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

[Austin Zheng]

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.
>>
>> -Joe
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>
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