<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="">I’d be interested in reading up on what the Oracle response was if you have links.<div class=""><br class=""></div><div class="">As you mentioned even Swift doesn’t get away from runtime type checking but in the two examples you mentioned - array out of bounds and casts - Swift makes use of the Optionals system to highlight that an operation may fail and allows the user to handle that failure. Covariance should have similar syntax support (for example use of optional chaining similar to optional protocol requirements to indicate that a call may fail due to incorrect types). For the compiler to understand when such failure is possible, some kind of covariance syntax would be required.<br class=""><div class=""><br class=""></div><div class="">As a related question, do you see covariance syntax as such a burden?</div><div class=""><br class=""></div><div class="">-Simon</div><div class=""><div class=""><div class=""><br class=""></div><div class=""><br class=""><div><blockquote type="cite" class=""><div class="">On 13 Jan 2016, at 12:47 PM, Howard Lovatt <<a href="mailto:howard.lovatt@gmail.com" class="">howard.lovatt@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class="">Yes you can annotate for covariance, invariance, and contravariance, both Java and Scala, allow all three. The problem is that the code becomes splattered with variance annotations. The Java people themselves have publicly regretted this and wished that they had made covariance the default. If you look at generic code invariance and covariance are by far the most common requirements; this proposal would address these common use case without burdening the programmer.<div class=""><br class=""></div><div class="">Swift, and no usable language, is completely statically typed. Examples in Swift of runtime type checking are array out of bounds and casts. There are other examples of non-type related runtime checks is Swift: numerical overflow, throws, using optionals to signal errors, and using enums to signal errors. I say use what is appropriate, static type checking if it is easy to do, otherwise runtime type checking. Note I am not proposing an unsafe language like C, it is still type checked.</div><div class=""><br class=""></div><div class="">There is a strong positive precedence for a type check on write, Java arrays (not Java `List`s). Arrays in Java may be passed covariantly, and this is extensively used. However if you attempt to write the wrong type into the array you will get an `ArrayStoreException`. In practice you don't get many `ArrayStoreException`, non of my code has ever had one. Its just not something you do in practice, as noted before contravariance is rare.</div><div class=""><br class=""></div><div class="">Thanks for you comments and I hope this eases your concerns,</div><div class=""><br class=""></div><div class=""> -- Howard.<br class=""><div class=""><br class=""></div></div></div><div class="gmail_extra"><br class=""><div class="gmail_quote">On 13 January 2016 at 11:33, Simon Pilkington <span dir="ltr" class=""><<a href="mailto:simonmpilkington@icloud.com" target="_blank" class="">simonmpilkington@icloud.com</a>></span> wrote:<br class=""><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word" class=""><div class="">The problem is that conceptually and behaviourally Box<Bottom> *is indeed not* a Box<Top> and cannot be treated the same as one. The proposal attempts to get around this difference with a runtime failure but this would result in very fragile code - you get passed a Box<Top> and want to pass it a subclass of Top, will it succeed, who knows. You probably would be able to check the types but the complier wouldn’t highlight that this is an operation that could potentially fail.</div><div class=""><br class=""></div><div class="">This seems to be very much against Swift’s goal of safety being enforced by the compiler as much as possible.</div><div class=""><br class=""></div><div class="">Java uses the wildcard syntax to highlight this conceptual and behavioural difference - Box<Bottom> is not covariant with Box<Top> but rather with Box<? extends Top>. The compiler can then enforce that a programmer doesn’t try to pass an incompatible type to a variable of such type. Even though this is a complication to the language (many Java programmers struggle with correctly using the wildcard syntax) I don’t see covariance for generics being added to Swift in a robust manner without some kind of similar syntax.</div><div class=""><br class=""></div><div class="">-Simon</div><div class=""><br class=""><div class=""><blockquote type="cite" class=""><div class=""><div class="h5"><div class="">On 12 Jan 2016, at 8:45 PM, Howard Lovatt via swift-evolution <<a href="mailto:swift-evolution@swift.org" target="_blank" class="">swift-evolution@swift.org</a>> wrote:</div><br class=""></div></div><div class=""><div style="word-wrap:break-word" class=""><div class=""><div class="h5"><div style="margin:0px;line-height:normal" class=""><font face="Menlo" class=""><span style="font-size:11px" class="">Currently you generics are invariant whereas function arguments etc. are covariant. I am suggesting that if the way generics are implemented is changed then they can be made covariant and that this will add considerable utility to Swift generics.