<div dir="ltr"><div>plx: wouldn't the same overload resolution strategy be appropriate here? "most specific choice, otherwise diagnostic"</div><div><br></div><div><br></div><div><br></div><div>separately:</div>Are there any subscripts in the standard library that would be throwing/generic but can't be?<div><br></div></div><div class="gmail_extra"><br><div class="gmail_quote">On Wed, Jun 22, 2016 at 9:13 AM, plx via swift-evolution <span dir="ltr"><<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>></span> wrote:<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"><div>Prefacing the below with a “I am well-aware this proposal likely won’t make it into Swift 3”:</div><div><br></div><div>A feature like this would be nice to use, but before I could get behind any proposal along these lines it I’d want to see it include an explicit strategy for disambiguation.</div><div><br></div><div>EG: in your example, your generic subscript uses `self[key]`, and presumably expects that to use the “original” subscript…and not the generic subscript being defined.</div><div><br></div><div>I think that’s reasonable in that specific case, but it doesn’t seem unreasonable to anticipate this proposal introducing ambiguities that would need explicit disambiguation…and for which explicit type annotation may not always be adequate to resolve (I could be wrong here, though). </div><div><br></div><div>This would need addressing (either showing they won’t be an issue, or providing a reliable disambiguation mechanism).</div><div><br></div><div>Relatedly, in-re: “rethrows”: if the syntax supported it, this kind of thing would be another way of tackling the "JSON problem":</div><div><br></div><div> subscript<T>(key: Key, transform: (Value) throws -> T) rethrows -> T {</div><div> guard let value = self[key] else { throw JSON.MissingKey(…) }</div><div> return try transform(value)</div><div> }</div><div><br></div><div>…so that e.g. you can write typical parsing-code as</div><div><br></div><div> let asUserID = UserID.init(untrustedString:) // <- assume this is a "throwing constructor"</div><div> let sender = try json[“sender”,asUserID] </div><div> let recipient = try json[“recipient”,asUserID]</div><div><br></div><div>…(modulo any syntax errors, etc.), which would benefit from a `rethrows` declaration.</div><div><br></div><div>That’s my 2c; thankfully (IMHO) there’s clearly a lot of time for this proposal to simmer.</div><div><br></div><div><blockquote type="cite"><div><div class="h5"><div>On Jun 20, 2016, at 1:10 PM, Robert Widmann via swift-evolution <<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>> wrote:</div><br></div></div><div><div><div class="h5"><div style="word-wrap:break-word">Good morning all. Attached is the proposal Harlan Haskins and I will be submitting shortly about adding generic and `throw`ing subscript declarations to the language. <div><br></div><div>Cheers,</div><div><br></div><div>~Robert Widmann<br><div><br></div><div><h1 style="font-size:2.25em;margin-right:0px;margin-bottom:16px;margin-left:0px;line-height:1.2;padding-bottom:0.3em;border-bottom-width:1px;border-bottom-style:solid;border-bottom-color:rgb(238,238,238);color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';margin-top:0px!important">Generic and Throwing Subscripts</h1><ul style="padding-left:2em;margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:14px"><li>Proposal: <a href="https://github.com/apple/swift-evolution/blob/master/proposals/NNNN-name.md" style="color:rgb(64,120,192);text-decoration:none" target="_blank">SE-NNNN</a></li><li>Author(s): <a href="https://github.com/harlanhaskins" style="color:rgb(64,120,192);text-decoration:none" target="_blank">Harlan Haskins</a> and <a href="https://github.com/codafi" style="color:rgb(64,120,192);text-decoration:none" target="_blank">Robert Widmann</a></li><li>Status: <span><a href="https://github.com/typelift/SwiftCheck/pull/168#rationale" style="color:rgb(64,120,192);text-decoration:none" target="_blank">Awaiting review</a></span></li><li>Review manager: TBD</li></ul><h2 style="margin-top:1em;margin-bottom:16px;line-height:1.225;font-size:1.75em;padding-bottom:0.3em;border-bottom-width:1px;border-bottom-style:solid;border-bottom-color:rgb(238,238,238);color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol'">Introduction</h2><p style="margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:14px">Currently, subscripts cannot be declared <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:11.899999618530273px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px">[re]throws</code> and cannot declare new generic parameters.