[swift-evolution] protocol-oriented integers (take 2)

[Dave Abrahams]

on Sun Jan 15 2017, Xiaodi Wu <xiaodi.wu-AT-gmail.com> wrote:

> On Sun, Jan 15, 2017 at 6:42 PM, David Sweeris <davesweeris at mac.com> wrote:
>
>>
>>
>>
>> 
>> On Jan 15, 2017, at 18:02, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>>
>> "Mathematically correct" integers behave just like Int in that there is
>> not a multiplicative inverse. What we're trying to do here is to determine
>> how much of what we know about mathematics is usefully modeled in the
>> standard library. The answer is not zero, because there is more than just
>> counting that people do with integers.
>>
>>
>> It's an interesting problem... When I was in school, "integer" division
>> "returned" a "quotient and remainder", a "fraction" (which, occasionally,
>> could be simplified to just an integer), or a "real". We never talked about
>> division in the context of "(Int, Int) -> Int", though. OTOH, I never took
>> any math classes past Differential Equations or Linear Algebra, either...
>> I'm *aware* of areas of math where you formally restrict yourself to the
>> kind of "(Int, Int) -> Int" operations we're doing here, but I don't
>> really know much about it. Is division even well-defined in that context?
>>
>> - Dave Sweeris
>>
>
> I'm no mathematician, and I'm not sure how to tackle the question of
> "well-defined." Hopefully someone who is more knowledgable can chime in
> here.
>
> But I'll have a go at replying to your point as it relates to the practical
> issue here. Two Int values can be "divided" to produce another Int, and
> that gives a predictable and well-understood result. It's an operation
> that's always going to be there--first, because it'd be insane to remove it
> since much working code relies on it, and second, because we're only
> re-designing integer protocols and not the concrete types. However, it _is_
> true that such an operation has very different semantics from division as
> you learned it in math.
>
> This is why I'm advocating for perhaps another look at the top of this
> integer protocol hierarchy. At the moment, `Arithmetic` offers reasonable
> semantic guarantees for a lot of things, but `/` has different semantics
> for integer types and floating point types 

Well, that really depends on how closely you look.  From one
point-of-view, floating point division and integer division *both*
produce approximate results.

> and is really closer to just syntax. Other mathematical types--which
> certainly the stdlib doesn't have to offer, but the stdlib protocol
> hierarchy shouldn't preclude their conformance to relevant protocols
> if it's possible--such as fractions and complex numbers, share with
> floating point types the semantics of `/` that qualify these types as
> fields. Dave A's question as to practical uses can probably best be
> answered in this way: to the extent that any generic algorithm relies
> on `/` having semantics and can be applied to fractions and real
> numbers, it would be useful to distinguish such an operation from
> integer "division."

That's not a bad answer.  Ruminating on this...

-- 
-Dave