<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="">I'm sure you heard this request like a billion times already, but I'd like to be official and make a proper proposal for that. So having said that, I hereby propose adding support for typed `throws` annotations.<div class=""><br class=""></div><div class="">If a function can throw, it is often known what type of error it may throw. Consider this piece of code:</div><div class=""><br class=""></div><div class=""><blockquote type="cite" class="">enum NetworkError: ErrorType {<br class=""><span class="Apple-tab-span" style="white-space:pre"> </span>case RequestTimeout<br class=""><span class="Apple-tab-span" style="white-space:pre"> </span>case UnreachableHost<br class="">}<br class=""><br class="">enum SearchError: ErrorType {<br class=""><span class="Apple-tab-span" style="white-space:pre"> </span>case InvalidQuery<br class="">}<br class=""><br class="">func search(query: String) throws {}<br class=""><br class="">func m() {<br class=""><span class="Apple-tab-span" style="white-space:pre"> </span>do {<br class=""><span class="Apple-tab-span" style="white-space:pre"> </span>try search("foo")<br class=""><span class="Apple-tab-span" style="white-space:pre"> </span>} catch SearchError.InvalidQuery {<br class=""><span class="Apple-tab-span" style="white-space:pre"> </span>print("your query is invalid")<br class=""><span class="Apple-tab-span" style="white-space:pre"> </span>} catch is NetworkError {<br class=""><span class="Apple-tab-span" style="white-space:pre"> </span>print("please check your internet connection")<br class=""><span class="Apple-tab-span" style="white-space:pre"> </span>} catch {<br class=""><span class="Apple-tab-span" style="white-space:pre"> </span>print("an unknown error occurred") // ???<br class=""><span class="Apple-tab-span" style="white-space:pre"> </span>}<br class="">}</blockquote></div><div class=""><br class=""></div><div class="">In order for `do-catch` to be exhaustive, the vague `catch` block is required by the compiler, and inside of it, one has literally no idea what kind of error they deal with. This is useless or even dangerous for error handling, because the last `catch` often looks like this:</div><div class=""><br class=""></div><div class=""><blockquote type="cite" class="">catch {</blockquote><blockquote type="cite" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>// don't know what that is, let's ignore that</blockquote><blockquote type="cite" class="">}</blockquote><br class=""></div><div class="">The situation above can be resolved by introducing typed error handling, so that the `search` function may be refactored to:</div><div class=""><br class=""></div><div class=""><blockquote type="cite" class="">func search(query: String) throws SearchError, NetworkError {}</blockquote><br class=""></div><div class="">Then, the requirement for last, opaque `catch` block can be completely removed as the compiler will make sure that only errors of the allowed types be thrown from `search`.</div><div class=""><br class=""></div><div class="">I will be happy to hear your comments on this.</div><div class=""><br class=""></div><div class="">–––––</div><div class=""><br class=""></div><div class="">As a bonus, I prepared a short FAQ for my proposal:</div><div class=""><br class=""></div><div class=""><div class=""><b class="">Q: What if my function throws multiple error types?</b></div></div><div class=""><br class=""></div><div class="">Use comma-separated list of error types.</div><div class=""><br class=""></div><div class=""><div class=""><blockquote type="cite" class="">func f() throws SomeError, OtherError {}</blockquote><br class=""></div><div class=""><b class="">Q: What if I `try` a function which throws something completely different?</b></div><div class=""><b class=""><br class=""></b></div><div class="">Then you have three possibilities:</div><div class=""><br class=""></div><div class="">1. You resolve the error in scope of the function, inside a `catch` block.</div><div class="">2. You convert it into a compatible type and `throw` it.</div><div class="">3. You annotate your `func` to be throwing that particular error as well.</div><div class=""><br class=""></div></div><div class=""><b class="">Q: What about `rethrows`?</b></div><div class=""><br class=""></div><div class="">`rethrows` should become generic and throw anything that the closure arguments can throw.</div><div class=""><br class=""></div><div class=""><blockquote type="cite" class="">func f(g: () throws SomeError -> Void, h: () throws OtherError -> Void) rethrows {}</blockquote><blockquote type="cite" class=""><br class=""></blockquote></div><blockquote type="cite" class=""><div class="">// behaves like</div><div class=""><br class=""></div><div class="">func f(g: () throws SomeError -> Void, h: () throws OtherError -> Void) throws SomeError, OtherError {}</div></blockquote><div class=""><br class=""></div><div class=""><b class="">Q: What if I want to use the old behavior?</b></div><div class=""><b class=""><br class=""></b></div><div class="">Just annotate as `throws ErrorType` and you can throw anything from there.</div><div class=""><br class=""></div><div class=""><blockquote type="cite" class="">func f() throws ErrorType {}</blockquote></div><div class=""><br class=""></div><div class=""><b class="">Q: Is the failure path really as important as the success path?</b></div><div class=""><br class=""></div><div class="">Yes, it is.</div><div class=""><div class="">
<div style="color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 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;" class=""><font color="#929292" class=""><br class="Apple-interchange-newline">Pozdrawiam – Regards,</font></div><div style="color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 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;" class=""><font color="#929292" class="">Adrian Kashivskyy</font></div>
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