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

Xiaodi Wu xiaodi.wu at gmail.com
Sun Dec 25 14:19:30 CST 2016

On Sun, Dec 25, 2016 at 3:07 PM, Adam Nemecek <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
> 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> 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> 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
>>> swift-evolution at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
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