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<!--StartFragment--><p class="MsoNormal">I guess it is the terminology of “exactly equivalent” is
where you are losing me because I do agree that our two code samples are
functionally equivalent but I would be hard pressed to say they are exactly
equivalent. </p><p class="MsoNormal">Before I go on, I hope it is all right to have this
discussion on this group because I do enjoy friendly discussions like
this. I believe it helps everyone to see
multiple points of view and to see advantages to both sides in a friendly
manner.</p><p class="MsoNormal">Lets say that a year from now we receive requirements that
some of our types will have three numbers while others will stay at two
numbers. We also have the requirement
that the math function will be preformed on all three numbers for the types
that contain three numbers. With
protocol extensions, using protocol composition and constraints we could rewrite our code like this:</p><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">protocol</span> TestProtocol1 {</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""> <span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">var</span> num1: <span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">Int</span> {<span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">get</span> <span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">set</span>}</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""> <span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">var</span> num2: <span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">Int</span> {<span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">get</span> <span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">set</span>}</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class="">}</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">protocol</span> TestProtocol2 {</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""> <span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">var</span> num3: <span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">Int</span> {<span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">get</span> <span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">set</span>}</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class="">}</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">extension</span> <span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">TestProtocol1</span> <span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">where</span> <span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">Self</span>: TestProtocol2 {</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""> <span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">func</span> doMath() -> <span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">Int</span> {</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""> <span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">return</span> <span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">num1</span> + <span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">num2</span> + <span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">num3</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""> }</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class="">}</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(79, 129, 135);" 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>TestProtocol1<span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> {</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""> <span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">func</span> doMath() -> <span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">Int</span> {</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""> <span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">return</span> <span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">num1</span> + <span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">num2</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""> }</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class="">}</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(79, 129, 135);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">struct</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> MyTypeOne: </span>TestProtocol1<span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">, </span>TestProtocol2<span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> {</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""> <span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">var</span> num1 = <span style="font-variant-ligatures: no-common-ligatures; color: #272ad8" class="">1</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""> <span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">var</span> num2 = <span style="font-variant-ligatures: no-common-ligatures; color: #272ad8" class="">2</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""> <span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">var</span> num3 = <span style="font-variant-ligatures: no-common-ligatures; color: #272ad8" class="">3</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class="">}</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">struct</span> MyTypeTwo: <span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">TestProtocol1</span> {</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""> <span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">var</span> num1 = <span style="font-variant-ligatures: no-common-ligatures; color: #272ad8" class="">1</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""> <span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">var</span> num2 = <span style="font-variant-ligatures: no-common-ligatures; color: #272ad8" class="">2</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class="">}</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">var</span> one = <span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">MyTypeOne</span>()</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">var</span> two = <span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">MyTypeTwo</span>()</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(209, 47, 27);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #3d1d81" class="">print</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">(</span>"MyTypeOne: <span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">\</span>(<span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">one</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">.