<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=""><br class=""><div><blockquote type="cite" class=""><div class="">On Jan 23, 2017, at 7:55 AM, Srđan Rašić 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 dir="ltr" class="">Hi Everyone,<div class=""><br class=""></div><div class="">I've opened a PR (<a href="https://github.com/apple/swift-evolution/pull/591" class="">https://github.com/apple/swift-evolution/pull/591</a>) proposing default generic arguments which I think would be nice addition to the language. They are also mentioned in "Generic manifesto". </div><div class=""><br class=""></div><div class="">The proposal is focusing around generic types. Generic functions are not coved by the proposal and I don't think that we need default generic arguments in generic functions as all the types are always part of the function signature so the compiler can always infer them. One corner case might be if using default argument values in which case support for default generic arguments in functions might be useful.</div></div></div></blockquote><div><br class=""></div><div>The proposal looks fairly straightforward and reasonable. One thing to think about is how it interacts with type inference. For example, consider these examples:</div><div><br class=""></div><div><span class="Apple-tab-span" style="white-space:pre"> </span>struct X<T = Int> { }</div><div><br class=""></div><div><span class="Apple-tab-span" style="white-space:pre"> </span>func f1() -> X<Double> { return X() }</div><div><br class=""></div><div><div><span class="Apple-tab-span" style="white-space: pre;"> </span>func f2() -> X<Int> { return X() }</div><div class=""><div><span class="Apple-tab-span" style="white-space: pre;"> </span>func f2() -> X<Double> { return X() }</div></div><div class=""><br class=""></div><div class=""><span class="Apple-tab-span" style="white-space:pre"> </span>func f3<T>(_: T) -> X<T> { return X() }</div><div class=""><br class=""></div></div><div><span class="Apple-tab-span" style="white-space:pre"> </span>let x1: X = f1() // okay: x1 has type X<Double>?</div><span class="Apple-tab-span" style="white-space: pre;"> </span>let x2: X = f2() // ambiguous?</div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>let x3a: X = f3(1.5) // okay: x3a has type X<Double>?</div><div class=""><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>let x3b: X = f3(1) // okay: x3a has type X<Int>?</div><div class=""><br class=""></div>The type checker already has some notion of “if you can’t infer a particular type, fill in a default” that is used for literals. That rule could be used here… or we could do something else. This should be discussed in the proposal.</div><div class=""><br class=""></div><div class="">Thanks for working on this!</div><div class=""><br class=""></div><div><span class="Apple-tab-span" style="white-space:pre"> </span>- Doug</div><div><br class=""></div><br class=""></body></html>