<br>There isn't a clear reason why they aren't as capable as full-fledged functions, so we propose<br>adding generic constraints and throwing semantics to subscripts.</p><h2 style="margin-top:1em;margin-bottom:16px;line-height:1.225;font-size:1.75em;padding-bottom:0.3em;border-bottom-width:1px;border-bottom-style:solid;border-bottom-color:rgb(238,238,238);color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol'">Motivation</h2><p style="margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:14px">On the throwing side, currently there are two ways to express a failing subscript:</p><ul style="padding-left:2em;margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:14px"><li>Return an <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:11.899999618530273px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px">Optional</code>, failing with <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:11.899999618530273px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px">nil</code>.</li><li>Call <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:11.899999618530273px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px">fatalError(_:)</code> on failure.</li></ul><p style="margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:14px">Both of these throw out useful information about the cause of the underlying error that using Swift's error handling mechanism can otherwise provide.</p><p style="margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:14px">As for generics, to take an example, it has become a common pattern among JSON decoding DSL libraries to express a throwing generic extension on <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:11.899999618530273px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px">Dictionary</code> like so</p><div style="margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:14px;overflow:visible!important"><pre style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:11.899999618530273px;margin-top:0px;margin-bottom:0px;line-height:1.45;word-wrap:normal;padding:16px;overflow:auto;background-color:rgb(247,247,247);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;word-break:normal"><span style="color:rgb(167,29,93)">extension</span> <span style="color:rgb(0,134,179)">Dictionary</span> {
<span style="color:rgb(167,29,93)">public</span> <span style="color:rgb(167,29,93)">func</span> <span style="color:rgb(121,93,163)">parse</span><T>(key: Key) throws <span style="color:rgb(167,29,93)">-></span> T {
<span style="color:rgb(167,29,93)">guard</span> <span style="color:rgb(167,29,93)">let</span> value <span style="color:rgb(167,29,93)">=</span> <span style="color:rgb(167,29,93)">self</span>[key] <span style="color:rgb(167,29,93)">else</span> {
<span style="color:rgb(167,29,93)">throw</span> JSONError<span style="color:rgb(167,29,93)">.</span>MissingKey(<span style="color:rgb(24,54,145)"><span>"</span><span>\(</span><span style="color:rgb(51,51,51)">key</span><span>)</span><span>"</span></span>)
}
<span style="color:rgb(167,29,93)">guard</span> <span style="color:rgb(167,29,93)">let</span> ofType <span style="color:rgb(167,29,93)">=</span> value <span style="color:rgb(167,29,93)">as?</span> T <span style="color:rgb(167,29,93)">else</span> {
<span style="color:rgb(167,29,93)">throw</span> JSONError<span style="color:rgb(167,29,93)">.</span>InvalidKey(key: <span style="color:rgb(24,54,145)"><span>"</span><span>\(</span><span style="color:rgb(51,51,51)">key</span><span>)</span><span>"</span></span>, expectedType: T<span style="color:rgb(167,29,93)">.</span><span style="color:rgb(167,29,93)">self</span>, foundType: value<span style="color:rgb(167,29,93)">.</span><span style="color:rgb(167,29,93)">dynamicType</span>)
}
<span style="color:rgb(167,29,93)">return</span> ofType
}
}
<span style="color:rgb(167,29,93)">public</span> <span style="color:rgb(167,29,93)">enum</span> JSONError: <span style="color:rgb(0,134,179)">ErrorType</span>, <span style="color:rgb(0,134,179)">CustomStringConvertible</span> {
