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

[Adam Nemecek]

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.



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.
>>
>>
>>
>>
>>
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>
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