<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">Hi Jon,<div class=""><br class=""></div><div class="">Thank you for the feedback, this is really valuable! A couple questions below.</div><div class=""><br class=""><div><blockquote type="cite" class=""><div class="">On Feb 17, 2017, at 8:50 PM, Jonathan Hull via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">Thoughts inline.</span><div class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;"><br class=""><div class=""><blockquote type="cite" class=""><div class="">On Feb 16, 2017, at 4:26 PM, Ben Cohen via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><p class="" style="box-sizing: border-box; margin-bottom: 16px; color: rgb(51, 51, 51); font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 14px; background-color: rgb(255, 255, 255); margin-top: 0px !important;">Hi swift-evolution,</p><p class="" style="box-sizing: border-box; margin-bottom: 16px; color: rgb(51, 51, 51); font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 14px; background-color: rgb(255, 255, 255); margin-top: 0px !important;">Following up on Ted’s post regarding the opening up of stage 2, I’m starting a thread to discuss improvements to the <code class="" style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 11.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">Dictionary</code> type.</p><p class="" style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(51, 51, 51); font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 14px; background-color: rgb(255, 255, 255);">Here is a list of commonly requested changes/enhancements to <code class="" style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 11.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">Dictionary</code>, all of which would probably be appropriate to put together into a single evolution proposal:</p><ul class="" style="box-sizing: border-box; padding-left: 2em; margin-top: 0px; margin-bottom: 16px; color: rgb(51, 51, 51); font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 14px; background-color: rgb(255, 255, 255);"><li class="" style="box-sizing: border-box;"><code class="" style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 11.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">init</code> from/<code class="" style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 11.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">merge</code> in a <code class="" style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 11.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">Sequence</code> of <code class="" style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 11.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">Key/Value</code> pairs (already raised as SE-100: <a href="https://github.com/apple/swift-evolution/blob/master/proposals/0100-add-sequence-based-init-and-merge-to-dictionary.md" class="" style="box-sizing: border-box; background-color: transparent; -webkit-text-decoration-skip: objects; color: rgb(64, 120, 192); text-decoration: none;">https://github.com/apple/swift-evolution/blob/master/proposals/0100-add-sequence-based-init-and-merge-to-dictionary.md</a>).</li></ul></div></div></blockquote><div class="">+1. I have wanted this since Swift 1.</div><br class=""><blockquote type="cite" class=""><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><ul class="" style="box-sizing: border-box; padding-left: 2em; margin-top: 0px; margin-bottom: 16px; color: rgb(51, 51, 51); font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 14px; background-color: rgb(255, 255, 255);"><li class="" style="box-sizing: border-box; margin-top: 0.25em;">make the <code class="" style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 11.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">Values</code> view collection a <code class="" style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 11.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">MutableCollection</code> (as in this PR: <a href="https://github.com/apple/swift-evolution/pull/555" class="" style="box-sizing: border-box; background-color: transparent; -webkit-text-decoration-skip: objects; color: rgb(64, 120, 192); text-decoration: none;">https://github.com/apple/swift-evolution/pull/555</a>).</li></ul></div></div></blockquote>I think Nate’s proposal covers this case well.</div><div class=""><br class=""><blockquote type="cite" class=""><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><ul class="" style="box-sizing: border-box; padding-left: 2em; margin-top: 0px; margin-bottom: 16px; color: rgb(51, 51, 51); font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 14px; background-color: rgb(255, 255, 255);"><li class="" style="box-sizing: border-box; margin-top: 0.25em;">Add a defaulting <code class="" style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 11.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">subscript</code> <code class="" style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 11.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">get</code> (e.g. <code class="" style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 11.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">counts[key, default: 0] += 1</code> or <code class="" style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 11.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">grouped(key, default:[]].append(value)</code>).</li></ul></div></div></blockquote>I am indifferent to this. I am happy using ??. I guess it could be slightly more efficient because it avoids wrapping and unwrapping the optional.