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? >> >> >> On Sun, Dec 25, 2016 at 1:24 PM, Chris Lattner <[email protected]> >> wrote: >> >> On Dec 25, 2016, at 12:54 PM, Adam Nemecek via swift-evolution < >> [email protected]> wrote: >> >> Does enabling a lot of small improvements that make APIs more ergonomic >> count as practical? >> >> >> Yes, that would count as practical, but Xiaodi’s question is just as >> important. *Which* APIs become more ergonomic? >> >> Here are a couple of more questions: >> >> 1) How does this square with Swift’s general philosophy to not default >> initialize values to “zero”? >> >> 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. >> >> -Chris >> >> >> On Sun, Dec 25, 2016 at 12:19 PM, Xiaodi Wu <[email protected]> wrote: >> >> On Sun, Dec 25, 2016 at 3:07 PM, Adam Nemecek <[email protected]> >> wrote: >> >> There's a book that provides quite a bit of info on this >> >> https://smile.amazon.com/Elements-Programming-Alexander-Stepanov/dp/ >> 032163537X?sa-no-redirect=1 >> >> They say that DefaultConstructible is one of the essential protocols on >> which most algorithms rely in one way or another. One of the authors is the >> designer of the C++ STL and basically the father of modern generics. >> >> This protocol is important for any algebraic structure that deals with >> the concept of appending or addition (as "zero" is one of the requirements >> of monoid). There isn't a good short answer to your question. It's a >> building block of algorithms. Think about why a RangeReplaceableCollection >> can provide you with a default constructor but a Collection can't. >> >> >> It's well and fine that most algorithms rely on the concept in one way or >> another. Yet the Swift standard library already implements many generic >> algorithms but has no DefaultConstructible, presumably because there are >> other protocols that guarantee `init()` and the algorithms being >> implemented don't need to be (practically speaking) generic over all >> DefaultConstructible types. My question is: what practical use cases are >> there for an explicit DefaultConstructible that are impractical today? >> >> >> On Sun, Dec 25, 2016 at 11:37 AM, Xiaodi Wu <[email protected]> wrote: >> >> Can you give some other examples of generic algorithms that would make >> use of this DefaultConstructible? I'm having trouble coming up with any >> other than reduce. >> On Sun, Dec 25, 2016 at 14:23 Adam Nemecek via swift-evolution < >> [email protected]> wrote: >> >> This protocol is present in C++ http://en.cppreference.com/w/cpp/concept/ >> DefaultConstructible as well as in Rust https://doc.rust-lang.org/std/ >> default/ >> >> It's the identity element/unit of a monoid or a zero. >> >> The Swift implementation is very simple (I'm open to different names) >> >> protocol DefaultConstructible { >> init() >> } >> >> A lot of the standard types could then be made to conform to this >> protocol. These include all the numeric types, collection types (array, >> set, dict), string, basically at least every type that currently has a >> constructor without any arguments. >> >> The RangeReplaceableCollection protocol would inherit from this protocol >> as well. >> >> This protocol would simplify a lot of generic algorithms where you need >> the concept of a zero (which shows up a lot) >> >> Once introduced, Sequence could define an alternative implementation of >> reduce where the initial result doesn't need to be provided as it can be >> default constructed. >> >> >> >> >> >> >> >> _______________________________________________ >> >> >> swift-evolution mailing list >> >> >> [email protected] >> >> >> https://lists.swift.org/mailman/listinfo/swift-evolution >> >> >> >> >> >> >> >> >> >> >> >> >> >> _______________________________________________ >> swift-evolution mailing list >> [email protected] >> https://lists.swift.org/mailman/listinfo/swift-evolution >> >> >> >> >> >> >> >> >> >> >> >> >> >> >> >> >> >> _______________________________________________ >> >> swift-evolution mailing list >> >> [email protected] >> >> https://lists.swift.org/mailman/listinfo/swift-evolution >> >>
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