</span><span style="font-variant-ligatures: no-common-ligatures; color: #31595d" class="">doMath</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">()</span>)"<span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">)</span></div><p class="MsoNormal"><span style="font-family: Menlo; font-size: 11px; color: rgb(61, 29, 129);" class="">print</span><span style="font-family: Menlo; font-size: 11px;" class="">(</span><span style="color: rgb(209, 47, 27); font-family: Menlo; font-size: 11px;" class="">"MyTypeTwo: </span><span style="font-family: Menlo; font-size: 11px;" class="">\</span><span style="color: rgb(209, 47, 27); font-family: Menlo; font-size: 11px;" class="">(</span><span style="font-family: Menlo; font-size: 11px; color: rgb(79, 129, 135);" class="">two</span><span style="font-family: Menlo; font-size: 11px;" class="">.</span><span style="font-family: Menlo; font-size: 11px; color: rgb(49, 89, 93);" class="">doMath</span><span style="font-family: Menlo; font-size: 11px;" class="">()</span><span style="color: rgb(209, 47, 27); font-family: Menlo; font-size: 11px;" class="">)"</span><span style="font-family: Menlo; font-size: 11px;" class="">)</span> </p><p class="MsoNormal">This example would print out:</p><div class=""><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><b class="">MyTypeOne: 6</b></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><b class="">MyTypeTwo: 3</b></div></div><p class="MsoNormal">We could create a global function and use the “is” operator
within the function to test if the type conforms to the TestProtocol2 protocol
or not however I believe that the protocol extensions approach using protocol
composition and constraints like this is much cleaner and easier to manage than
the global function approach. </p><p class="MsoNormal">So I would definitely agree that functionally they can be equivalent but, in my opinion, I just have trouble agreeing that they are exactly
equivalent. <o:p class=""></o:p></p><p class="MsoNormal"><br class=""></p><p class="MsoNormal">Jon</p>
<!--EndFragment--><div><blockquote type="cite" class=""><div class="">On Feb 16, 2016, at 10:36 PM, Daniel Tartaglia <<a href="mailto:danielt1263@gmail.com" class="">danielt1263@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><meta http-equiv="Content-Type" content="text/html charset=utf-8" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">And your example is exactly equivalent to the below:<div class=""><br class=""></div><div class=""><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">protocol</span> TestProtocol {</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span class="Apple-tab-span" style="white-space:pre"> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">var</span> num1: <span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">Int</span> {<span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">get</span> <span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">set</span>}</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span class="Apple-tab-span" style="white-space:pre"> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">var</span> num2: <span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">Int</span> {<span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">get</span> <span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">set</span>}</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class="">}</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">func</span> doMath(t: <span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">TestProtocol</span>) -> <span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">Int</span> {</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span class="Apple-tab-span" style="white-space:pre"> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">return</span> t.<span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">num1</span> + t.<span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">num2</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class="">}</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">struct</span> MyTypeOne: <span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">TestProtocol</span> {</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span class="Apple-tab-span" style="white-space:pre"> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">var</span> num1 = <span style="font-variant-ligatures: no-common-ligatures; color: #272ad8" class="">1</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span class="Apple-tab-span" style="white-space:pre"> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">var</span> num2 = <span style="font-variant-ligatures: no-common-ligatures; color: #272ad8" class="">2</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class="">}</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">struct</span> MyTypeTwo: <span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">TestProtocol</span> {</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span class="Apple-tab-span" style="white-space:pre"> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">var</span> num1 = <span style="font-variant-ligatures: no-common-ligatures; color: #272ad8" class="">1</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span class="Apple-tab-span" style="white-space:pre"> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">var</span> num2 = <span style="font-variant-ligatures: no-common-ligatures; color: #272ad8" class="">2</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span class="Apple-tab-span" style="white-space:pre"> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">func</span> doMath() -> <span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">Int</span> {</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(79, 129, 135);" class=""><span style="" class=""><span class="Apple-tab-span" style="white-space:pre"> </span></span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">return</span><span style="" class=""> </span>num2<span style="" class=""> - </span>num1</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>}</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class="">}</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">var</span> one = <span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">MyTypeOne</span>()</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">var</span> two = <span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">MyTypeTwo</span>()</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(209, 47, 27);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #3d1d81" class="">print</span><span style="" class="">(</span>"MyTypeOne: <span style="" class="">\</span>(<span style="font-variant-ligatures: no-common-ligatures; color: #31595d" class="">doMath</span><span style="" class="">(</span><span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">one</span><span style="" class="">)</span>)"<span style="" class="">)</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(209, 47, 27);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #3d1d81" class="">print</span><span style="" class="">(</span>"MyTypeTwo: <span style="" class="">\</span>(<span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">two</span><span style="" class="">.