[swift-evolution] [Proposal draft] Enhanced floating-point protocols

Thorsten Seitz tseitz42 at icloud.com
Thu Apr 21 08:06:38 CDT 2016

I totally agree with Dave Sweeris. He has put the issue quite nicely.

Being able to add protocol conformance to a protocol would indeed solve this problem (having well designed protocols in the standard library would still be nice, though, so many thanks for the pointers, Dave A, that looks really interesting!).


> Am 20.04.2016 um 23:23 schrieb Dave via swift-evolution <swift-evolution at swift.org>:
>> On Apr 20, 2016, at 1:15 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
>> on Tue Apr 19 2016, Thorsten Seitz <swift-evolution at swift.org> wrote:
>>> I'd like to have something like Summable with 'add', 'adding' and 'zero' being a
>>> separate protocol as well as somthing like Multiplicative with 'multiply',
>>> 'multiplied' and 'one' being a separate protocol, because these are universally
>>> interesting for other cases, e.g. Summable would be useful for defining path
>>> lengths in a graph library.
>>> Would you mind adding that to the proposal?
>> I suspect you may be headed into the realm of
>> protocols-as-bags-of-syntax.
> You say that like it’s a bad thing… Am I missing the point? (NOT a rhetorical question)
> Speaking only for myself, I want stuff broken up into many simpler protocols (which `Arithmetic` and such conform to) because there are many algorithms which only require small parts of the larger protocol. What if I wanted to add a function that sums all the elements in a collection?
> extension CollectionType where Generator.Element: Arithmetic {
>     func sum() -> Generator.Element {
>         var s = Generator.Element()
>         for e in self {
>             s.add(e)
>         }
>         return s
>     }
> }
> Yeah, it works, but if I want to get the sum of a collection of some custom type, I have to implement *all* of `Arithmetic`, as opposed to just the two parts of it (`init()` and `.add(:)`) that the algorithm actually uses. It’d be both simpler for the users and *more to the point of the function*, if it could be written like this:
> extension CollectionType where Generator.Element: Addable { // "Addable" instead of "Arithmetic"
>     ... // No change here
> }
> Now, if Swift allowed you to add protocol conformance to protocols:
> protocol Addable {
>     ... // Relevant subset of `Arithmetic`
> }
> #for T in Arithmetic { // "#for" because this is clearly macro-ish, and # seems to be what we've settled on for that
>     extension T : Addable {} // Don't need anything here since `Arithmetic` already has everything in `Addable`
> }
> … then this all becomes a somewhat moot point. If some generic function only needs a subset of a protocol’s functionality, said function’s author could do this “#for T …” song & dance, and their function would be defined with the minimum constraints.
>> Look at pages 14 and 24 of
>> <http://www.cs.indiana.edu/pub/techreports/TR638.pdf>—which does the job
>> right for C++—and you'll see why.
> COOL!!! Thanks for posting that, it looks like it’ll be a great read! :-D
> - Dave Sweeris
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