<div><br></div><div><br></div><div><br><div class="gmail_quote"><div>On Sun, Feb 19, 2017 at 2:34 PM Anton Zhilin via swift-evolution <<a href="mailto:swift-evolution@swift.org">swift-evolution@swift.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div class="gmail_msg"><div class="m_-5940626585113039258markdown-here-wrapper gmail_msg"><p style="margin:0px 0px 1.2em!important" class="gmail_msg">Now that I think about it, generic throws does not exactly cover <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline" class="gmail_msg">rethrows</code>.<br class="gmail_msg">Firstly, <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline" class="gmail_msg">rethrows</code> has semantic information that function itself does not throw—it would be lost.</p></div></div></blockquote><div><br></div><div>That's not true. Parametric polymorphism guarantees that rethrows and polymorphic throw are the same. </div><div><br></div><div>For example, you can prove that as a consequence of parametricity that there is only one (pure) function in the of the set of all functions with the type ``forall A. A -> A'' and furthermore that it is the identity function.</div><div><br></div><div>The intuition behind this is that you (meaning the fiction; imagine being a function!) cannot construct your own value of "A" since you don't have any information about what "A" is. The only place to get an "A" is from your argument.</div><div><br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div class="gmail_msg"><div class="m_-5940626585113039258markdown-here-wrapper gmail_msg"><p style="margin:0px 0px 1.2em!important" class="gmail_msg"></p>
<p style="margin:0px 0px 1.2em!important" class="gmail_msg">Secondly, <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline" class="gmail_msg">rethrows</code> allows a function with multiple throwing function parameters to become non-throwing iff all the arguments are non-throwing.<br class="gmail_msg">How would you express it with generics?<br class="gmail_msg">In the proposal, in this case you are required to provide a non-generic supertype of all used error types. This type can’t just turn into <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline" class="gmail_msg">Never</code> automatically.</p>
<p style="margin:0px 0px 1.2em!important" class="gmail_msg">One solution would be to retain <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline" class="gmail_msg">rethrows</code> as an additional attribute.<br class="gmail_msg">It would help with semantic information, and resulting error will be able to turn into <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline" class="gmail_msg">Never</code> as a special case—now that we know, that this function doesn’t throw that error itself.</p></div></div><div class="gmail_msg"><div class="m_-5940626585113039258markdown-here-wrapper gmail_msg">
<p style="margin:0px 0px 1.2em!important" class="gmail_msg">2017-02-19 0:16 GMT+03:00 Martin Waitz <<a href="mailto:tali@admingilde.org" class="gmail_msg" target="_blank">tali@admingilde.org</a>>:</p>
<p style="margin:0px 0px 1.2em!important" class="gmail_msg"></p><div class="m_-5940626585113039258markdown-here-exclude gmail_msg"><p class="gmail_msg"></p><blockquote class="gmail_quote gmail_msg" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word" class="gmail_msg"><br class="gmail_msg"><div class="gmail_msg"><span class="gmail_msg"><blockquote type="cite" class="gmail_msg"><div class="gmail_msg">Am 18.02.2017 um 17:37 schrieb Matthew Johnson via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="gmail_msg" target="_blank">swift-evolution@swift.org</a>>:</div><br class="m_-5940626585113039258m_-5212946143225501370m_-3606562864695778611Apple-interchange-newline gmail_msg"><div class="gmail_msg"><div style="word-wrap:break-word" class="gmail_msg">Thank you for taking the time to put this proposal together Anton! I really want to see typed throws make it into Swift 4. This will be a very nice feature to have.<div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg">I noticed that you included Joe Groff’s idea of replacing `rethrows` by making every function have an error type which is by default `Never` for non-throwing functions and `Error` for throwing functions that do not specify an error type. </div><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg">I want to urge you to consider updating the proposal to take this direction now rather than later. This is a breaking change which means the longer we wait the harder it is to justify. In fact, I think incorporating the breaking change could increase the chances of it being accepted for Swift 4. Without that it is a purely additive change and those are not being given priority in the Swift 4 release.</div></div></div></blockquote><div class="gmail_msg"><br class="gmail_msg"></div></span><div class="gmail_msg">Seconded.</div><div class="gmail_msg">With typed throwing function parameters, it makes a lot of sense to be able to specify the rethrown type, based on the function given as parameter.<br class="gmail_msg"></div></div></div></blockquote><p class="gmail_msg"></p></div><p style="margin:0px 0px 1.2em!important" class="gmail_msg"></p>
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