[swift-evolution] [Proposal] Explicit Synthetic Behaviour

Nevin Brackett-Rozinsky nevin.brackettrozinsky at gmail.com
Tue Sep 12 10:21:11 CDT 2017

Tony makes an excellent point, and I agree. At some point in the future we
should consider introducing a “transient” attribute for caches and other
non-essential properties. That will make generated conformances more
powerful while simultaneously reducing boilerplate.


On Tue, Sep 12, 2017 at 10:39 AM, Tony Allevato via swift-evolution <
swift-evolution at swift.org> wrote:

> On Mon, Sep 11, 2017 at 10:05 PM Gwendal Roué <gwendal.roue at gmail.com>
> wrote:
>> There is this sample code by Thorsten Seitz with a cached property which
>> is quite simple and clear : https://lists.swift.org/
>> pipermail/swift-evolution/Week-of-Mon-20170911/039684.html
>> This is the sample code that had me enter the "worried" camp.'
> I really like Thorsten's example, because it actually proves that
> requiring explicit derivation is NOT the correct approach here. (Let's set
> aside the fact that Optionals prevent synthesis because we don't have
> conditional conformances yet, and assume that we've gotten that feature as
> well for the sake of argument.)
> Let's look at two scenarios:
> 1) Imagine I have a value type with a number of simple Equatable
> properties. In a world where synthesis is explicit, I tell that value type
> to "derive Equatable". Everything is fine. Later, I decide to add some
> cache property like in Thorsten's example, and that property just happens
> to also be Equatable. After doing so, the correct thing to do would be to
> remove the "derive" part and provide my custom implementation. But if I
> forget to do that, the synthesized operator still exists and applies to
> that type. If you're arguing that "derive Equatable" is better because its
> explicitness prevents errors, you must also accept that there are possibly
> just as many cases where that explicitness does *not* prevent errors.
> 2) Imagine I have a value type with 10 Equatable properties and one
> caching property that also happens to be Equatable. The solution being
> proposed here says that I'm better off with explicit synthesis because if I
> conform that type to Equatable without "derive", I get an error, and then I
> can provide my own custom implementation. But I have to provide that custom
> implementation *anyway* to ignore the caching property even if we don't
> make synthesis explicit. Making it explicit hasn't saved me any work—it's
> only given me a compiler error for a problem that I already knew I needed
> to resolve. If we tack on Hashable and Codable to that type, then I still
> have to write a significant amount of boilerplate for those custom
> operations. Furthermore, if synthesis is explicit, I have *more* work
> because I have to declare it explicitly even for types where the problem
> above does not occur.
> So, making derivation explicit is simply a non-useful dodge that doesn't
> solve the underlying problem, which is this: Swift's type system currently
> does not distinguish between Equatable properties that *do* contribute to
> the "value" of their containing instance vs. Equatable properties that *do
> not* contribute to the "value" of their containing instance. It's the
> difference between behavior based on a type and additional business logic
> implemented on top of those types.
> So, what I'm trying to encourage people to see is this: saying "there are
> some cases where synthesis is risky because it's incompatible with certain
> semantics, so let's make it explicit everywhere" is trying to fix the wrong
> problem. What we should be looking at is *"how do we give Swift the
> additional semantic information it needs to make the appropriate decision
> about what to synthesize?"*
> That's where concepts like "transient" come in. If I have an
> Equatable/Hashable/Codable type with 10 properties and one cache property,
> I *still* want the synthesis for those first 10 properties. I don't want
> the presence of *one* property to force me to write all of that boilerplate
> myself. I just want to tell the compiler which properties to ignore.
> Imagine you're a stranger reading the code to such a type for the first
> time. Which would be easier for you to quickly understand? The version with
> custom implementations of ==, hashValue, init(from:), and encode(to:) all
> covering 10 or more properties that you have to read through to figure out
> what's being ignored (and make sure that the author has done so correctly),
> or the version that conforms to those protocols, does not contain a custom
> implementation, and has each transient property clearly marked? The latter
> is more concise and "transient" carries semantic weight that gets buried in
> a handwritten implementation.
> Here's a fun exercise—you can actually write something like "transient"
> without any additional language support today: https://gist.github.com/
> allevato/e1aab2b7b2ced72431c3cf4de71d306d. A big drawback to this
> Transient type is that it's not as easy to use as an Optional because of
> the additional sugar that Swift provides for the latter, but one could
> expand it with some helper properties and methods to sugar it up the best
> that the language will allow today.
> I would wager that this concept, either as a wrapper type or as a built-in
> property attribute, would solve a significant majority of cases where
> synthesis is viewed to be "risky". If we accept that premise, then we can
> back to our slice of pie and all we're left with in terms of "risky" types
> are "types that contain properties that conform to a certain protocol but
> are not really transient but also shouldn't be included verbatim in
> synthesized operations". I'm struggling to imagine a type that fits that
> description, so if they do exist, it's doubtful that they're a common
> enough problem to warrant introducing more complexity into the protocol
> conformance system.
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