</span></font></div><div style="margin:0px;line-height:normal" class=""><font face="Menlo" class=""><span style="font-size:11px" class=""><br class=""></span></font></div><div style="margin:0px;line-height:normal" class=""><font face="Menlo" class=""><span style="font-size:11px" class="">1st a demonstration of the current situation of invariant generics:</span></font></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)" class=""><span class=""><br class=""></span></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)" class=""><span class=""> </span>// Current system</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class=""> <span style="color:#bb2ca2" class="">class</span> Top {}</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class=""> <span style="color:#bb2ca2" class="">class</span> Bottom: <span style="color:#4f8187" class="">Top</span> {}</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;min-height:13px" class=""><br class=""></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class=""> <span style="color:#bb2ca2" class="">struct</span> Box<T: AnyObject> {</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class=""> <span style="color:#bb2ca2" class="">var</span> value: <span style="color:#703daa" class="">T</span></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class=""> <span style="color:#bb2ca2" class="">init</span>(<span style="color:#bb2ca2" class="">_</span> initialValue: <span style="color:#703daa" class="">T</span>) {</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class=""> <span style="color:#4f8187" class="">value</span> = initialValue;</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class=""> }</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class=""> }</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;min-height:13px" class=""><br class=""></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class=""> <span style="color:#bb2ca2" class="">let</span> boxB = <span style="color:#4f8187" class="">Box</span>(<span style="color:#4f8187" class="">Bottom</span>())</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)" class=""><span class=""> </span>// let boxT: Box<Top> = boxB // Covariance currently not allowed</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;min-height:13px" class=""><br class=""></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;min-height:13px" class="">The key point is although `Bottom` 'is a’ `Top`, `Box<Bottom>` *is not* a `Box<Top>`.</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;min-height:13px" class=""><br class=""></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;min-height:13px" class="">I am suggesting:</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;min-height:13px" class=""><br class=""></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;min-height:13px" class="">1. That `Box<Bottom>` should be a `Box<Top>` (covariance).</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;min-height:13px" class="">2. An implementation that allows the above covariance.</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;min-height:13px" class="">3. That protocols are made generic, i.e. `protocol Box<T> { var value: T { get set } }` and that this mechanism replaces associated types for protocols.</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;min-height:13px" class=""><br class=""></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)" class=""><span class=""> </span>// Proposal:</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)" class=""><span class=""> </span>// 1. No change to Box, i.e. programmer would just write Box as before</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)" class=""><span class=""> </span>// 2. Code transformed by comiler with write check for each specific, generic type instance</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)" class=""><span class=""> </span>// Best approximation of resulting code in current Swift to demonstrate spirit of idea:</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;min-height:13px" class=""><br class=""></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)" class=""><span class=""> </span>// Compiler writes a universal form using the upper bound (it writes the underlyting representation).</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)" class=""><span class=""> </span>// In practice this would be called `Box` but used `BoxAnyObject` to indicate that it has a generic argument bounded by `AnyObject`.</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class=""> <span style="color:#bb2ca2" class="">struct</span> BoxAnyObject {</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)" class=""><span class=""> </span>// Generated from generic argument `<T: AnyObject>`.</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)" class=""><span class=""> </span><span style="color:#bb2ca2" class="">let</span><span class=""> T: </span><span style="color:#703daa" class="">AnyObject</span><span class="">.Type </span>// Store the actual type.</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;min-height:13px" class=""> <br class=""></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)" class=""><span class=""> </span>// Generated from stored property `var value: T` and noting that `T`'s upper bound is `AnyObject`.