menlo;" class="">== </span><font color="#008F00" face="Menlo" class="">RecursivelySliceableCollection</font><span style="font-family: Menlo;" class="">,</span></div>
<div class=""><font color="#008F00" face="Menlo" class="">RecursivelyIndexableCollection</font><span style="font-family: Menlo;" class="">.</span><font color="#008F00" face="Menlo" class="">Indices</font><span style="font-family: Menlo;" class="">.</span><font color="#008F00" face="Menlo" class="">Index</font> <span style="font-family: Menlo;" class="">== </span><font color="#008F00" face="Menlo" class="">RecursivelyIndexableCollection</font><span style="font-family: Menlo;" class="">.</span><font color="#008F00" face="Menlo" class="">Index</font><span style="font-family: Menlo;" class="">,</span></div>
<div class=""><span style="font-family: Menlo;" class=""><br class=""></span></div>
<div class=""><font color="#941751" face="Menlo" class="">struct</font> <span style="font-family: Menlo;" class="">Mirror
{</span></div>
<div class=""><font color="#797979" face="Menlo" class="">///
...</font></div>
<div class=""><font color="#941751" face="Menlo" class="">init</font><span style="font-family: Menlo;" class=""><</span><font color="#008F00" face="Menlo" class="">Subject</font><span style="font-family: Menlo;" class="">,</span> <font color="#008F00" face="Menlo" class="">C</font><span style="font-family: Menlo;" class="">: </span><font color="#008F00" face="Menlo" class="">RecursivelySliceableCollection</font> <span style="font-family: Menlo;" class="">& </span><font color="#008F00" face="Menlo" class="">RecursivelyIndexableCollection</font><span style="font-family: Menlo;" class="">, </span><font color="#941751" face="Menlo" class="">where</font> <font color="#008F00" face="Menlo" class="">C</font><span style="font-family: Menlo;" class="">.</span><font color="#008F00" face="Menlo" class="">Element</font> <span style="font-family: Menlo;" class="">==</span> <font color="#008F00" face="Menlo" class="">Mirror</font><span style="font-family: Menlo;" class="">.</span><font color="#008F00" face="Menlo" class="">Child</font><span style="font-family: Menlo;" class="">>(</span><span style="font-family: Menlo;" class="">_
subject:</span><span style="font-family: Menlo;" class=""> </span><font color="#008F00" style="font-family: Menlo;" class="">Subject</font><span style="font-family: Menlo;" class="">,
children:</span><span style="font-family: Menlo;" class=""> </span><font color="#008F00" style="font-family: Menlo;" class="">C</font><span style="font-family: Menlo;" class="">,
displayStyle:</span><span style="font-family: Menlo;" class=""> </span><font color="#008F00" style="font-family: Menlo;" class="">Mirror</font><span style="font-family: Menlo;" class="">.</span><font color="#008F00" style="font-family: Menlo;" class="">DisplayStyle</font><span style="font-family: Menlo;" class="">?
=</span><span style="font-family: Menlo;" class=""> </span><font color="#941751" style="font-family: Menlo;" class="">default</font><span style="font-family: Menlo;" class="">,
ancestorRepresentation:</span><span style="font-family: Menlo;" class=""> </span><font color="#008F00" style="font-family: Menlo;" class="">Mirror</font><span style="font-family: Menlo;" class="">.</span><font color="#008F00" style="font-family: Menlo;" class="">AncestorRepresentation</font><span style="font-family: Menlo;" class=""> </span><span style="font-family: Menlo;" class="">=</span><span style="font-family: Menlo;" class=""> </span><font color="#941751" style="font-family: Menlo;" class="">default</font><span style="font-family: Menlo;" class="">)</span></div>
<div class=""><font color="#797979" face="Menlo" class="">///
...</font></div>
<div class=""><span style="font-family: Menlo;" class="">}</span></div>
<div class=""><br class=""></div>
<div class="">Even considering that the proposal SE-0157 (<a href="https://github.com/apple/swift-evolution/blob/master/proposals/0157-recursive-protocol-constraints.md" class="">https://github.com/apple/swift-evolution/blob/master/proposals/0157-recursive-protocol-constraints.md</a>)
is going to make this specific use case a non-issue, the principle
applies to all cases where there are commonly used complex
constraints that don't necessarily involve recursive
constraints.</div>
<div class=""><br class=""></div>
<div class=""><b class="">Specializing Generic-Only Protocols For
Non-Generic Use</b></div>
<div class=""><br class=""></div>
<div class="">An additional feature that would prove to be very
useful would be to make a constrained protocol alias be a
non-generic-only protocol if the constraints of the alias
declaration specify a same-type requirement for all its associated
types, while defaulted associated types would also count.</div>
<div class="">Example:</div>
<div class=""><br class=""></div>
<div class=""><font face="Menlo" class=""><font color="#941751" class="">protocol</font> <font color="#008F00" class="">Consumer</font> {</font></div>
<div class=""><font face="Menlo" class=""><font color="#941751" class="">associatedtype</font> <font color="#008F00" class="">Consumable</font></font></div>
<div class=""><font face="Menlo" class=""><font color="#941751" class="">mutating</font> <font color="#941751" class="">func</font> consume(_ consumable: <font color="#008F00" class="">Consumable</font>) <font color="#941751" class="">throws</font></font></div>
<div class=""><font face="Menlo" class="">}</font></div>
<div class=""><font face="Menlo" class=""><br class=""></font></div>
<div class=""><font face="Menlo" class=""><font color="#941751" class="">var</font> consumer0: <font color="#008F00" class="">Consumer</font> <font color="#797979" class="">// </font></font><span style="font-family: Menlo;" class=""><i class=""><font color="#FF2600" class="">error: Consumer is
only usable in a generic context</font></i></span></div>
<div class=""><font face="Menlo" class=""><br class=""></font></div>
<div class=""><font face="Menlo" class=""><font color="#941751" class="">typealias</font> <font color="#008F00" class="">CharacterConsumer</font> = <font color="#008F00" class="">Consumer</font> <font color="#941751" class="">where </font> </font><span style="color: rgb(0, 143, 0); font-family: Menlo;" class="">Character</span><font face="Menlo" class=""><font color="#008F00" class="">Consumer</font>.<font color="#008F00" class="">Consumable</font> == </font><span style="color: rgb(0, 143, 0); font-family: Menlo;" class="">Character</span></div>
<div class=""><font face="Menlo" class=""><br class=""></font></div>
<div class=""><font face="Menlo" class=""><font color="#941751" class="">var</font> consumer1: </font><span style="color: rgb(0, 143, 0); font-family: Menlo;" class="">Character</span><font face="Menlo" class=""><font color="#008F00" class="">Consumer</font> <font color="#797979" class="">// OK</font></font></div>
<div class=""><br class=""></div>
<div class="">The current workaround would be to declare a new
protocol with protocol inheritance clauses and a <font color="#941751" face="Menlo" class="">where</font> clause, but the major
downside is that it introduces a completely new protocol that is
not compatible with any context that expects the underlying
protocols and their constraints.</div>
<div class=""><br class=""></div>
<div class="">Regards,</div>
<div class="">Gor Gyolchanyan.</div>
<div class=""><br class=""></div>
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