<div dir="ltr"><div class="gmail_default" style="font-family:georgia,serif">Vatsal thinks the default works as a default value keyword, which restricts that the implementation of the function must have a default value. The default value is not set in the protocol. It is a restrict keyword. </div><div class="gmail_default" style="font-family:georgia,serif"><br></div><div class="gmail_default" style="font-family:georgia,serif">Vatsal thinks that will be a good idea, as that will make Protocol like, </div><div class="gmail_default" style="font-family:georgia,serif"><br></div><div class="gmail_default" style="font-family:georgia,serif">protocol A {</div><div class="gmail_default" style="font-family:georgia,serif"> func foo()</div><div class="gmail_default" style="font-family:georgia,serif"> func foo(bar: String)</div><div class="gmail_default" style="font-family:georgia,serif">}</div><div class="gmail_default" style="font-family:georgia,serif"><br></div><div class="gmail_default" style="font-family:georgia,serif">simplified as </div><div class="gmail_default" style="font-family:georgia,serif"><br></div><div class="gmail_default" style="font-family:georgia,serif">protocol A {</div><div class="gmail_default" style="font-family:georgia,serif"> func foo(bar: String = default)</div><div class="gmail_default" style="font-family:georgia,serif">}</div><div class="gmail_default" style="font-family:georgia,serif"><br></div><div class="gmail_default" style="font-family:georgia,serif">So if you implement protocol A</div><div class="gmail_default" style="font-family:georgia,serif"><br></div><div class="gmail_default" style="font-family:georgia,serif">struct X:A {</div><div class="gmail_default" style="font-family:georgia,serif"> func foo(bar: String) { // will show an error as protocol A restricts a default value</div><div class="gmail_default" style="font-family:georgia,serif"> ..</div><div class="gmail_default" style="font-family:georgia,serif"> }</div><div class="gmail_default" style="font-family:georgia,serif">}</div><div class="gmail_default" style="font-family:georgia,serif"><br></div><div class="gmail_default" style="font-family:georgia,serif">However, there is also a pitfall here. If foo() is not a foo(bar:String) with a default value, but some operations without using the bar parameter? Then the logic is not the same. You can't define another func foo() as the compiler can not distinguish from foo() to foo(bar:String = "John") with default value.</div><div class="gmail_default" style="font-family:georgia,serif"><br></div><div class="gmail_default" style="font-family:georgia,serif">zhaoxin</div></div><div class="gmail_extra"><br><div class="gmail_quote">On Sat, Jan 16, 2016 at 7:35 PM, Goffredo Marocchi <span dir="ltr"><<a href="mailto:panajev@gmail.com" target="_blank">panajev@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="auto"><div>I still think that, except in certain very generic cases, default methods are something I would be wary of being easily abused.</div><div><br></div><div>Protocols, Java style interfaces, they allow users to focus only on a generic behaviour/contract without having to rely or being able to rely and/or make bonding assumptions on any implementation details of the type conforming to the protocol. Default methods in a protocol still seem to go in the opposite direction although they do offer a lot of convenience and open up new styles.<br><br>Sent from my iPhone</div><div><div class="h5"><div><br>On 16 Jan 2016, at 11:04, Haravikk via swift-evolution <<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>> wrote:<br><br></div><blockquote type="cite"><div><div>I think that the point of allowing defaults in protocols is so that you can assume a default for all types conforming to that protocol. To use your example, if you receive an instance of Name, you can only call printSomething() without arguments if you test that it is an instance of type Name. If instead you test its conformance to the Good protocol (which you might do if there are a lot of different types conforming to Good) then you have to provide a value, because you can’t infer that every possible implementation will have a default.</div><div><br></div><div>Regarding this proposal however I think it might be useful to have a distinction between a protocol function that specifies a default value for all implementations (that they must all conform to) versus one that specifies that implementations must have a default value, but not what that value must be.