on Sun Dec 25 2016, Daniel Leping <[email protected]> wrote:
> You are right, usually it's required to implement a protocol which is not a > good approach. The best is plain objects which can be used independently of > ORM if needed (as DTOs, i.e.). > > I was thinking of DefaultConstructable as a protocol automatically applied > to any class/struct having a default init, which is really logical for > me. Swift doesn't do implicit conformance. It always has to be declared explicitly. I'm pretty sure Doug Gregor can explain why better than I could. > On Mon, 26 Dec 2016 at 9:41 Xiaodi Wu <[email protected]> wrote: > >> On Sun, Dec 25, 2016 at 10:50 PM, Daniel Leping >> <[email protected]> >> wrote: >> >> Ok, an example from ORM. You have an entity factory with a virtual (read, >> overloadable in the subclasses) method populating the properties. >> DefaultConstructable is a great choice here. Otherwise you will have to >> force the users of your ORM to implement a certain protocol, which you most >> probably would like to avoid. >> >> >> Sorry--I'm not very familiar with using Swift for ORM purposes. Why do you >> want to avoid having your users conform to a certain protocol? Wouldn't the >> users of your ORM have to conform to `DefaultConstructible` then? I'm >> looking at Swift ORMs, and all require users to conform to a protocol or >> inherit from a base class, typically named `Model` or similar. From a quick >> Google search: >> >> https://vapor.github.io/documentation/fluent/model.html >> https://github.com/blitzagency/amigo-swift >> >> >> In general I think the best showcase is generic factories. >> >> On Mon, 26 Dec 2016 at 9:02 Xiaodi Wu <[email protected]> wrote: >> >> On Sun, Dec 25, 2016 at 10:18 PM, Daniel Leping >> <[email protected]> >> wrote: >> >> Usually it's a generic function that needs to return a value from some >> other function or a default value (zero) in a case of some conditions. >> Optional value is an arguable solution in quite some scenarios. Afaik, >> sometimes it can be used for optional resolution. >> >> >> Right, I'd agree that Optional is the idiomatic way to do it. Afaict, >> there's not much you can do with a default value that you couldn't with >> nil, unless you have some guarantee as to _what_ that default is; however, >> I'd expect that in every case that you can rely on a guarantee about a >> default value which would be more useful than nil, it's going to require >> more specific knowledge of your type than an all-encompassing >> `DefaultConstructible` can provide. >> >> Also, generic factories. Widely used in ORM solutions. >> >> >> Can you elaborate on this? Why is Optional not a solution here? >> >> >> As mentioned above, algorythmical stuff that requires Zero. >> >> >> I'm still not convinced there exist credible use cases that need to be >> generic over both collections and floating point, for instance. In fact, in >> my experience, there are few math-heavy algorithms where one can ignore >> even the distinction between integers and binary floating point. By the >> time you get down to matrix math, you start to run into difficulties that >> require separate implementations for Float and Double. >> >> On Mon, 26 Dec 2016 at 8:38 Xiaodi Wu <[email protected]> wrote: >> >> Can you give some examples of what you used this approach to do? >> >> >> On Sun, Dec 25, 2016 at 9:49 PM, Daniel Leping >> <[email protected]> >> wrote: >> >> +1 to this approach. I remember I had to create it on my own for my >> projects. Would be nice to have it out of the box. >> >> On Mon, 26 Dec 2016 at 8:11 Adam Nemecek via swift-evolution < >> [email protected]> wrote: >> >> > Yes, those particular types have initializers that take no arguments. >> That does not address my question. You merely restated your initial >> observation that many types in Swift have implemented `init()`. >> >> Right, it's an empirical argument. >> >> > I didn't think the value returned by `init()` was regarded as any sort >> of zero--or even any sort of "default." In fact, some types in Foundation >> have a static property called `default` distinct from `init()`. >> >> Let's not talk about those then. This would not apply to every single type >> in existence, as I've stated previously. >> >> > It gives you something different every time. How can this be squared >> with your stated motivation regarding concepts of zero and concepts of >> equality? >> >> Due to the fact that it's a resource, not a value. As I've stated above, >> not all of this applies to types that are more resource-like. >> >> > Or, it's what you get because that's the most trivial possible string. >> Quite simply, I do not think the designer of most types that implement >> `init()` have paused to wonder whether the value that you get is the >> identity element associated with the most useful and prominent operation >> that can be performed on that type. I certainly never have. >> >> This is an appeal to tradition. >> >> > The statement I wrote was in JavaScript, so I'm not sure what you mean >> by returning an optional. `[].reduce((a, b) => a + b)` results in