[swift-evolution] [Pitch] Add the DefaultConstructible protocol to the standard library

[Chris Lattner]

On Dec 25, 2016, at 12:54 PM, Adam Nemecek via swift-evolution <swift-evolution at swift.org> 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 <xiaodi.wu at gmail.com <mailto:xiaodi.wu at gmail.com>> wrote:
> On Sun, Dec 25, 2016 at 3:07 PM, Adam Nemecek <adamnemecek at gmail.com <mailto:adamnemecek at gmail.com>> 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 <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 <xiaodi.wu at gmail.com <mailto:xiaodi.wu at gmail.com>> 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 <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> This protocol is present in C++ http://en.cppreference.com/w/cpp/concept/DefaultConstructible <http://en.cppreference.com/w/cpp/concept/DefaultConstructible> as well as in Rust https://doc.rust-lang.org/std/default/ <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
> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
> https://lists.swift.org/mailman/listinfo/swift-evolution <https://lists.swift.org/mailman/listinfo/swift-evolution>
> 
> 
> 
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20161225/66cd69cd/attachment.html>