</span><span style="font-variant-ligatures: no-common-ligatures; color: #31595d" class="">doMath</span><span style="" class="">()</span>)"<span style="" class="">)</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(79, 129, 135);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">var</span><span style="" class=""> t: </span>TestProtocol<span style="" class=""> = </span>two</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(209, 47, 27);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #3d1d81" class="">print</span><span style="" class="">(</span>"TestProtocol <span style="" class="">\</span>(<span style="font-variant-ligatures: no-common-ligatures; color: #31595d" class="">doMath</span><span style="" class="">(</span><span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">t</span><span style="" class="">)</span>)"<span style="" class="">)</span></div><div class=""><span style="" class=""><br class=""></span></div><div class="">What the protocol extension gives you, all that it gives you, is a different syntax for doing the exact same thing. Where your code shines IMHO is when you want to then use the return value as a parameter to yet another function.</div><div class=""><br class=""></div><div class=""><div style="margin: 0px; line-height: normal; color: rgb(49, 89, 93);" class=""><span style="font-family: Menlo; font-size: 11px; color: rgb(79, 129, 135);" class="">two</span><span style="font-family: Menlo; font-size: 11px;" class="">.</span><font face="Menlo" class=""><span style="font-size: 11px;" class="">doMath</span></font><span style="font-family: Menlo; font-size: 11px;" class="">().</span><font face="Menlo" class=""><span style="font-size: 11px;" class="">handleResult</span></font><span style="" class=""><font face="Menlo" class=""><span style="font-size: 11px;" class="">() </span></font>is far cleaner, and better expresses what is happening, than<font face="Menlo" class=""><span style="font-size: 11px;" class=""> </span></font></span><font face="Menlo" class=""><span style="font-size: 11px;" class="">handleResult</span></font><span style="font-family: Menlo; font-size: 11px;" class="">(</span><font face="Menlo" class=""><span style="font-size: 11px;" class="">doMath</span></font><span style="font-family: Menlo; font-size: 11px;" class="">(</span><span style="font-family: Menlo; font-size: 11px; color: rgb(79, 129, 135);" class="">one</span><span style="font-family: Menlo; font-size: 11px;" class="">))</span></div></div><div class=""><span style="" class=""><br class=""></span></div><div class=""><span style="" class="">All IMHO of course.</span></div><div class=""><br class=""></div><div class=""><blockquote type="cite" class=""><div class="">On Feb 16, 2016, at 9:28 PM, Jon Hoffman <<a href="mailto:hoffman.jon@gmail.com" class="">hoffman.jon@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><meta http-equiv="Content-Type" content="text/html charset=utf-8" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">You are correct that if you are using the protocol type to interact with the instance then the method defined in the protocol extension will be called however if it is type casted to a concrete type which has a method that overrides the method in the protocol extension then the method in the concrete type would be called. This following code illustrates this:<div class=""><br class=""></div><div class=""><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">protocol</span> TestProtocol {</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""> <span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">var</span> num1: <span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">Int</span> {<span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">get</span> <span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">set</span>}</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""> <span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">var</span> num2: <span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">Int</span> {<span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">get</span> <span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">set</span>}</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class="">}</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(79, 129, 135);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">extension</span><span style="" class=""> </span>TestProtocol<span style="" class=""> {</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""> <span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">func</span> doMath() -> <span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">Int</span> {</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""> <span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">return</span> <span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">num1</span> + <span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">num2</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""> }</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class="">}</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">struct</span> MyTypeOne: <span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">TestProtocol</span> {</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""> <span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">var</span> num1 = <span style="font-variant-ligatures: no-common-ligatures; color: #272ad8" class="">1</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""> <span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">var</span> num2 = <span style="font-variant-ligatures: no-common-ligatures; color: #272ad8" class="">2</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class="">}</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">struct</span> MyTypeTwo: <span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">TestProtocol</span> {</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""> <span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">var</span> num1 = <span style="font-variant-ligatures: no-common-ligatures; color: #272ad8" class="">1</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""> <span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">var</span> num2 = <span