<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=""><div class="">Thanks for you considerate reply. My concern over the proliferation of “sugar proposals” is a general one. This proposal has more merit and general utiliity than many others. I have never used a throwing function in a guard statement that was not itself in a throwing function, but I can see that it could possibly be common in some code. Wrapping a guard statement and all the code that uses variables set in the guard in a do/catch is sub-optimal.</div><div class=""><br class=""></div><div><blockquote type="cite" class=""><div class="">On Jul 8, 2017, at 4:16 PM, Benjamin Spratling 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 class=""><br class=""><blockquote type="cite" class="">On Jul 8, 2017, at 4:08 PM, Christopher Kornher <<a href="mailto:ckornher@me.com" class="">ckornher@me.com</a>> wrote:<br class=""><br class="">I am opposed to this proposal because it muddies up the language to support what is essentially an edge case. The standard way to exit a function early because an exception is thrown is to make the function itself throw, if it is part of a larger operation. The addition of a few lines of try/catch code is not a great burden and makes the termination of an an exception very clear.<br class=""></blockquote><br class="">I’ve read your email, but haven’t digested it fully. One thing I agree with is that most functions which call throwing functions don’t actually use a do…catch block, but instead are merely marked “throws” and the error is propagated back through the stack. Once I seriously started coding functions with errors, I realized I almost always wanted my errors to reach my view-controller or my business logic so I could present separate UI if a real error occurred, and often my error message depended on the details of the error instance.<br class=""><br class=""><blockquote type="cite" class="">`guard` statements are generally used to set variables that are needed in the body of a function. Using them to save a few lines of exception handing code is a very different use. There is no need to mix two relatively clean syntaxes for a few edge cases and increase cognitive load one more time, <br class=""></blockquote><br class=""><br class="">I disagree with your conclusion on this point.<br class="">The “guard” syntax is specifically designed to achieve early return (and placing code associated with early return at the point where it happens) and cleanly installing the returned value into the surrounding scope. So far it has been used to achieve early return only with optionals, true. But is that inherent to ‘guard’, or is it merely because that’s the only way it has been used? The guard does set variables that are needed in the body of the function, and that’s exactly why using guard with values returned from throwing functions makes so much sense, because it does exactly the same thing in a general sense. The “do”…”catch” structure is intentionally designed differently, to place the “happy path” in one place and place the returns in another place. I think with guard/else, we’re seeing developers who can handle less cognitive loading find it easier to reason about early return than grouping failures after the happy path. This proposal hopes to introduce that better language architecture to the catching of errors.<br class=""></div></div></blockquote><div><br class=""></div>All catches don’t have to exit the outer scope, so using guard only handles a subset <br class=""><div><br class=""></div><div>It think that creating the terse try/catch for simple cases has multiple advantages:</div><div><br class=""></div><div><span class="Apple-tab-span" style="white-space:pre"> </span>1) I think that it addresses your desire for a simple way to use throwing functions easily in guard statements.</div><div><br class=""></div><div><div><span class="Apple-tab-span" style="white-space: pre;"> </span>2) It avoids having to change the guard syntax to accomplish this</div><div><br class=""></div><div><span class="Apple-tab-span" style="white-space:pre"> </span>3) It is useful for handling simple one line try/catch constructs in less space in a way that should not seem too foreign to Swift developers.</div><div><span class="Apple-tab-span" style="white-space: pre;"> </span></div><div><span class="Apple-tab-span" style="white-space:pre"> </span>4) It simplifies code that currently uses nested do/try/catch constructs. Even though this is rare, it introduces significant “rightward drift”.</div><div class=""><br class=""></div></div><div><span class="Apple-tab-span" style="white-space:pre"> </span>5) It can used to return early from void throwing functions easily. e.g. : </div><div><br class=""></div><div><span class="Apple-tab-span" style="white-space:pre"> </span>```</div></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div><div><span class="Apple-tab-span" style="white-space:pre"> </span>guard try foo( ) catch { return }</div></div></blockquote><div><div><span class="Apple-tab-span" style="white-space:pre"> </span>```</div><div><span class="Apple-tab-span" style="white-space:pre"> </span></div><div><span class="Apple-tab-span" style="white-space:pre"> </span>Multiple void throwing functions would probably be better handled by a do/catch block, but there is no danger of needing values from these functions because there are none:</div><div><br class=""></div><div><span class="Apple-tab-span" style="white-space:pre"> </span>```</div></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div><div><span class="Apple-tab-span" style="white-space:pre"> </span>do {</div></div><div><div><span class="Apple-tab-span" style="white-space:pre"> </span>try fn1()</div></div><div><div><span class="Apple-tab-span" style="white-space:pre"> </span>try fn2()</div></div><div><div><span class="Apple-tab-span" style="white-space:pre"> </span>} catch {</div></div><div><div><span class="Apple-tab-span" style="white-space:pre"> </span>// keep going, return or call a non-returning function, since throw is already handled by declaring a throwing enclosing function.