</div><div class=""><br class=""><blockquote type="cite" class=""><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><ul class="" style="box-sizing: border-box; padding-left: 2em; margin-top: 0px; margin-bottom: 16px; color: rgb(51, 51, 51); font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 14px; background-color: rgb(255, 255, 255);"><li class="" style="box-sizing: border-box; margin-top: 0.25em;">Add a group by-like init to create a <code class="" style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 11.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">Dictionary<K,[V]></code> from a sequence of <code class="" style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 11.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">V</code> and a closure <code class="" style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 11.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">(V)->K</code>.</li></ul></div></div></blockquote>+1. I would use this.</div><div class=""><br class=""><blockquote type="cite" class=""><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><ul class="" style="box-sizing: border-box; padding-left: 2em; margin-top: 0px; margin-bottom: 16px; color: rgb(51, 51, 51); font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 14px; background-color: rgb(255, 255, 255);"><li class="" style="box-sizing: border-box; margin-top: 0.25em;">Add <code class="" style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 11.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">Dictionary.filter</code> to return a <code class="" style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 11.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">Dictionary</code>.</li></ul></div></div></blockquote>+1.</div><div class=""><br class=""><blockquote type="cite" class=""><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><ul class="" style="box-sizing: border-box; padding-left: 2em; margin-top: 0px; margin-bottom: 16px; color: rgb(51, 51, 51); font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 14px; background-color: rgb(255, 255, 255);"><li class="" style="box-sizing: border-box; margin-top: 0.25em;">Add <code class="" style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 11.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">Dictionary.mapValues</code> to return a <code class="" style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 11.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">Dictionary</code> (can be more efficiently implemented than composition as the storage layout remains the same).</li></ul></div></div></blockquote><div class="">+1000. I have also been asking for this since the beginning. I built my own version (and use it frequently), but as you say, the standard library can do it much more efficiently.</div><div class=""><br class=""></div><div class="">I would also like to see an in-place version as well.</div><div class=""><br class=""></div><div class="">One design detail. Even though it is only mapping the values, I would like it to pass the key to the closure as well. It occasionally figures in to the mapping logic.</div></div></div></div></blockquote><div><br class=""></div>Do you have any examples you can share where you use the key in this type of map? I'm not contesting its usefulness; it would just be helpful to see some real-world usage.<br class=""><div><br class=""></div><blockquote type="cite" class=""><div class=""><div class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;"><div class=""><blockquote type="cite" class=""><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><ul class="" style="box-sizing: border-box; padding-left: 2em; margin-top: 0px; margin-bottom: 16px; color: rgb(51, 51, 51); font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 14px; background-color: rgb(255, 255, 255);"><li class="" style="box-sizing: border-box; margin-top: 0.25em;">Add <code class="" style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 11.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">capacity</code> property and <code class="" style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 11.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">reserveCapacity()</code> method.</li></ul></div></div></blockquote><div class="">+0.5. I could see it being useful occasionally.</div><br class=""><blockquote type="cite" class=""><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><ul class="" style="box-sizing: border-box; padding-left: 2em; margin-top: 0px; margin-bottom: 16px; color: rgb(51, 51, 51); font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 14px; background-color: rgb(255, 255, 255);"><li class="" style="box-sizing: border-box; margin-top: 0.25em;">Have <code class="" style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 11.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">Dictionary.removeAtIndex</code> return the <code class="" style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 11.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">Index</code> of the next entry.</li></ul></div></div></blockquote><div class="">No opinion on this.</div><br class=""><blockquote type="cite" class=""><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><ul class="" style="box-sizing: border-box; padding-left: 2em; margin-top: 0px; margin-bottom: 16px; color: rgb(51, 51, 51); font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 14px; background-color: rgb(255, 255, 255);"><li class="" style="box-sizing: border-box; margin-top: 0.25em;">(once we have conditional conformance) Make dictionaries with <code class="" style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 11.