<span style="color:rgb(167,29,93)">case</span> InvalidKey(key: <span style="color:rgb(0,134,179)">String</span>, expectedType: <span style="color:rgb(0,134,179)">Any</span><span style="color:rgb(167,29,93)">.</span><span style="color:rgb(167,29,93)">Type</span>, foundType: <span style="color:rgb(0,134,179)">Any</span><span style="color:rgb(167,29,93)">.</span><span style="color:rgb(167,29,93)">Type</span>)
<span style="color:rgb(167,29,93)">case</span> MissingKey(<span style="color:rgb(0,134,179)">String</span>)
<span style="color:rgb(167,29,93)">public</span> <span style="color:rgb(167,29,93)">var</span> description: <span style="color:rgb(0,134,179)">String</span> {
<span style="color:rgb(167,29,93)">switch</span> <span style="color:rgb(167,29,93)">self</span> {
<span style="color:rgb(167,29,93)">case</span> <span style="color:rgb(167,29,93)">.</span>InvalidKey(<span style="color:rgb(167,29,93)">let</span> key, <span style="color:rgb(167,29,93)">let</span> expected, <span style="color:rgb(167,29,93)">let</span> found):
<span style="color:rgb(167,29,93)">return</span> <span style="color:rgb(24,54,145)"><span>"</span>Invalid key <span>\"</span><span>\(</span><span style="color:rgb(51,51,51)">key</span><span>)</span><span>\"</span>. Expected value of type <span>\"</span><span>\(</span><span style="color:rgb(51,51,51)">expected</span><span>)</span><span>\"</span>, found <span>\"</span><span>\(</span><span style="color:rgb(51,51,51)">found</span><span>)</span><span>\"</span>.<span>"</span></span>
<span style="color:rgb(167,29,93)">case</span> <span style="color:rgb(167,29,93)">.</span>MissingKey(<span style="color:rgb(167,29,93)">let</span> key):
<span style="color:rgb(167,29,93)">return</span> <span style="color:rgb(24,54,145)"><span>"</span>Key <span>\(</span><span style="color:rgb(51,51,51)">key</span><span>)</span> not found.<span>"</span></span>
}
}
}</pre></div><p style="margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:14px">Given this, one can decode JSON with the full support of native type inference and exception handling. But when working with the DSL, one would expect to be able to express this as a subscript on <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:11.899999618530273px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px">Dictionary</code>, allowing the following:</p><div style="margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:14px;overflow:visible!important"><pre style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:11.899999618530273px;margin-top:0px;margin-bottom:0px;line-height:1.45;word-wrap:normal;padding:16px;overflow:auto;background-color:rgb(247,247,247);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;word-break:normal"><span style="color:rgb(150,152,150)">//...</span>
<span style="color:rgb(167,29,93)">extension</span> <span style="color:rgb(0,134,179)">Dictionary</span> {
<span style="color:rgb(167,29,93)">public</span> <span style="color:rgb(167,29,93)">subscript</span><span style="color:rgb(167,29,93)"><</span>T<span style="color:rgb(167,29,93)">></span>(key: Key) <span style="color:rgb(167,29,93)">throws</span> <span style="color:rgb(167,29,93)">-></span> T {
<span style="color:rgb(167,29,93)">guard</span> <span style="color:rgb(167,29,93)">let</span> value <span style="color:rgb(167,29,93)">=</span> <span style="color:rgb(167,29,93)">self</span>[key] <span style="color:rgb(167,29,93)">else</span> {
<span style="color:rgb(167,29,93)">throw</span> JSONError<span style="color:rgb(167,29,93)">.</span>MissingKey(<span style="color:rgb(24,54,145)"><span>"</span><span>\(</span><span style="color:rgb(51,51,51)">key</span><span>)</span><span>"</span></span>)
}
<span style="color:rgb(167,29,93)">guard</span> <span style="color:rgb(167,29,93)">let</span> ofType <span style="color:rgb(167,29,93)">=</span> value <span style="color:rgb(167,29,93)">as?</span> T <span style="color:rgb(167,29,93)">else</span> {
<span style="color:rgb(167,29,93)">throw</span> JSONError<span style="color:rgb(167,29,93)">.</span>InvalidKey(key: <span style="color:rgb(24,54,145)"><span>"</span><span>\(</span><span style="color:rgb(51,51,51)">key</span><span>)</span><span>"</span></span>, expectedType: T<span style="color:rgb(167,29,93)">.</span><span style="color:rgb(167,29,93)">self</span>, foundType: value<span style="color:rgb(167,29,93)">.</span><span style="color:rgb(167,29,93)">dynamicType</span>)
