<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div><br><br>Sent from my iPhone</div><div><br>On Feb 20, 2017, at 5:51 PM, Anton Zhilin <<a href="mailto:antonyzhilin@gmail.com">antonyzhilin@gmail.com</a>> wrote:<br><br></div><blockquote type="cite"><div><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote">2017-02-21 1:21 GMT+03:00 Matthew Johnson <span dir="ltr"><<a href="mailto:matthew@anandabits.com" target="_blank">matthew@anandabits.com</a>></span>:<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><div>Thanks for the links. I scanned through them somewhat quickly and didn’t see anything that specifically said `Never` should conform to all protocols. Did you see that specifically? I only saw mentions of it being a bottom type and therefore a subtype of all types, which I think is a bit different.</div><div><br></div><div>I think a big part of the confusion here revolves around the distinction between a type `T` being a subtype of another type `U` and `Type<T>` being a subtype of `Type<U>` (using the syntax in your metatype refactoring proposal). I’m not an expert in this area, but I suspect that `Never` can be a subtype of all existential types but without requiring it to actually *conform* to all protocols. Any non-instance protocol requirements are not available on existentials (afaik).</div></div></div></blockquote><div> </div><div>I didn't fully understand about metatypes, but otherwise yes indeed.</div><div><br></div><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>Yes, I understood the example and it’s a good one. What I’m wondering is what benefit you actually get from this. There are two places where this default initializer could be used:</div><div><br></div><div>1. In `seq` itself. But it seems highly dubious to throw an error you know nothing about. Why does `seq` need the ability to construct an error of the same type as a function given to it without knowing anything more about that error type. Is there a use case for this?</div><div>2. In callers of `seq`. Why would the caller care if the error type that `seq` can throw has a default initializer? Is there a use case for this?</div><div><br></div><div>In other words, why do you want to specify that the type of error that might be thrown must have a default initializer? I can’t think of any possible circumstance where this would be valuable.</div><div><br></div><div>The same question can be asked of any other static requirements. What are the use cases? These seem highly theoretical to me. Maybe I’m missing something, but if so I would like to see an example of how it is *used*, not just how you would need to write an extra overload without `rethrows`.</div></div></blockquote><div><br></div><div>Seems highly theoretical to me as well.</div><div><br></div><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>There is a potentially more practical benefit of keeping rethrows. If a function is declared with `rethrows` we know that the function itself does not throw. It only throws if one of its arguments throw when it invokes them. This is a subtle but important difference. For example, users calling a rethrowing function know that *they* have control over whether or not the call *actually* throws. The caller might pass a couple of functions that *can* throw but in this particular case are known not to throw. That could influence how the caller handles errors in the surrounding scope.</div></div></blockquote></div><br></div><div class="gmail_extra">Agreed. Now I lean towards leaving the proposal as is.</div></div></div></blockquote><div><br></div><div>I've been continuing to think through these generic rethrowing functions and have identified a problem I think we want to address before submitting a proposal for review. Please hold off until I have a chance to write a post describing the problem and laying out our options.</div><br><blockquote type="cite"><div><div class="gmail_extra"><br><div class="gmail_quote">2017-02-21 1:21 GMT+03:00 Matthew Johnson <span dir="ltr"><<a href="mailto:matthew@anandabits.com" target="_blank">matthew@anandabits.com</a>></span>:<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"><br><div><span class=""><blockquote type="cite"><div>On Feb 20, 2017, at 11:14 AM, Anton Zhilin <<a href="mailto:antonyzhilin@gmail.com" target="_blank">antonyzhilin@gmail.com</a>> wrote:</div><br class="m_-3978293884672421488Apple-interchange-newline"><div><div dir="ltr"><div class="m_-3978293884672421488markdown-here-wrapper"><p style="margin:0px 0px 1.2em!important">2017-02-20 18:23 GMT+03:00 Matthew Johnson <<a href="mailto:matthew@anandabits.com" target="_blank">matthew@anandabits.com</a>>:</p><div style="margin:0px 0px 1.2em!important"><br class="m_-3978293884672421488webkit-block-placeholder"></div><div class="m_-3978293884672421488markdown-here-exclude"><div><br class="m_-3978293884672421488webkit-block-placeholder"></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div style="word-wrap:break-word"><br><div><span class="m_-3978293884672421488gmail-"><blockquote type="cite"><div>On Feb 20, 2017, at 3:58 AM, Anton Zhilin <<a href="mailto:antonyzhilin@gmail.com" target="_blank">antonyzhilin@gmail.com</a>> wrote:</div><div><div dir="ltr"><div class="m_-3978293884672421488gmail-m_-1259088571132064262markdown-here-wrapper"><p style="margin:0px 0px 1.2em">But that raises another concern. In a previous discussion, it was taken for granted that <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">Never</code> should conform to all protocols</p></div></div></div></blockquote></span><div>Do you have a pointer to this discussion? I must have missed it.