+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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