an >> error in JavaScript. In Swift, such a function may also be implemented with >> a precondition that the array is not empty and would not return an optional. >> >> I was talking about their analogous swift implementations. >> >> > Can you give an example of an algorithm dealing with tensors where you >> would use a `DefaultConstructible` generic over all types that have >> `init()`, as opposed to working with the existing `Integer`, >> `FloatingPoint`, and other numerical protocols? >> >> If it's implemented as either nested collections or numbers. >> >> >> >> On Sun, Dec 25, 2016 at 6:00 PM, Xiaodi Wu <[email protected]> wrote: >> >> On Sun, Dec 25, 2016 at 7:30 PM, Adam Nemecek <[email protected]> >> wrote: >> >> > Is it well settled, either in Swift or in C++/Rust/etc., that the value >> returned by a default initializer/constructor is regarded as an identity >> element or zero? >> >> Int() == 0, String() == "" so to some extent by convention, a lot of types >> have a default value as is. >> >> >> Yes, those particular types have initializers that take no arguments. That >> does not address my question. You merely restated your initial observation >> that many types in Swift have implemented `init()`. >> >> I didn't think the value returned by `init()` was regarded as any sort of >> zero--or even any sort of "default." In fact, some types in Foundation have >> a static property called `default` distinct from `init()`. In Rust, the >> Default trait requires a function called `default()`, which is documented >> as being useful when you want "some kind of default value, and don't >> particularly care what it is." >> >> I was asking whether there's some understanding, of which I've been >> unaware, that the result of `init()` (or the equivalent in other languages) >> is expected to be some sort of zero or an identity element. I'm not aware >> of any evidence to that effect. Are you? >> >> > Is the thread that I get by writing `let t = Thread()` some kind of zero >> in any reasonable sense of the word? >> >> DefaultConstructibility makes less sense for types that represent some >> sort of resource but make sense for things that are values. But even in >> this case, Thread() gives you a default value for example if you are >> working with a resizable collection of threads. >> >> >> It gives you something different every time. How can this be squared with >> your stated motivation regarding concepts of zero and concepts of equality? >> >> A better question is why does thread currently implement a default >> constructor? >> >> >> It's an initializer that takes no arguments, because none are needed for a >> new thread. How else would you write it? >> >> > Do you mean to argue that for an integer the additive identity should be >> considered "more prominent and useful" than the multiplicative identity? >> I'm not aware of any mathematical justification for such a conclusion. >> >> I do. The justification is that if I call the default constructor of Int >> currently, I get the value of 0. >> >> >> This is backwards. Why do you believe that the value you obtain from >> `init()` is intended to be an identity element at all, let alone the most >> important one? (It's also circular reasoning. Since `init()` only ever >> gives you one value at a time, by your reasoning it demonstrates that every >> type must have one "more prominent and useful" identity, which is begging >> the question.) >> >> Which means that the binary operation must be addition. >> >> >> Based on the value of `Int.init()`, you conclude that addition of integers >> is a "more prominent and useful" operation than multiplication? Again, this >> is backwards. Rather, we know that each numerical type belongs to multiple >> ring algebras; there is no basis for reckoning any as "more useful." Since >> `init()` can only ever give us one value at a time, we know that `init()` >> cannot give a value that is a meaningful default with respect to any >> particular operation. >> >> If I call String() I get "" which is the identity of the + String >> operation. >> >> >> Or, it's what you get because that's the most trivial possible string. >> Quite simply, I do not think the designer of most types that implement >> `init()` have paused to wonder whether the value that you get is the >> identity element associated with the most useful and prominent operation >> that can be performed on that type. I certainly never have. >> >> > Going to your original example, I should add: other languages provide a >> version of `reduce` that doesn't require an initial result (for instance, >> JavaScript). In JavaScript, `[1, 2, 3].reduce((a, b) => a + b)` uses the >> element at array index 0 as the initial result, and the accumulator >> function is invoked starting with the element at array index 1. This is >> precisely equivalent to having `reduce` use the additive identity as the >> default initial result when + is the accumulator function and the >> multiplicative identity when * is the accumulator function (with the >> accumulator function being invoked starting with the element at array