[swift-evolution] [pitch] Comparison Reform

[Xiaodi Wu]

On Sat, Apr 22, 2017 at 7:02 PM, Matthew Johnson <matthew at anandabits.com>
wrote:

>
>
> Sent from my iPad
>
> On Apr 22, 2017, at 4:53 PM, Xiaodi Wu via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> On Sat, Apr 22, 2017 at 4:14 PM, Dave Abrahams <dabrahams at apple.com>
> wrote:
>
>>
>> on Tue Apr 18 2017, Xiaodi Wu <xiaodi.wu-AT-gmail.com> wrote:
>>
>> > On Tue, Apr 18, 2017 at 10:40 AM, Ben Cohen via swift-evolution <
>> > swift-evolution at swift.org> wrote:
>> >
>> >>
>> >> On Apr 17, 2017, at 9:40 PM, Chris Lattner via swift-evolution <
>> >> swift-evolution at swift.org> wrote:
>> >>
>> >>
>> >> On Apr 17, 2017, at 9:07 AM, Joe Groff via swift-evolution <
>> >> swift-evolution at swift.org> wrote:
>> >>
>> >>
>> >> On Apr 15, 2017, at 9:49 PM, Xiaodi Wu via swift-evolution <
>> >> swift-evolution at swift.org> wrote:
>> >>
>> >> For example, I expect `XCTAssertEqual<T : FloatingPoint>(_:_:)` to be
>> >> vended as part of XCTest, in order to make sure that
>> `XCTAssertEqual(resultOfComputation,
>> >> Double.nan)` always fails.
>> >>
>> >>
>> >> Unit tests strike me as an example of where you really *don't* want
>> level
>> >> 1 comparison semantics. If I'm testing the output of an FP operation, I
>> >> want to be able to test that it produces nan when I expect it to, or
>> that
>> >> it produces the right zero.
>> >>
>> >>
>> >> I find it very concerning that == will have different results based on
>> >> concrete vs generic type parameters.  This can only lead to significant
>> >> confusion down the road.  I’m highly concerned about situations where
>> >> taking a concrete algorithm and generalizing it (with generics) will
>> change
>> >> its behavior.
>> >>
>> >>
>> >> It is already the case that you can start with a concrete algorithm,
>> >> generalize it, and get confusing results – just with a different
>> starting
>> >> point. If you start with a concrete algorithm on Int, then generalize
>> it to
>> >> all Equatable types, then your algorithm will have unexpected behavior
>> for
>> >> floats, because these standard library types fail to follow the rules
>> >> explicitly laid out for conforming to Equatable.
>> >>
>> >> This is bad. Developers need to be able to rely on those rules. The
>> >> standard library certainly does:
>> >>
>> >> let a: [Double] = [(0/0)]
>> >> var b = a
>> >>
>> >> // true, because fast path buffer pointer comparison:
>> >> a == b
>> >>
>> >> b.reserveCapacity(10) // force a reallocation
>> >>
>> >> // now false, because memberwise comparison and nan != nan,
>> >> // violating the reflexivity requirement of Equatable:
>> >> a == b
>> >>
>> >>
>> >> Maybe we could go through and special-case all the places in the
>> standard
>> >> library that rely on this, accounting for the floating point behavior
>> >> (possibly reducing performance as a result). But we shouldn't expect
>> users
>> >> to.
>> >>
>> >
>> > I was not thinking about the issue illustrated above, but this is
>> > definitely problematic to me.
>> >
>> > To be clear, this proposal promises that `[0 / 0 as Double]` will be
>> made
>> > to compare unequal with itself, yes?
>>
>> Nope.
>>
>> As you know, equality of arrays is implemented generically and based on
>> the equatable conformance of their elements.  Therefore, two arrays of
>> equatable elements are equal iff the conforming implementation of
>> Equatable's == is true for all elements.
>>
>> > It is very clear that here we are working with a concrete FP type and
>> > not in a generic context, and thus all IEEE FP behavior should apply.
>>
>> I suppose that's one interpretation, but it's not the right one.
>>
>> If this were C++, it would be different, because of the way template
>> instantiation works: in a generic context like the == of Array, the
>> compiler would look up the syntactically-available == for the elements
>> and use that.  But Swift is not like that; static lookup is done at the
>> point where Array's == is compiled, and it only finds the == that's
>> supplied by the Element's Equatable conformance.
>>
>> This may sound like an argument based on implementation details of the
>> language, and to some extent it is.  But that is also the fundamental
>> nature of the Swift language (and one for which we get many benefits),
>> and it is hopeless to paper over it.  For example, I can claim that all
>> doubles are equal to one another:
>>
>>   9> func == (lhs: Double, rhs: Double) -> Bool { return true }
>>  10> 4.0 == 1.0
>> $R2: Bool = true
>>  11> [4.0] == [1.0]  // so the arrays should be equal too!
>> $R3: Bool = false
>>
>> Another way to look at this is that Array is not a numeric vector, and
>> won't be one no matter what you do ([1.0] + [2.0] => [1.0, 2.0]).  So it
>> would be wrong for you to expect it to reflect the numeric properties of
>> its elements.
>>
>
> I understand that's how the generic Array<T> would work, but the proposal
> as written promises FP-aware versions of these functions. That is to say, I
> would expect the standard library to supply an alternative implementation
> of equality for Array<T where T : FloatingPoint>.
>
>
> Are you suggesting it implies that Array's Equatable conformance is
> implemented differently when T: FloatingPoint?  Is it even possible to
> provide such an implementation given the current restriction that bans
> overlapping conformance?  (If there is a technique for implementing
> something like this in Swift 4 I would love to learn it)
>

I don't believe it's possible, but there is compiler magic being proposed
to make this protocol stick together, so we are not bound by the limits of
Swift 4-expressible designs.
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