<html><head><meta http-equiv="Content-Type" content="text/html charset=us-ascii"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">The following situation has come up for me now and then: I want to work with groups of types that share a common behavior, but that behavior is not sourced in the implementation of the Self type but rather in a secondary type. Specifically, could Swift be extended to introduce a protocol requirement that discusses how a type is used by a secondary type and not the kind of member provided directly by the type. For example:<div class=""><div class=""><div class=""><br class=""></div><div class=""><div style="margin: 0px; font-size: 15px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">// These are all numbers</span></div><div style="margin: 0px; font-size: 15px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">let</span><span style="font-variant-ligatures: no-common-ligatures" class=""> int32: </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">Int32</span><span style="font-variant-ligatures: no-common-ligatures" class=""> = </span><span style="font-variant-ligatures: no-common-ligatures; color: #272ad8" class="">1; </span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(187, 44, 162);" class="">let</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> int8: </span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(112, 61, 170);" class="">Int8</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> = </span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(39, 42, 216);" class="">1</span></div><div style="margin: 0px; font-size: 15px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">let</span><span style="font-variant-ligatures: no-common-ligatures" class=""> double: </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">Double</span><span style="font-variant-ligatures: no-common-ligatures" class=""> = </span><span style="font-variant-ligatures: no-common-ligatures; color: #272ad8" class="">1.0; </span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(187, 44, 162);" class="">let</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> cgfloat: </span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(112, 61, 170);" class="">CGFloat</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> = </span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(39, 42, 216);" class="">1.0; </span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(187, 44, 162);" class="">let</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> float: </span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(112, 61, 170);" class="">Float</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> = </span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(39, 42, 216);" class="">1</span></div><div style="margin: 0px; font-size: 15px; line-height: normal; font-family: Menlo; min-height: 21px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""></span><br class=""></div><div style="margin: 0px; font-size: 15px; line-height: normal; font-family: Menlo; min-height: 21px;" class="">// They can all be converted to Double using Double.init</div><div style="margin: 0px; font-size: 15px; line-height: normal; font-family: Menlo; color: rgb(112, 61, 170);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">Double</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">(</span><span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">int32</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">); </span><span style="font-variant-ligatures: no-common-ligatures;" class="">Double</span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(0, 0, 0);" class="">(</span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(79, 129, 135);" class="">int8</span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(0, 0, 0);" class="">); </span><span style="font-variant-ligatures: no-common-ligatures;" class="">Double</span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(0, 0, 0);" class="">(</span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(79, 129, 135);" class="">double</span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(0, 0, 0);" class="">); </span><span style="font-variant-ligatures: no-common-ligatures;" class="">Double</span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(0, 0, 0);" class="">(</span><span style="color: rgb(79, 129, 135); font-variant-ligatures: no-common-ligatures;" class="">cgfloat</span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(0, 0, 0);" class="">); </span><span style="font-variant-ligatures: no-common-ligatures;" class="">Double</span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(0, 0, 0);" class="">(</span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(79, 129, 135);" class="">float</span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(0, 0, 0);" class="">)</span></div><div style="margin: 0px; font-size: 15px; line-height: normal; font-family: Menlo; min-height: 21px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""></span><br class=""></div><div style="margin: 0px; font-size: 15px; line-height: normal; font-family: Menlo; min-height: 21px;" class="">// A heterogeneous collection cannot be unified into a single protocol</div><div style="margin: 0px; font-size: 15px; line-height: normal; font-family: Menlo; color: rgb(79, 129, 135);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">let</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> foo: [</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">Any</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">] = [</span><span style="font-variant-ligatures: no-common-ligatures" class="">int32</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">, </span><span style="font-variant-ligatures: no-common-ligatures" class="">int8</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">, </span><span style="font-variant-ligatures: no-common-ligatures" class="">double</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">, </span><span style="font-variant-ligatures: no-common-ligatures" class="">cgfloat</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">, </span><span style="font-variant-ligatures: no-common-ligatures" class="">float</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">]</span></div><div style="margin: 0px; font-size: 15px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">foo</span><span style="font-variant-ligatures: no-common-ligatures" class="">.map{ </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">Double</span><span style="font-variant-ligatures: no-common-ligatures" class="">($0) } // Can't work, Any doesn't make any sense here</span></div><div class=""><br class=""></div><div class="">The kind of thing I am looking for is something like this:</div></div><div class=""><br class=""></div><div class=""><div style="margin: 0px; font-size: 15px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">protocol</span><span style="font-variant-ligatures: no-common-ligatures" class=""> DoubleSource {</span></div><div style="margin: 0px; font-size: 15px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> Double.