</div></div></div></blockquote><div><br class="m_-3978293884672421488webkit-block-placeholder"></div></div><div style="margin:0px 0px 1.2em!important"><br class="m_-3978293884672421488webkit-block-placeholder"></div><p style="margin:0px 0px 1.2em!important"><a href="http://discourse.natecook.com/t/idea-change-noreturn-func-f-to-func-f-noreturn/1000" target="_blank">Here</a> is the discussion where the idea of “empty” type originated.<br>Some messages on the topic ended up being <a href="http://discourse.natecook.com/t/idea-repurpose-void/1406" target="_blank">there</a>.</p><p style="margin:0px 0px 1.2em!important"><a href="http://discourse.natecook.com/t/idea-repurpose-void/1406" target="_blank">This</a> is the earliest mention of usage of this empty type for <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">rethrows</code> I could find.<br>Some related messages are <a href="http://discourse.natecook.com/t/draft-change-noreturn-to-unconstructible-return-type/1765/16" target="_blank">here</a> as well.</p><div><br></div></div></div></div></blockquote><blockquote type="cite"><div><div dir="ltr"><div class="m_-3978293884672421488markdown-here-wrapper"><p style="margin:0px 0px 1.2em!important">We called this type <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">NoReturn</code> and meant it to be the <em>bottom type</em>, i.e. subtype of all types, meaning that if you have an instance of <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">NoReturn</code>—which can only happen in unreachable sections of code—then you can convert it to any type. It should have worked like this:</p>
<pre style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;font-size:1em;line-height:1.2em;margin:1.2em 0px"><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;white-space:pre-wrap;overflow:auto;border-radius:3px;border:1px solid rgb(204,204,204);padding:0.5em 0.7em;display:block!important">func fatalError() -> Never
func divide(a: Int, b: Int) -> Int {
if b == 0 {
let n: Never = fatalError()
return n as Int
}
return a / b
}
</code></pre><p style="margin:0px 0px 1.2em!important">I pushed the idea of replacing <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">rethrows</code> with <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">Never</code>, inspired by Haskell. Although Haskell doesn’t have static function requirements and initializer requirements.</p><div><br></div></div></div></div></blockquote><div><br></div></span><div>Thanks for the links. I scanned through them somewhat quickly and didn’t see anything that specifically said `Never` should conform to all protocols. Did you see that specifically? I only saw mentions of it being a bottom type and therefore a subtype of all types, which I think is a bit different.</div><div><br></div><div>I think a big part of the confusion here revolves around the distinction between a type `T` being a subtype of another type `U` and `Type<T>` being a subtype of `Type<U>` (using the syntax in your metatype refactoring proposal). I’m not an expert in this area, but I suspect that `Never` can be a subtype of all existential types but without requiring it to actually *conform* to all protocols. Any non-instance protocol requirements are not available on existentials (afaik).</div><span class=""><br><blockquote type="cite"><div><div dir="ltr"><div class="m_-3978293884672421488markdown-here-wrapper"><div style="margin:0px 0px 1.2em!important"><br class="m_-3978293884672421488webkit-block-placeholder"></div><div class="m_-3978293884672421488markdown-here-exclude"><div><br class="m_-3978293884672421488webkit-block-placeholder"></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div style="word-wrap:break-word"><div><span class="m_-3978293884672421488gmail-"><blockquote type="cite"><div><div dir="ltr"><div class="m_-3978293884672421488gmail-m_-1259088571132064262markdown-here-wrapper"><p style="margin:0px 0px 1.2em">, because if one obtains an instance of <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">Never</code> (and they won’t), then everything is possible. But now we say that <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">Never</code> can’t conform to <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">Default</code>, because this would break its very invariant. Also it can’t conform to any protocol with static members or initializers.</p></div></div></div></blockquote></span><div>It seems highly problematic to me to say that never conforms to any protocol with non-instance requirements.</div></div></div></blockquote><div><br class="m_-3978293884672421488webkit-block-placeholder"></div></div><div style="margin:0px 0px 1.2em!important"><br class="m_-3978293884672421488webkit-block-placeholder"></div><p style="margin:0px 0px 1.2em!important">Here is an example with instance requirements only:</p>
<pre style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;font-size:1em;line-height:1.2em;margin:1.2em 0px"><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;white-space:pre-wrap;overflow:auto;border-radius:3px;border:1px solid rgb(204,204,204);padding:0.5em 0.7em;display:block!important">protocol MakesPizza {
func cook() -> Pizza
}
extension Never : MakesPizza {
func cook() -> Pizza {
// this method will never be called anyway
burnThisComputer()
}
}
let maestroLaPizza = isHeAtWork ? validMaestro : (fatalError("something went wrong") as MakesPizza)
maestroLaPizza.cook()
