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.send { color:#77bb77; }
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.error { color:#AA0000; }</style></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class="bloop_markdown"><p>I meant to say “… more than one throwing error type …”</p>
<p></p></div><div class="bloop_original_html"><style>body{font-family:Helvetica,Arial;font-size:13px}</style><div id="bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;"><br></div> <br> <div id="bloop_sign_1487368134070016000" class="bloop_sign"><div style="font-family:helvetica,arial;font-size:13px">-- <br>Adrian Zubarev<br>Sent with Airmail</div></div> <br><p class="airmail_on">Am 17. Februar 2017 um 22:48:35, Adrian Zubarev (<a href="mailto:adrian.zubarev@devandartist.com">adrian.zubarev@devandartist.com</a>) schrieb:</p> <blockquote type="cite" class="clean_bq"><span><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div></div><div>
<title></title>
<div class="bloop_markdown">
<p>No actually not, it was made up by the assumption that the
proposed syntax would have more than one throwing that which was
clarified by others to be incorrect. ;)</p>
<p>Thanks again for clarification.</p>
</div>
<div class="bloop_original_html">
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<br></div>
<br>
<div id="bloop_sign_1487368037940445952" class="bloop_sign">
<div style="font-family:helvetica,arial;font-size:13px">
-- <br>
Adrian Zubarev<br>
Sent with Airmail</div>
</div>
<br>
<p class="airmail_on">Am 17. Februar 2017 um 22:45:35, Joe Groff
(<a href="mailto:jgroff@apple.com">jgroff@apple.com</a>)
schrieb:</p>
<blockquote type="cite" class="clean_bq">
<div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">
<div><span><br class=""></span>
<div>
<blockquote type="cite" class="">
<div class=""><span>On Feb 17, 2017, at 11:03 AM, Adrian Zubarev
via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:</span></div>
<span><br class="Apple-interchange-newline"></span>
<div class="">
<div class="bloop_markdown" style="font-family: Helvetica, Arial; font-size: 13px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: 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; background-color: rgb(254, 254, 254);">
<p style="margin: 15px 0px; -webkit-margin-before: 0px;" class="">
<span>I suggest we need to find a way to shorten the list of the
possible error types with a the help of<span class="Apple-converted-space"> </span><code style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(234, 234, 234); margin: 0px 2px; padding: 0px 5px; word-break: normal; word-wrap: normal; -webkit-margin-before: 0px;" class="">typeallias</code></span></p>
<pre style="margin: 15px 0px; font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(204, 204, 204); overflow: auto; padding: 4px 8px; word-break: normal; word-wrap: normal;" class=""><code class="swift" style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 0px; margin: 0px; padding: 0px; word-break: normal; word-wrap: normal; -webkit-margin-before: 0px;">extension MyError1: Error { ... }
extension MyError2: Error { ... }
extension MyError3: Error { ... }
typealias MyErrors = MyError1 | MyError2 | MyError3
func foo() throws(MyErrors) -> MyResult
func bar<T : Error>(_: () throws(T) -> Void) rethrows(MyErrors, T) -> MyResult
</code></pre></div>
</div>
</blockquote>
Do you actually need that? Experience in other languages like Rust
and Haskell that use Result-based error propagation suggests that a
single error type is adequate, and beneficial in many ways. If
nothing else, you could `Either` your way to multiple errors if you
really needed to.</div>
<div><br class=""></div>
<div>IMO, if we accept a single error type per function, there
could be a simpler model for this. We could say that the `throws`
type is a generic parameter of all function types, and it defaults
to the uninhabited `Never` type for functions that don't
throw.</div>
<div><br class=""></div>
<div>() -> () == () throws Never -> ()</div>
<div>() throws -> () == () throws Error -> ()</div>
<div><br class=""></div>
<div>In this model, you'd get many benefits:</div>
<div><br class=""></div>
<div>- `rethrows` could become first-class, reducing down to just
polymorphic `throws`:</div>
<div><br class=""></div>
<div>func foo(_: () throws -> ()) rethrows // Swift 3</div>
<div>func foo<T: Error>(_: () throws T -> ()) throws T //
Swift X</div>
func foo<T: Error>(_: () throws T -> ()) throws
Either<MyErrors, T>
<div class=""><br class=""></div>
<div class="">- Protocols could abstract over error handling; for
instance, we could support throwing sequences:</div>
<div class=""><br class=""></div>
<div class="">protocol IteratorProtocol {</div>
<div class=""> associatedtype Element</div>
<div class=""> associatedtype Error: Swift.Error =
Never</div>
<div class=""><br class=""></div>
<div class=""> mutating func next() throws Error ->
Element?</div>
<div class="">}</div>
<div class=""><br class=""></div>
<div class="">Separate of the type system model, the type
*checking* model also deserves thorough consideration. Propagating
the effects of possibly multiple error types propagating within a
`do` block is much trickier than doing so as a single "throws" or
not bit, especially if you want to be able to use type context in
`catch` patterns or to implicitly propagate a narrower `throws`
type out of the enclosing function.</div>
<div class=""><br class=""></div>
<div class="">-Joe</div>
</div>
</div>
</blockquote>
</div>
<div class="bloop_markdown"></div>
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