</div><div><br></div><div>For example, to have a fixed and altered default we currently we have to do things like this:</div><div><br></div><div><div>protocol Protocol {</div><div><span style="white-space:pre-wrap"> </span>func functionWithSpecificDefault(argument:String)</div><div><span style="white-space:pre-wrap"> </span>func functionWithAnyDefault(argument:String)</div><div>}</div></div><div><br></div><div>extension Protocol {</div><div><span style="white-space:pre-wrap"> </span>func functionWithSpecificDefault() { self.functionWithSpecificDefault(“Foo”) }</div><div><span style="white-space:pre-wrap"> </span>func functionWithAnyDefault() { self.functionWithAnyDefault(“Foo”) }</div><div>}</div><div><br></div><div>class MyClass : Protocol {</div><span style="white-space:pre-wrap"> </span>func functionWithSpecificDefault(argument:String) { /* Implementation here */ }<div><span style="white-space:pre-wrap"> </span>func functionWithAnyDefault(argument:String) { /* Implementation here */ }</div><div><br><div><span style="white-space:pre-wrap"> </span>func functionWithAnyDefault() { self.functionWithAnyDefault(“Bar”) } // Override default</div><div>}</div><div><br></div><div>Which could be replaced by:</div><div><br></div><div>protocol Protocol {</div><div><span style="white-space:pre-wrap"> </span>func functionWithSpecificDefault(argument:String = “Foo")</div><div><span style="white-space:pre-wrap"> </span>func functionWithAnyDefault(argument:String = default)</div><div>}</div><div><br></div><div>class MyClass : Protocol {</div><span style="white-space:pre-wrap"> </span>func functionWithSpecificDefault(argument:String = “Foo") { /* Implementation here */ }<div><span style="white-space:pre-wrap"> </span>func functionWithAnyDefault(argument:String = “Bar") { /* Implementation here */ }</div><div>}</div><div><br></div><div>However, this has the added advantage that implementing functionWithSpecificDefault with a default other than “Foo” would cause a compiler error, while doing so for functionWithAnyDefault would not (but specifying no default at all would, as one is required).</div><br><div><blockquote type="cite"><div>On 16 Jan 2016, at 10:15, 肇鑫 via swift-evolution <<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>> wrote:</div><br><div><div dir="ltr"><div class="gmail_default" style="font-family:georgia,serif">No. Although you protocol's function doesn't has a default parameter value. Your implementation does. So you don't need to define another func function() in your protocol.</div><div class="gmail_default" style="font-family:georgia,serif"><br></div><div class="gmail_default"><div class="gmail_default"><font face="georgia, serif">protocol Good {</font></div><div class="gmail_default"><font face="georgia, serif"> func printSomething(something:String)</font></div><div class="gmail_default"><font face="georgia, serif">}</font></div><div class="gmail_default"><font face="georgia, serif"><br></font></div><div class="gmail_default"><font face="georgia, serif">struct Name:Good {</font></div><div class="gmail_default"><font face="georgia, serif"> func printSomething(something: String = "John") {</font></div><div class="gmail_default"><font face="georgia, serif"> print(something)</font></div><div class="gmail_default"><font face="georgia, serif"> }</font></div><div class="gmail_default"><font face="georgia, serif">}</font></div><div class="gmail_default"><font face="georgia, serif"><br></font></div><div class="gmail_default"><font face="georgia, serif">Name().printSomething()</font></div><div class="gmail_default"><font face="georgia, serif"><br></font></div><div class="gmail_default"><font face="georgia, serif">above code works.</font></div><div class="gmail_default"><font face="georgia, serif"><br></font></div><div class="gmail_default"><font face="georgia, serif">zhaoxin</font></div></div></div><div class="gmail_extra"><br><div class="gmail_quote">On Sat, Jan 16, 2016 at 6:05 PM, Vatsal Manot <span dir="ltr"><<a href="mailto:vatsal.manot@yahoo.com" target="_blank">vatsal.manot@yahoo.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word"><div>It serves as a better (if not simpler) substitute for the following pattern:</div><div> </div><span>protocol Protocol<br></span><span>{<br></span><span> typealias Argument<br></span><span> <br></span><span> func function()<br></span><span> func function(_: Argument)<br></span><div><div><span>}<br></span><span> <br></span><div><div><blockquote type="cite"><div>On 16-Jan-2016, at 3:29 PM, 肇鑫 <<a href="mailto:owenzx@gmail.com" target="_blank">owenzx@gmail.com</a>> wrote:</div><br><div><div dir="ltr"><div class="gmail_default" style="font-family:georgia,serif">I wonder where is the good for a protocol designer on this?<br></div><div class="gmail_default" style="font-family:georgia,serif"><br></div><div class="gmail_default" style="font-family:georgia,serif">zhaoxin</div><div class="gmail_extra"><br><div class="gmail_quote">On Sat, Jan 16, 2016 at 5:23 PM, Vatsal Manot via swift-evolution <span dir="ltr"><<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Currently, the following the code fails with multiple errors:<br>
<br>
protocol Protocol<br>
{<br>
typealias Argument<br>
<br>
func function(argument: Argument = default)<br>
}<br>
<br>
I propose that we allow protocols to require functions with default parameter values. I can’t see any disadvantages to this, and the change would only be additive.<br>
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</blockquote></div><br><br clear="all"><div><br></div>-- <br><div><div dir="ltr"><div><br>Owen Zhao<br></div></div></div>
</div></div>
</div></blockquote></div><br></div></div></div></div></blockquote></div><br><br clear="all"><div><br></div>-- <br><div><div dir="ltr"><div><br>Owen Zhao<br></div></div></div>
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