style="font-variant-ligatures: no-common-ligatures; color: #272ad8" class="">2</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""> <span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">func</span> doMath() -> <span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">Int</span> {</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""> <span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">return</span> <span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">num2</span> - <span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">num1</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""> }</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class="">}</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">var</span> one = <span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">MyTypeOne</span>()</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">var</span> two = <span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">MyTypeTwo</span>()</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(209, 47, 27);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #3d1d81" class="">print</span><span style="" class="">(</span>"MyTypeOne: <span style="" class="">\</span>(<span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">one</span><span style="" class="">.</span><span style="font-variant-ligatures: no-common-ligatures; color: #31595d" class="">doMath</span><span style="" class="">()</span>)"<span style="" class="">)</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(209, 47, 27);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #3d1d81" class="">print</span><span style="" class="">(</span>"MyTypeTwo: <span style="" class="">\</span>(<span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">two</span><span style="" class="">.</span><span style="font-variant-ligatures: no-common-ligatures; color: #31595d" class="">doMath</span><span style="" class="">()</span>)"<span style="" class="">)</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(79, 129, 135);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">var</span><span style="" class=""> t: </span>TestProtocol<span style="" class=""> = </span>two</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(209, 47, 27);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #3d1d81" class="">print</span><span style="" class="">(</span>"TestProtocol <span style="" class="">\</span>(<span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">t</span><span style="" class="">.</span><span style="font-variant-ligatures: no-common-ligatures; color: #31595d" class="">doMath</span><span style="" class="">()</span>)"<span style="" class="">)</span></div></div><div class=""><br class=""></div><div class="">The output from this code would be:</div><div class=""><br class=""></div><div class=""><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><b class="">MyTypeOne: 3</b></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><b class="">MyTypeTwo: 1</b></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><b class="">TestProtocol 3</b></div></div><div class=""><br class=""></div><div class="">This shows that when calling the doMath() on the MyTypeTwo instance calls the method defined in the MyTypeTwo structure however if we call the method on the same instance but it being typecast as the protocol will call the method in the protocol.</div><div class=""><br class=""></div><div class="">For the most part we would access the types though the interface provided by the protocol however using the “is” keyword we can check if a type is an instance of a particular concrete type or use the typecast operator to downcast the instance. This would give us access to the method that is defined in the concrete type. This can certainly be useful in certain situations. I do agree that these situation should be avoided however it is something that can be done which we can not do with global functions. I was just trying to list some of the differences between protocol extensions and global functions. Honestly, and this is my personal opinion, I believe there is significant difference between protocol extensions and global functions: avoiding the global scope, using constraints, better code organization to name a couple, overriding, self parameter, better code manageability just to name some of them.</div><div class=""><br class=""></div><div class="">Protocol Extensions are there so we can add functionality to a group of types that conform to a specific protocol that meets any constraints defined by the extension. This allows us to avoid duplicate code similar to what we did with super classes in the OOP world. However with protocols we can create very specific protocols rather than large monolithic superclasses. Without protocol extensions we would have to revert to creating global functions however, in my opinion once again, the protocol extensions is a better option. That is what I was trying to get across.</div><div class=""><br class=""></div><div class="">Jon</div><div class=""> </div><div class=""><br class=""><div class=""><blockquote type="cite" class=""><div class="">On Feb 16, 2016, at 8:30 PM, Brent Royal-Gordon <<a href="mailto:brent@architechies.com" class="">brent@architechies.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div class=""><blockquote type="cite" class="">We can override functionality provided from a protocol extension if we need specific functionality for a particular type.<br class=""></blockquote><br class="">No you can't. Protocol extension methods (that is, methods declared only in an extension, as opposed to default implementations, which are both declared in the protocol and defined in an extension) are statically dispatched, so calling them on the protocol witness will always use the extension's implementation, even if some concrete type conforming to the protocol has a different implementation with the same name and signature.<br class=""><br class="">(I've worked on a proposal in the past that would require keywords to make this behavior clearer, but I've had trouble getting it through.)<br class=""><br class="">In any case, I'm pretty sure that's what Daniel means when he says extension methods are exactly equivalent to global functions. Sure, they appear after a dot, have an implicit self parameter, and are scoped to a particular type, but there's nothing dynamic about their behavior—they're not overridable in any useful sense.<br class=""><br class="">-- <br class="">Brent Royal-Gordon<br class="">Architechies<br class=""><br class=""></div></div></blockquote></div><br class=""></div></div></div></blockquote></div><br class=""></div></div></div></blockquote></div><br class=""></body></html>