</span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">init</span><span style="font-variant-ligatures: no-common-ligatures" class="">(</span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">Self</span><span style="font-variant-ligatures: no-common-ligatures" class="">)</span></div><div style="margin: 0px; font-size: 15px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">}</span></div></div><div class=""><br class=""></div><div class="">In other words, the functionality constraint is not provided by the base type but by a second type to which the base type is a parameter.</div><div class=""><br class=""></div><div class="">My use case is for unrelated types (that is, there's no inheritance relationship like you'd find in `UISwitch` and `UISlider`, for example -- both of which are `UIView` and `UIControl`), where there is a secondary type that implements behavior with the same signature for these separate types, such as the Double initializer. Where this pops up the most is in working with Sprite/SceneKit, GamePlayKit, QuartzCore, Accelerate, unifying my numeric values so I can mix and match calls and clean up the flow where some calls require CGPoint, others need float2, etc. Ideally I would be able to </div><div class=""><br class=""></div><div class=""><div style="margin: 0px; font-size: 15px; line-height: normal; font-family: Menlo; color: rgb(79, 129, 135);" class=""><div style="margin: 0px; line-height: normal;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">extension</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> </span><span style="font-variant-ligatures: no-common-ligatures" class="">DoubleSource</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> {</span></div><div style="margin: 0px; line-height: normal;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">func</span><span style="font-variant-ligatures: no-common-ligatures" class=""> bar() -> </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">T</span><span style="font-variant-ligatures: no-common-ligatures" class=""> {</span></div><div style="margin: 0px; line-height: normal; color: rgb(0, 132, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">let</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> value = </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">Double</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">.</span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">init</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">(</span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">self</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">)</span></div><div style="margin: 0px; line-height: normal; color: rgb(0, 132, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> // do something with value; return T of some type</span></div><div style="margin: 0px; line-height: normal;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> }</span></div><div style="margin: 0px; line-height: normal;" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">}</span></div><div style="margin: 0px; font-size: 18px; line-height: normal;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><br class=""></span></div></div><div style="margin: 0px; font-size: 15px; line-height: normal; font-family: Menlo; color: rgb(79, 129, 135);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: rgb(187, 44, 162);" class="">let</span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(0, 0, 0);" class=""> foo: [</span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(112, 61, 170);" class="">DoubleSource</span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(0, 0, 0);" class="">] = [</span><span style="font-variant-ligatures: no-common-ligatures;" class="">int32</span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(0, 0, 0);" class="">, </span><span style="font-variant-ligatures: no-common-ligatures;" class="">int8</span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(0, 0, 0);" class="">, </span><span style="font-variant-ligatures: no-common-ligatures;" class="">double</span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(0, 0, 0);" class="">, </span><span style="font-variant-ligatures: no-common-ligatures;" class="">cgfloat</span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(0, 0, 0);" class="">, </span><span style="font-variant-ligatures: no-common-ligatures;" class="">float</span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(0, 0, 0);" class="">]</span></div><div style="margin: 0px; font-size: 15px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: rgb(79, 129, 135);" class="">foo</span><span style="font-variant-ligatures: no-common-ligatures;" class="">.map{ </span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(112, 61, 170);" class="">bar</span><span style="font-variant-ligatures: no-common-ligatures;" class="">($0) } // would work</span></div></div><div class=""><br class=""></div><div class="">Would something like this be a valuable direction to extend Swift? Is it something found in other languages? Does this kind of requirement have a name? Is it *pragmatically* possible to introduce it in Swift?</div></div><div class=""><br class=""></div><div class="">Thanks in advance for your thoughts and insights.</div><div class=""><br class=""></div><div class="">-- E</div><div class=""><br class=""></div></div></body></html>