}
<span style="color:rgb(167,29,93)">return</span> ofType
}
}</pre></div><p style="margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:14px">We believe this is an even more natural way to write these kinds of libraries in Swift and that bringing subscript member declarations up to par with functions is a useful addition to the language as a whole.</p><h2 style="margin-top:1em;margin-bottom:16px;line-height:1.225;font-size:1.75em;padding-bottom:0.3em;border-bottom-width:1px;border-bottom-style:solid;border-bottom-color:rgb(238,238,238);color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol'">Proposed solution</h2><p style="margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:14px">Add the ability to introduce new generic parameters and mark <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:11.899999618530273px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px">throws</code> and <span style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:11.899999618530273px;background-color:rgba(0,0,0,0.0392157)">rethrows </span>on subscript members.</p><h2 style="margin-top:1em;margin-bottom:16px;line-height:1.225;font-size:1.75em;padding-bottom:0.3em;border-bottom-width:1px;border-bottom-style:solid;border-bottom-color:rgb(238,238,238);color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol'">Detailed design</h2><p style="margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:14px">This change will modify and add the following productions in the Swift grammar</p><div style="margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:14px;overflow:visible!important"><pre style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:11.899999618530273px;margin-top:0px;margin-bottom:0px;line-height:1.45;word-wrap:normal;padding:16px;overflow:auto;background-color:rgb(247,247,247);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;word-break:normal">GRAMMAR OF A SUBSCRIPT DECLARATION
subscript-declaration → subscript-head subscript-result code-block
subscript-declaration → subscript-head subscript-result getter-setter-block
subscript-declaration → subscript-head subscript-result getter-setter-keyword-block
<span style="background-color:rgb(255,236,236);color:rgb(189,44,0)">-subscript-head → attributes(opt) declaration-modifiers(opt) subscript parameter-clause</span>
<span style="background-color:rgb(234,255,234);color:rgb(85,165,50)">+subscript-head → attributes(opt) declaration-modifiers(opt) generic-parameter-clause(opt) subscript parameter-clause</span>
<span style="background-color:rgb(234,255,234);color:rgb(85,165,50)">+subscript-result → -> attributes(opt) throws(opt) type</span>
<span style="background-color:rgb(234,255,234);color:rgb(85,165,50)">+subscript-result → -> attributes(opt) rethrows(opt) type</span></pre></div><hr style="min-height:4px;overflow:hidden;margin:16px 0px;background-color:rgb(231,231,231);border:0px none;padding:0px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:14px"><h1 style="font-size:2.25em;margin:1em 0px 16px;line-height:1.2;padding-bottom:0.3em;border-bottom-width:1px;border-bottom-style:solid;border-bottom-color:rgb(238,238,238);color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol'">Rationale</h1><div style="margin-top:0px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:14px;margin-bottom:0px!important">On [Date], the core team decided to <span>(TBD)</span> this proposal.<br>When the core team makes a decision regarding this proposal,<br>their rationale for the decision will be written here.</div></div></div></div></div></div><span class="">_______________________________________________<br>swift-evolution mailing list<br><a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a><br><a href="https://lists.swift.org/mailman/listinfo/swift-evolution" target="_blank">https://lists.swift.org/mailman/listinfo/swift-evolution</a><br></span></div></blockquote></div><br></div><br>_______________________________________________<br>
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<br></blockquote></div><br></div>