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<div class="moz-cite-prefix">For reference, here are some links to
previous discussions on related topics:<br>
<br>
1) Kevin Ballard in December 2015: <a
href="https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20151228/004635.html">Proposal:
CollectionType.cycle property for an infinite sequence</a><br>
<br>
2) Ben Cohen in February 2016: <a
href="https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170213/032120.html">Sequence/Collection
Enhancements</a> (one of the enhancements that Ben proposed to
consider for adding to the standard library is a <tt>cycle</tt>
method for <tt>Collection</tt> and/or <tt>Sequence</tt>. I don't
think there is a more specific proposal for this yet, but at least
we know it's on the core team's radar.<br>
<br>
On 02.05.2017 04:34, Karl Wagner via swift-evolution wrote:<br>
</div>
<blockquote
cite="mid:A52C9587-8FF0-42CE-86EA-267981BB76C2@springsup.com"
type="cite">Currently, we have the Repeated<T> type, which
presents a single element as though it were a Collection.
<div class="">
<div class=""><br class="">
</div>
<blockquote style="margin: 0 0 0 40px; border: none; padding:
0px;" class="">
<div class=""><font class="" face="Courier">> for i in
repeatElement(1, count: 3) { print(i) }</font></div>
<div class=""><font class="" face="Courier">1</font></div>
<div class=""><font class="" face="Courier">1</font></div>
<div class=""><font class="" face="Courier">1</font></div>
<div class=""><font class="" face="Courier"><br class="">
</font></div>
<div class=""><font class="" face="Courier">> for i in
repeatElement([1, 2, 3], count: 3) { print(i) }</font></div>
<div class=""><font class="" face="Courier">[1, 2, 3]</font></div>
<div class=""><font class="" face="Courier">[1, 2, 3]</font></div>
<div class=""><font class="" face="Courier">[1, 2, 3]</font></div>
</blockquote>
<div class=""><br class="">
</div>
<div class="">However, we lack the ability for Collections to
repeat their contents in a single list; basically, some kind
of “flatMap” to repeatElement’s “map”. So I’d like to pitch a
new API for repeated values. </div>
<div class=""><br class="">
</div>
<div class="">- We would add a RepeatCollection<C:
Collection> type, which loops over its base Collection a
certain number of times (or until a maximum ‘count’). </div>
<div class=""> Implementation might look something like this (<a
moz-do-not-send="true"
href="https://gist.github.com/karwa/5228974a0b4dfd000a916f0aac2721c6"
class="">https://gist.github.com/karwa/5228974a0b4dfd000a916f0aac2721c6</a>),
except that we’d add some optimised map(), filter() and
contains() functions which apply the algorithm once to the
base and multiply the result.</div>
<div class=""><br class="">
</div>
<div class="">- We would add 3 new functions to all Collections:</div>
<div class=""><br class="">
</div>
</div>
<blockquote style="margin: 0 0 0 40px; border: none; padding:
0px;" class="">
<div class="">
<div class=""><font class="" face="Courier">/// Repeats the
collection <i class="">itself</i> N times.</font></div>
</div>
<div class="">
<div class=""><font class="" face="Courier">///</font></div>
</div>
<div class="">
<div class=""><font class="" face="Courier">func repeated(_
times: Int) ->
RepeatCollection<CollectionOfOne<Self>></font></div>
</div>
<div class="">
<div class=""><font class="" face="Courier"><br class="">
</font></div>
</div>
<div class="">
<div class=""><font class="" face="Courier">/// Repeats the
collection’s <i class="">contents</i> N times.</font></div>
</div>
<div class="">
<div class=""><font class="" face="Courier">///</font></div>
</div>
<div class="">
<div class=""><font class="" face="Courier">func
repeatElements(_ times: Int) ->
RepeatCollection<Self></font></div>
</div>
<div class="">
<div class=""><font class="" face="Courier"><br class="">
</font></div>
</div>
<div class="">
<div class=""><font class="" face="Courier">/// Loops the
collection’s contents to present a Collection of length N.</font></div>
</div>
<div class="">
<div class=""><font class="" face="Courier">///</font></div>
