[swift-evolution] [pitch] Comparison Reform

Ben Cohen ben_cohen at apple.com
Tue Apr 18 10:40:47 CDT 2017

> 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.

This is a bump in the rug – push it down in one place, it pops up in another. I feel like this proposal at least moves the bump to where fewer people will trip over it. I think it highly likely that the intersection of developers who understand enough about floating point to write truly correct concrete code, but won’t know about or discover the documented difference in generic code, is far smaller than the set of people who hit problems with the existing behavior.

 A more comprehensive solution, with additional protocols or overloads, representation of unordered comparison etc, might be able to flatten the rug completely, but probably at the cost of introducing complexity that could act as a barrier to entry into the world of writing generic code.

> -Chris
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