index >> 0). It does not require a DefaultConstructible protocol. What more >> ergonomic solution could be implemented using a monoidic wrapper type? >> >> These two will have different signatures. The reduce you describe returns >> an optional, >> >> >> The statement I wrote was in JavaScript, so I'm not sure what you mean by >> returning an optional. `[].reduce((a, b) => a + b)` results in an error >> in JavaScript. In Swift, such a function may also be implemented with a >> precondition that the array is not empty and would not return an optional. >> >> the other one would returns the default value. >> >> >> In what scenario would you prefer to propagate a default after reducing a >> potential empty collection _without supplying an explicit default_ for that >> operation? This would certainly violate the Swift convention of not >> defaulting to zero and, I suspect, most users of Swift would not regard >> that as ergonomic at all. >> >> >> Fundamentally the default constructibles are useful in numerical >> computations e..g. dealing with tensors. >> >> >> Can you give an example of an algorithm dealing with tensors where you >> would use a `DefaultConstructible` generic over all types that have >> `init()`, as opposed to working with the existing `Integer`, >> `FloatingPoint`, and other numerical protocols? (I should also add, FWIW, I >> have never seen a generic algorithm written for integers or FP types that >> has preferred the use of `T()` over `0`.) >> >> >> On Sun, Dec 25, 2016 at 3:30 PM, Xiaodi Wu <[email protected]> wrote: >> >> On Sun, Dec 25, 2016 at 5:27 PM, Adam Nemecek <[email protected]> >> wrote: >> >> > *Which* APIs become more ergonomic? >> >> I'll get back to this question in a second if I may. This would be a >> longer discussion and I first want to make sure that before we get into the >> details that there is a possibility of this being introduced (I'm asking if >> violating the no zero defaults is more important than slightly more >> ergonomic APIs). But to give a broad answer I think that the concept of a >> zero is closely related to the concept of equality (and all the things that >> build up on equality such as comparability and negation). >> >> > 1) How does this square with Swift’s general philosophy to not default >> initialize values to “zero”? >> >> I actually wasn't aware of this philosophy. Despite this philosophy, look >> at how many types actually currently implement a default constructor. >> >> >> (Not a rhetorical question:) Is it well settled, either in Swift or in >> C++/Rust/etc., that the value returned by a default initializer/constructor >> is regarded as an identity element or zero? Is the thread that I get by >> writing `let t = Thread()` some kind of zero in any reasonable sense of the >> word? >> >> >> Also can I ask what's the motivation behind this philosophy? >> I think that in Swift, default constructibility makes complete sense for >> (most?) structs, maybe less so for classes. >> >> > 2) To your original example, it isn’t immediately clear to me that >> reduce should choose a default identity. Some types (e.g. integers and FP) >> belong to multiple different ring algebras, and therefore have different >> identity values that correspond to the relevant binary operations. >> >> This is a good point that I've considered as well but felt that for the >> most part, there is one particular identity and associated operation that >> is more prominent and useful than others. Furthermore, modeling different >> algebras isn't mutually exclusive with writing generic algorithms that rely >> on this protocol, you can always introduce some monoidic wrapper type that >> defines the more appropriate default value and operation. >> >> >> Do you mean to argue that for an integer the additive identity should be >> considered "more prominent and useful" than the multiplicative identity? >> I'm not aware of any mathematical justification for such a conclusion. >> >> Going to your original example, I should add: other languages provide a >> version of `reduce` that doesn't require an initial result (for instance, >> JavaScript). In JavaScript, `[1, 2, 3].reduce((a, b) => a + b)` uses the >> element at array index 0 as the initial result, and the accumulator >> function is invoked starting with the element at array index 1. This is >> precisely equivalent to having `reduce` use the additive identity as the >> default initial result when + is the accumulator function and the >> multiplicative identity when * is the accumulator function (with the >> accumulator function being invoked starting with the element at array index >> 0). It does not require a DefaultConstructible protocol. What more >> ergonomic solution could be implemented using a monoidic wrapper type? >> >> >> > _______________________________________________ > swift-evolution mailing list > [email protected] > https://lists.swift.org/mailman/listinfo/swift-evolution > -- -Dave _______________________________________________ swift-evolution mailing list [email protected] https://lists.swift.org/mailman/listinfo/swift-evolution