</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)" class=""><span class=""> </span><span style="color:#bb2ca2" class="">private</span><span class=""> </span><span style="color:#bb2ca2" class="">var</span><span class=""> _value: </span><span style="color:#703daa" class="">AnyObject</span><span class=""> </span>// Access the stored property through a setter so that type can be checked</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class=""> <span style="color:#bb2ca2" class="">var</span> value: <span style="color:#703daa" class="">AnyObject</span> {</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class=""> <span style="color:#bb2ca2" class="">get</span> {</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class=""> <span style="color:#bb2ca2" class="">return</span> <span style="color:#4f8187" class="">_value</span></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class=""> }</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class=""> <span style="color:#bb2ca2" class="">set</span> {</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)" class=""><span class=""> </span>// In all functions check that args declared as `T` are actually a `T` or a sub-type.</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)" class=""><span class=""> </span>// Note: `is` only works with type literal and there is no `>=` operator for types :(.</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)" class=""><span class=""> </span>// `is` would need changing or `>=` for types adding, nearest at moment `==`.</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(209,47,27)" class=""><span class=""> </span><span style="color:#3d1d81" class="">precondition</span><span class="">(</span><span style="color:#4f8187" class="">T</span><span class=""> == </span><span style="color:#008400" class="">/* >= */</span><span class=""> newValue.</span><span style="color:#bb2ca2" class="">dynamicType</span><span class="">, </span>"Type of newValue, <span class="">\</span>(<span class="">newValue.</span><span style="color:#bb2ca2" class="">dynamicType</span>), is not a sub-type of generic type T, <span class="">\</span>(<span style="color:#4f8187" class="">T</span>)"<span class="">)</span></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class=""> <span style="color:#4f8187" class="">_value</span> = newValue</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class=""> }</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class=""> }</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;min-height:13px" class=""> <br class=""></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)" class=""><span class=""> </span>// Generated from `init(_ initialValue: T)` and noting that `T`'s upper bound is `AnyObject`.</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class=""> <span style="color:#bb2ca2" class="">init</span>(<span style="color:#bb2ca2" class="">_</span> lowestCommonDeclaredT: <span style="color:#703daa" class="">AnyObject</span>.Type, <span style="color:#bb2ca2" class="">_</span> initialValue: <span style="color:#703daa" class="">AnyObject</span>) {</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class=""> <span style="color:#4f8187" class="">T</span> = lowestCommonDeclaredT</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class=""> <span style="color:#4f8187" class="">_value</span> = initialValue</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class=""> }</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class=""> }</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;min-height:13px" class=""><br class=""></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)" class=""><span class=""> </span>// Demonstrate that all `Box`es are the same size and therefore can be bitwise copied</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)" class=""><span class=""> </span>// Compiler supplies lowest-common, declared, generic type for all the `T`s in the `init` call.</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)" class=""><span class=""> </span><span style="color:#bb2ca2" class="">var</span><span class=""> bT = </span><span style="color:#4f8187" class="">BoxAnyObject</span><span class="">(</span><span style="color:#4f8187" class="">Top</span><span class="">.</span><span style="color:#bb2ca2" class="">self</span><span class="">, </span><span style="color:#4f8187" class="">Top</span><span class="">()) </span>// In practice user would write `let bT = Box(Top())`.</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)" class=""><span class=""> </span><span style="color:#4f8187" class="">bT</span><span class="">.</span><span style="color:#4f8187" class="">T</span><span class=""> </span>// Top.Type</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(61,29,129)" class=""><span class=""> </span>sizeofValue<span class="">(</span><span style="color:#4f8187" class="">bT</span><span class="">) </span><span style="color:#008400" class="">// 16</span></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;min-height:13px" class=""><br class=""></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)" class=""><span class=""> </span><span style="color:#bb2ca2" class="">var</span><span class=""> bB = </span><span style="color:#4f8187" class="">BoxAnyObject</span><span class="">(</span><span style="color:#4f8187" class="">Bottom</span><span class="">.</span><span style="color:#bb2ca2" class="">self</span><span class="">, </span><span style="color:#4f8187" class="">Bottom</span><span class="">()) </span>// In practice user would write `let bB = Box(Bottom())`.</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)" class=""><span class=""> </span><span style="color:#4f8187" class="">bB</span><span class="">.</span><span style="color:#4f8187" class="">T</span><span class=""> </span>// Bottom.Type</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(61,29,129)" class=""><span class=""> </span>sizeofValue<span class="">(</span><span style="color:#4f8187" class="">bB</span><span class="">) </span><span style="color:#008400" class="">// 16</span></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;min-height:13px" class=""><br class=""></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)" class=""><span class=""> </span>// Demonstration covariance.