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">Equatable</code> values <code class="" style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 11.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">Equatable</code>.</li></ul></div></div></blockquote><div class="">+1</div><br class=""><blockquote type="cite" class=""><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><p class="" style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(51, 51, 51); font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 14px; background-color: rgb(255, 255, 255);">Please reply here with any comments or questions on the above list, or any additions you believe are important that are missing from it.</p></div></div></blockquote><div class="">I would also like to see a version of map which returns a dictionary and handles key collisions:</div><div class=""><br class=""></div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>let newDict = myDict.map(collision: {k,v1,v2 in v2}) { (k,v) in ... }</div><div class=""><br class=""></div><div class="">The collision parameter would take a throwing closure and handle the case of a key conflict (by returning the value to use, throwing, or trapping). It would have a default value so that it would only have to be specified if a different behavior was desired. </div><div class=""><br class=""></div><div class="">In advanced cases, the collision could be used to accumulate values together. Because of this, I would actually like to see this on *collection* (not just dictionary). The map closure is handed each element of the sequence (which in the case of dictionary is a key/value tuple), and expects a return value of a key/value tuple. The collision block is called when a key is returned which has already been used to figure out what value to use. This might choose a winner, or it could act like reduce, building a value from the components.</div></div></div></div></blockquote><div><br class=""></div><div>I think the uses you're describing are handled by the merging initializer proposed in SE-100:</div><div><span class="Apple-tab-span" style="white-space:pre"> </span><a href="https://github.com/apple/swift-evolution/blob/master/proposals/0100-add-sequence-based-init-and-merge-to-dictionary.md" class="">https://github.com/apple/swift-evolution/blob/master/proposals/0100-add-sequence-based-init-and-merge-to-dictionary.md</a></div><div><br class=""></div><div>For example, you can use a combining closure to select specific elements when the keys collide:</div><div><br class=""></div></div></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><div><div>let duplicates: DictionaryLiteral = ["a": 1, "b": 2, "a": 3, "b": 4]</div></div></div><div class=""><div><div>// Using the first value only</div></div></div><div class=""><div><div>Dictionary(merging: duplicates, combine: { (first, _) in first }) // ["b": 2, "a": 1]</div></div></div><div class=""><div><div>// Using the maximum value</div></div></div><div class=""><div><div>Dictionary(merging: duplicates, combine: max) // ["b": 4, "a": 3]</div></div></div></blockquote><div class=""><div><div><br class=""></div><div>or to calculate the frequencies of values in a sequence:</div><div><br class=""></div></div></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><div><div>extension Sequence where Iterator.Element: Hashable {</div></div></div><div class=""><div><div><div class=""> func frequencies() -> [Iterator.Element: Int] {</div></div></div></div><div class=""><div><div><div class=""> return Dictionary(merging: self.lazy.map { v in (v, 1) }, combine: +)</div></div></div></div><div class=""><div><div><div class=""> }</div></div></div></div><div class=""><div><div><div class="">}</div></div></div></div><div class=""><div><div><div class="">[1, 2, 2, 3, 1, 2, 4, 5, 3, 2, 3, 1].frequencies()</div></div></div></div><div class=""><div><div>// [2: 4, 4: 1, 5: 1, 3: 3, 1: 3]</div></div></div></blockquote><div class=""><div><div><br class=""></div><div>Could you take a look and see if that provides the functionality you're looking for?</div><br class=""><blockquote type="cite" class=""><div class=""><div class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;"><div class=""><div class="">As a concrete example of what this allows, I could take in an array of words/strings [“apple”, “aardvark”, …] and then do the following to get a count of how many words start with each letter:</div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span></div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>let letterFrequency = words.map(collision:{$1+$2}) { (String($0.characters.first ?? “”) ,1)}</div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span></div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>print(letterFrequency[“a”]) //number of words starting with “a"</div><div class=""><br class=""></div><div class="">I am ok using a term other than ‘map' if that is easier on the compiler, but I would like this functionality. At the simple end, it allows map functionality for both keys and values. At the advanced end, it acts as a categorizing reduce over a sequence/collection.</div><div class=""><br class=""></div><div class="">You can even trivially implement the proposed groupBy with it (this is on Collection, but you could do an init on dict the same way):</div><div class=""><br class=""></div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>func grouped<K>(by categorizer: (Element)->K ) -> [K:Element] {</div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>return self.map(collision:{$1+$2}) {(categorizer($0), [$0])}</div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>}</div></div></div></div></blockquote><br class=""></div></div><div>Thanks!</div><div>Nate</div></body></html>