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

[Dave Abrahams]

on Tue Apr 26 2016, Tony Allevato <swift-evolution at swift.org> 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.
>
> 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.
>
> 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.

The main reasons to route through a single generic operator
implementation are:

* User experience; we want to cut down the number of overloads of any
  operator to a manageable set, in part because they live in the global
  namespace.  When you look at a list of functions in the global
  namespace, seeing fifty instances of `func +` is not helpful.

* Type checker speed.  Having all of these overloads around has
  historically put a strain on the type checker and made compilation
  slow.  That may be less true today than it once was, though.


HTH,

-- 
Dave