</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)" class=""><span class=""> </span><span style="color:#4f8187" class="">bT</span><span class=""> = </span><span style="color:#4f8187" class="">bB</span><span class=""> </span>// Compiler would check covariance of declared generic types.</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)" class=""><span class=""> </span><span style="color:#4f8187" class="">bT</span><span class="">.</span><span style="color:#4f8187" class="">T</span><span class=""> </span>// Bottom.Type</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;min-height:13px" class=""><br class=""></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)" class=""><span class=""> </span>// Demonstrate generic returned type</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)" class=""><span class=""> </span>// Compiler would add cast to declared, generic type.</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)" class=""><span class=""> </span><span style="color:#4f8187" class="">bB</span><span class="">.</span><span style="color:#4f8187" class="">value</span><span class=""> </span><span style="color:#bb2ca2" class="">as</span><span class="">! </span><span style="color:#4f8187" class="">Bottom</span><span class=""> </span>// In practice user would write `bB.value`.</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;min-height:13px" class=""><br class=""></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)" class=""><span class=""> </span>// Demonstrate type safety</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)" class=""><span class=""> </span><span style="color:#4f8187" class="">bT</span><span class=""> = </span><span style="color:#4f8187" class="">BoxAnyObject</span><span class="">(</span><span style="color:#4f8187" class="">Top</span><span class="">.</span><span style="color:#bb2ca2" class="">self</span><span class="">, </span><span style="color:#4f8187" class="">Top</span><span class="">()) </span>// In practice user would write `bT = Box(Top())`.</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class=""> <span style="color:#4f8187" class="">bT</span>.<span style="color:#4f8187" class="">value</span> = <span style="color:#4f8187" class="">Top</span>() <span style="color:#008400" class="">// OK</span></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)" class=""><span class=""> </span>// bT.value = Bottom() // Doesn't work at present because need `>=` for types, but would work in practice</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)" class=""><span class=""> </span>// bB.value = Top() // Runtime error - wrong type</div><div class=""><br class=""></div><div class="">The implications of this proposal are:</div><div class=""><br class=""></div><div class="">1. The compiler can statically type check a read from a stored property.</div><div class="">2. A write to a stored property is type checked at runtime.</div><div class="">3. Protocols can be made generic instead of having an associated type and then they become a proper type with dynamic dispatch.</div><div class="">4. Generic protocols can be a type just like non-generic protocols, structs, and classes and unlike associated type protocols that can only be a generic constraint.</div><div class="">5. The awkwardness of dealing with associated type generics is replaced by a more powerful and easier to understand semantic of a type, just like the other types.</div><div class="">6. There is a lot of ‘non-obvoius’, long code, for example `inits`, that use a `where` clause to constrain an associated type protocol, this would be unnecessary.</div><div class="">7. There are whole types, `AnySequence`, `AnyGenerator`, etc., that would be replaced by a generic protocols, `Sequence`, `Generator`, etc.</div><div class=""><br class=""></div><div class="">Advantages:</div><div class=""><br class=""></div><div class="">1. Covariant generics are a powerful addition to the language.</div><div class="">2. Generics’ invariance are inconsistent with the rest of the language.</div><div class="">3. Generic protocols would become a ‘proper’ type and you could have arrays and fields of a generic protocol.</div><div class="">4. There are many threads on swift-evolution looking at how protocols can be made into a ‘proper’ type or at least a concept that is easier to understand.</div><div class=""><br class=""></div><div class="">Compatibility:</div><div class=""><br class=""></div><div class="">1. This would be a major change since associated types in protocols would be replaced by generics.</div><div class="">2. The new implementation of generics might break some existing `struct` and `class` code, for example if it is dependent on the exact size of an object because the class will have extra fields, one for each generic type, and therefore will be larger.</div><div class=""><br class=""></div><div class="">Disadvantages:</div><div class=""><br class=""></div><div class="">1. Major change.</div><div class="">2. Object size increases.</div><div class=""><br class="">Thanks in advance for any comments,</div><div class=""><br class=""></div><div class=""> — Howard.</div><div class=""><br class=""></div><div class="">PS This is part of a collection of proposals previously presented as “Protocols on Steroids”.</div>
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