[swift-evolution] Universal Equatability, Hashability, and Comparability

Zach Waldowski zach at waldowski.me
Tue Mar 8 16:57:55 CST 2016

I completely agree with Austin here. Automatic derivation (perhaps
through the same mechanisms Joe is talking about) would be a nice
enhancement, but I find it refreshing and advantageous for simple value
types to have very little automatic behavior.

Cheers!  Zachary Waldowski  zach at waldowski.me

On Tue, Mar 8, 2016, at 05:15 PM, Austin Zheng via swift-evolution wrote:
> I would prefer Equatable and Hashable to remain opt-in, and for us to
> add better support for automatic deriving of implementation.
> For something like printing the representation of an object to a
> string, there exists a "not wrong" mapping of every possible value to
> a string. That is, if my FooStruct doesn't provide a custom
> description, having the runtime convert it to something like
> "(FooStruct instance)" is still a valid mapping. It might not be
> useful, but it's not wrong.
> I don't think the same applies for equatability. The universal default
> behavior for equating two objects is either correct or incorrect, and
> it's not possible to know beforehand which is which. One of the
> wonderful things about the current Swift system is that (modulo some
> exceptional cases) only things explicitly meant to be equatable with
> each other are comparable. We avoid the object-oriented pitfall in
> which 'equality' means two different things - equality of value if you
> implemented an override properly; a default 'equality of instance'
> otherwise (which might be right or wrong). Of course, the same pitfall
> wouldn't necessarily apply in our case, but the problem of having a
> 'default' == impl that allows a developer to falsely assume their type
> is being properly compared (or not think about it at all) would still
> be present.
> Best, Austin
> On Tue, Mar 8, 2016 at 2:02 PM, Brian Pratt via swift-evolution <swift-
> evolution at swift.org> wrote:
>> Definitely a +1 on the basics. When you get inheritance involved,
>> does that complicates things a little bit?
>> Let's say I have a subclass instance that has corresponding fields
>> with a superclass instance. Is it equal to said super-class instance
>> using just member-wise comparisons? Would that be problematic? In
>> Scala you'd often use a reference to an "equality contract" object
>> type in order to get "transitive" equality between subclasses and
>> superclasses, which definitely feels like a step backwards from the
>> current protocol-driven approach.
>> On Tue, Mar 8, 2016 at 2:54 PM, Joe Groff via swift-evolution <swift-
>> evolution at swift.org> wrote:
>>> (starting a new thread by DaveA's request)
There's a definition of equality that makes sense as a default for
nearly every type in our system:
- Basic types like IntNN, FloatNN, String, etc. have domain-
  defined equality,
- Structs and tuples can be considered equal if their corresponding
  fields are equal,
- Enums can be considered equal if they carry the same, equal payload,
- Class references can be considered equal if they refer to the
  same instance,
- Metatypes can be considered equal if they represent the same type, and
- Existentials can be considered equal if they carry equal values of the
  same dynamic type.
and similarly, reasonable hash code implementations could be synthesized
by applying a standard hash combine operation over the components, and a
default ordering  could be assigned to values of every type. I think
it's worth considering whether Equatable, Hashable, and/or Comparable,
instead of being explicit protocols, should become universal behavior
like 'print', with customization points to override the default
behavior. If Equatable and Hashable behavior were universal, that would
solve many of the common problems people currently have trying to work
with heterogeneous containers. In object-oriented frameworks, including
Cocoa, Java, and .NET, it is common for the root (NS)Object class to
provide default equality and hashing operations. There are of course
some tradeoffs:
- Universal behavior would require us to either generate code for '==',
  'hashValue', and/or '<' for every type, or provide sufficient
  reflection info for a common runtime implementation to do it. The reflection-
  based approach may be reasonable for print(), since dumping reflection
  info only reduces the quality of the default logging behavior, but
  '==' and 'hashValue' are more essential to proper behavior, so relying
  on reflection might be too slow, and would be brittle when we
  introduce the ability to drop reflection info.
- Type safety with '==' is important to prevent accidental '1 == "1"'
  type comparsions, and a fully generic 'func ==<T>(x: T, y: T) -> Bool'
  could potentially allow those sorts of mixed-type comparisons by
  accident. Language rules that constrained when generic parameters can
  be resolved to supertypes might help here.
- Function types in Swift do not provide a ready equality operation.
  We could provide a default implementation that always returns
  'false', perhaps.
- A Comparable ordering can be dreamt up for many types, but it's not
  always a stable ordering, or a desired one. Many people have
  complained that 'nil < .Some(1)' works for optionals, for instance,
  ordering 'nil' below Some values. We could use pointer identity to
  order class instances and types, but this wouldn't be a stable
  ordering across process runs. That might be good enough for ordered
  collections like search trees, but is weaker than what many people
  expect '<' to do.
It's my feeling that Equatable and Hashable would make a lot of sense as
universal operations; I'm not so sure about Comparable.
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