</code></pre><p style="margin:0px 0px 1.2em!important">In this way, <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">Never</code> can conform to any protocol with only instance requirements.</p></div></div></div></blockquote></span><div>Sure.</div><span class=""><br><blockquote type="cite"><div><div dir="ltr"><div class="m_-3978293884672421488markdown-here-wrapper"><div style="margin:0px 0px 1.2em!important"><br class="m_-3978293884672421488webkit-block-placeholder"></div><div class="m_-3978293884672421488markdown-here-exclude"><div><br class="m_-3978293884672421488webkit-block-placeholder"></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div style="word-wrap:break-word"><div><span class="m_-3978293884672421488gmail-"><blockquote type="cite"><div><div dir="ltr"><div class="m_-3978293884672421488gmail-m_-1259088571132064262markdown-here-wrapper"><p style="margin:0px 0px 1.2em">But then basically, <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">Never</code> trick can’t be used when we request anything more than <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">Error</code> from generic error type (with static members or initializers). So this approach turns out to be more limiting than <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">rethrows</code>.<br></p></div></div></div></blockquote></span><div>Can you elaborate here? If you require a function to throw an error type that has non-instance requirements then you would necessarily be restricting callers to provide a throwing function. It is not possible to express such a function with `rethrows`. You can’t talk about the error type at all. If you could talk about the error type and were able to constrain it in this way `rethrows` would necessarily have to exhibit the same behavior as the generic version. The behavior arises out of the constraint you are applying, not the mechanism by which you forward the type.</div></div></div></blockquote><div><br class="m_-3978293884672421488webkit-block-placeholder"></div></div><div style="margin:0px 0px 1.2em!important"><br class="m_-3978293884672421488webkit-block-placeholder"></div><p style="margin:0px 0px 1.2em!important">With <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">rethrows</code> approach:</p>
<pre style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;font-size:1em;line-height:1.2em;margin:1.2em 0px"><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;white-space:pre-wrap;overflow:auto;border-radius:3px;border:1px solid rgb(204,204,204);padding:0.5em 0.7em;display:block!important">protocol BaseError : Error {
init()
}
func seq<E1, E2>(f: () throws(E1) -> (), g: () throws(E2) -> ()) rethrows(BaseError)
where E1: BaseError, E2: BaseError { ... }
</code></pre><p style="margin:0px 0px 1.2em!important">With <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">Never</code> approach, we have to create two separate functions for the same effect, because <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">Never</code> does not fit in <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">BaseError</code>:</p>
<pre style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;font-size:1em;line-height:1.2em;margin:1.2em 0px"><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;white-space:pre-wrap;overflow:auto;border-radius:3px;border:1px solid rgb(204,204,204);padding:0.5em 0.7em;display:block!important">func seq<E1, E2>(f: () throws(E1) -> (), g: () throws(E2) -> ()) throws(BaseError)
where E1: BaseError, E2: BaseError {
// It never actually throws E1() or E2() itself, but this fact can't be reflected in the signature
}
func seq(f: () -> (), g: () -> ()) {
// repeat the body
}
</code></pre><p style="margin:0px 0px 1.2em!important">That’s where loss of information (which I meantioned earlier) hurts: we can’t apply magic and say “if <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">E1</code> and <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">E2</code> are <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">Never</code> then <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">seq</code> does not throw. Because it <strong>can</strong> throw anyway.</p><p style="margin:0px 0px 1.2em!important">Well, I’m just repeating myself, at least I gave a bit more complete example :)</p>
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</div></blockquote></span></div><br><div>Yes, I understood the example and it’s a good one. What I’m wondering is what benefit you actually get from this. There are two places where this default initializer could be used:</div><div><br></div><div>1. In `seq` itself. But it seems highly dubious to throw an error you know nothing about. Why does `seq` need the ability to construct an error of the same type as a function given to it without knowing anything more about that error type. Is there a use case for this?</div><div>2. In callers of `seq`. Why would the caller care if the error type that `seq` can throw has a default initializer? Is there a use case for this?</div><div><br></div><div>In other words, why do you want to specify that the type of error that might be thrown must have a default initializer? I can’t think of any possible circumstance where this would be valuable.</div><div><br></div><div>The same question can be asked of any other static requirements. What are the use cases? These seem highly theoretical to me. Maybe I’m missing something, but if so I would like to see an example of how it is *used*, not just how you would need to write an extra overload without `rethrows`.</div><div><br></div><div>There is a potentially more practical benefit of keeping rethrows. If a function is declared with `rethrows` we know that the function itself does not throw. It only throws if one of its arguments throw when it invokes them. This is a subtle but important difference. For example, users calling a rethrowing function know that *they* have control over whether or not the call *actually* throws. The caller might pass a couple of functions that *can* throw but in this particular case are known not to throw. That could influence how the caller handles errors in the surrounding scope.</div><div><br></div><div><br></div><div><br></div><div><br></div></div></blockquote></div><br></div>
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