</div>
<div class="">
<div class=""><font class="" face="Courier">func
repeatElements(count: Int) ->
RepeatCollection<Self></font></div>
</div>
</blockquote>
<div class="">
<div class=""><br class="">
</div>
<div class="">- We would replace the existing Repeated<T>
type with a typealias to
RepeatCollection<CollectionOfOne<T>></div>
<div class="">- The existing, top-level repeatElement(T, Int)
function <i class="">could</i> stay, but could also be
replaced with an incantation on CollectionOfOne. I’m fairly
ambivalent about this point - it’d be nice to see the function
go, but the replacement also isn’t obvious.</div>
<div class=""><br class="">
</div>
<div class="">Example usage of the new API:</div>
<div class=""><br class="">
</div>
<blockquote style="margin: 0 0 0 40px; border: none; padding:
0px;" class="">
<div class=""><font class="" face="Courier">// Equivalent to
repeatElement(1, count: 3)</font></div>
<div class=""><font class="" face="Courier"><br class="">
</font></div>
<div class=""><font class="" face="Courier">> for i in
CollectionOfOne(1).repeatElements(3).forEach { print(i) }</font></div>
<div class=""><font class="" face="Courier">1</font></div>
<div class=""><font class="" face="Courier">1</font></div>
<div class=""><font class="" face="Courier">1</font></div>
<div class=""><font class="" face="Courier"><br class="">
</font></div>
<div class=""><font class="" face="Courier">// Equivalent to
repeatElement([1, 2, 3], count: 3)</font></div>
<div class=""><font class="" face="Courier"><br class="">
</font></div>
<div class="">
<div class=""><font class="" face="Courier">> for i in
[1, 2, 3].repeated(3).forEach { print(i) }</font></div>
<div class=""><font class="" face="Courier">[1, 2, 3]</font></div>
<div class=""><span style="font-family: Courier;" class="">[1,
2, 3]</span></div>
<div class=""><span style="font-family: Courier;" class="">[1,
2, 3]</span></div>
</div>
<div class=""><span style="font-family: Courier;" class=""><br
class="">
</span></div>
<div class=""><span style="font-family: Courier;" class="">//
New, flat repetition</span></div>
<div class=""><font class="" face="Courier"><br class="">
</font></div>
<div class=""><font class="" face="Courier">> for i in [1,
2, 3].repeatElements(3) { print(i) }</font></div>
<div class=""><font class="" face="Courier">1</font></div>
<div class=""><font class="" face="Courier">2</font></div>
<div class=""><font class="" face="Courier">3</font></div>
<div class=""><font class="" face="Courier">1</font></div>
<div class=""><font class="" face="Courier">2</font></div>
<div class=""><font class="" face="Courier">3</font></div>
<div class=""><font class="" face="Courier">1</font></div>
<div class=""><font class="" face="Courier">2</font></div>
<div class=""><font class="" face="Courier">3</font></div>
<div class=""><font class="" face="Courier"><br class="">
</font></div>
<div class="">
<div class=""><span style="font-family: Courier;" class="">//
New, flat repetition</span></div>
<div class=""><font class="" face="Courier"><br class="">
</font></div>
<div class=""><font class="" face="Courier">> for i in
[1, 2, 3].repeatElements(count: 4) { print(i) }</font></div>
<div class=""><font class="" face="Courier">1</font></div>
<div class=""><font class="" face="Courier">2</font></div>
<div class=""><font class="" face="Courier">3</font></div>
<div class=""><font class="" face="Courier">1</font></div>
<div class=""><br class="">
</div>
</div>
<div class=""><font class="" face="Courier">// Additional
benefit: you can now repeat slices!</font></div>
<div class=""><font class="" face="Courier"><br class="">
</font></div>
<div class=""><font class="" face="Courier">>
String(“yellow”.characters.dropFirst().dropLast().repeat(times:
3))</font></div>
<div class=""><font class="" face="Courier">“elloelloello"</font></div>
<div class=""><br class="">
</div>
<div class=""><br class="">
</div>
</blockquote>
Thoughts?</div>
<div class=""><br class="">
</div>
<div class="">- Karl</div>
</blockquote>
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