[swift-evolution] [swift-evolution-announce] [Review] SE-0067: Enhanced Floating Point Protocols

Nicola Salmoria nicola.salmoria at gmail.com
Tue Apr 26 12:36:53 CDT 2016

On Tue, Apr 26, 2016 at 4:28 PM, Tony Allevato <allevato at google.com> wrote:

> On Sun, Apr 24, 2016 at 2:57 AM Nicola Salmoria via swift-evolution <
> swift-evolution at swift.org> wrote:
>> > > func isEqual(to other: Self) ->Bool
>> > > func isLess(than other: Self) ->Bool
>> > > func isLessThanOrEqual(to other: Self) ->Bool
>> >
>> > I'm still not sure why these are methods instead of operators.
>> I think this is an *excellent* choice, and I hope it is the first step to
>> completely removing operators from protocols.
>> IMHO throwing operators into protocols is inconsistent and confusing.
>> Having regular methods and a single generic version of the operator that
>> calls down on the type’s methods is clearer and guarantees that generic
>> code can avoid ambiguities by calling the methods directly, instead of
>> having to rely only on heavily overloaded global operators.
> I personally disagree on this point. To me, a protocol describes a set of
> requirements for a type to fulfill, which includes things other than
> methods. Just as a protocol can define initializers, properties, and
> associated types that a type must define in order to conform, it makes
> sense that a protocol would also define which operators a conforming type
> must support.

Well, I'm not sure about that. A protocol describes what a type can do, so
it's debatable whether a global function is within this scope.

Operators are magically special: you can declare them inside a protocol and
require them to be available for conformance, even if they don't belong to
the type. You can't do the same thing for normal global functions, yet
conceptually global functions and operators are the same thing.

> Introducing a mapping between names and operators poses a few problems:
> – IMO, they are overly verbose and add noise to the definition. This makes
> the language look less clean (I'm getting visions of NSDecimalNumber).
> – They expose two ways to accomplish the same thing (writing
> `x.isEqual(to: y)` and `x == y`).
> – Do certain operators automatically get mapped to method names with
> appropriate signatures across all types, or does a conforming type still
> have to provide that mapping by implementing the operators separately? If
> it's the latter, that's extra work for the author of the type writing the
> protocol. If it's the former, does it make sense to automatically push
> these operators for all types? Should any type that has an `add` method
> automatically get `+` as a synonym as well? That may not be desirable.

The difference at the protocol declaration is between:

protocol Equatable {
    func ==(lhs: Self, rhs: Self) -> Bool


protocol Equatable {
    func isEqual(to other: Self) -> Bool

func ==<T: Equatable>(lhs: T, rhs: T) -> Bool {
    return lhs.isEqual(to: rhs)

so the latter is a bit more verbose, but arguably clearer in intent, and
not different from how you would define any generic global function using a
protocol, or from how you can define protocol extensions with default
implementations that take advantage of the protocol's core methods.

The difference for the conformance is between:

extension Foo : Equatable { }

func ==(lhs: Foo, rhs: Foo) -> Bool {
    return <comparison>


extension Bar : Equatable {
    func isEqual(to: Bar) -> Bool {
        return <comparison>

the former way to define the conformance can be confusing to newbies. The
latter is straightforward and consistent with the usual way to adopt a

The == operator looks exactly the same at its use points, but the way how
it's implemented is different.
In the former case, it's many overloads of a global function, which can
stress the compiler's type inference and doesn't offer an obvious way to
disambiguate in case of ambiguities.
In the latter case, there is only one generic definition of ==, which
automatically applies to all types that conform to the protocol.


> I'm very supportive of the floating-point protocol proposal in general,
> but I feel the arithmetic and comparison operations should be exposed by
> operators alone and not by methods, where there is a suitable operator that
> has the intended meaning.
>>>> Nicola
>> _______________________________________________
>> swift-evolution mailing list
>> swift-evolution at swift.org
>> https://lists.swift.org/mailman/listinfo/swift-evolution
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