</div></div><div><div><span class="Apple-tab-span" style="white-space:pre"> </span>// No varibles are needed by the outer block because none are set</div></div><div><div><span class="Apple-tab-span" style="white-space:pre"> </span>// So it is not clearly a guard-like statement</div></div><div><div><span class="Apple-tab-span" style="white-space:pre"> </span>}<span class="Apple-tab-span" style="white-space:pre"> </span> </div></div></blockquote><div><div><span class="Apple-tab-span" style="white-space:pre"> </span>```</div><div><br class=""></div><div><span class="Apple-tab-span" style="white-space:pre"> </span>I did not think of this before, but perhaps we could allow `do` to be replaced with `guard`, thereby allowing values to escape to the outer scope, while still ensuring an early exit:</div><div><br class=""></div><div><div><span class="Apple-tab-span" style="white-space: pre;"> </span>```</div></div></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div><div><div><span class="Apple-tab-span" style="white-space: pre;"> </span>guard {</div></div></div><div><div><div><span class="Apple-tab-span" style="white-space: pre;"> </span>try fn1()</div></div></div><div><div><div><span class="Apple-tab-span" style="white-space: pre;"> </span>try fn2()</div></div></div><div><div><div><span class="Apple-tab-span" style="white-space:pre"> </span>let x = fn3()</div></div></div><div><div><div><span class="Apple-tab-span" style="white-space: pre;"> </span>} catch {</div></div></div><div><div><div><span class="Apple-tab-span" style="white-space: pre;"> </span>// Must exit</div></div></div><div><div><div><span class="Apple-tab-span" style="white-space: pre;"> </span>} else {</div></div></div><div><div><div><span class="Apple-tab-span" style="white-space:pre"> </span>// Must exit</div></div></div><div><div><div><span class="Apple-tab-span" style="white-space: pre;"> </span>}<span class="Apple-tab-span" style="white-space: pre;"> </span> </div></div></div></blockquote><div><div><div><span class="Apple-tab-span" style="white-space: pre;"> </span>```</div><div>I am not sure that “leaky” braces are a good idea, so perhaps some other character could be used to indicate a non-scope or whatever you want to call it:</div><div><br class=""></div><div><div><span class="Apple-tab-span" style="white-space: pre;"> </span>```</div></div></div></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div><div><div><div><span class="Apple-tab-span" style="white-space: pre;"> </span>guard <your favorite character here></div></div></div></div><div><div><div><div><span class="Apple-tab-span" style="white-space: pre;"> </span>try fn1()</div></div></div></div><div><div><div><div><span class="Apple-tab-span" style="white-space: pre;"> </span>try fn2()</div></div></div></div><div><div><div><div><span class="Apple-tab-span" style="white-space: pre;"> </span>let x = fn3()</div></div></div></div><div><div><div><div><span class="Apple-tab-span" style="white-space: pre;"> </span> <another favorite character here> catch {</div></div></div></div><div><div><div><div><span class="Apple-tab-span" style="white-space: pre;"> </span>// Must exit</div></div></div></div><div><div><div><div><span class="Apple-tab-span" style="white-space: pre;"> </span>} else {</div></div></div></div><div><div><div><div><span class="Apple-tab-span" style="white-space: pre;"> </span>// Must exit</div></div></div></div><div><div><div><div><span class="Apple-tab-span" style="white-space: pre;"> </span>}<span class="Apple-tab-span" style="white-space: pre;"> </span> </div></div></div></div></blockquote><div><div><div><div><span class="Apple-tab-span" style="white-space: pre;"> </span>```</div><div class="">This would make the language even harder to read, so just using braces is probably a better idea.</div></div><div><br class=""></div><div class="">This would change the guard syntax slightly, but is a straightforward extrapolation of do/catch and guard, I think. Of course, this could replace the existing guard syntax entirely and its use of semicolons, if we want to go that far…</div><div class=""><br class=""></div><div class="">Allowing this syntax only if one of the expressions throws is possibly a good backward-compatible solution that would avoid redundant guard syntaxes.</div><div class=""><br class=""></div><div class="">Anyway there are lot of possibilities here. We are not forced to extend the guard statement as it exists today. The current guard statement syntax was quite controversial when it was introduced and extending it may not be the best option to do what you want.</div><div class=""><br class=""></div><div class="">- Chris</div></div><div><br class=""></div><div><br class=""></div><div><br class=""></div><br class=""><blockquote type="cite" class=""><div class=""><div class=""><br class="">-Ben Spratling<br class=""><br class="">_______________________________________________<br class="">swift-evolution mailing list<br class=""><a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a><br class="">https://lists.swift.org/mailman/listinfo/swift-evolution<br class=""></div></div></blockquote></div><br class=""></body></html>