[swift-evolution] [Proposal] Explicit Synthetic Behaviour

Tony Allevato tony.allevato at gmail.com
Tue Sep 12 09:38:23 CDT 2017

On Mon, Sep 11, 2017 at 10:05 PM Gwendal Roué <gwendal.roue at gmail.com>

>> This doesn't align with how Swift views the role of protocols, though.
>> One of the criteria that the core team has said they look for in a protocol
>> is "what generic algorithms would be written using this protocol?"
>> AutoSynthesize doesn't satisfy that—there are no generic algorithms that
>> you would write with AutoEquatable that differ from what you would write
>> with Equatable.
>> And so everybody has to swallow implicit and non-avoidable code synthesis
>> and shut up?
> That's not what I said. I simply pointed out one of the barriers to
> getting a new protocol added to the language.
> Code synthesis is explicitly opt-in and quite avoidable—you either don't
> conform to the protocol, or you conform to the protocol and provide your
> own implementation. What folks are differing on is whether there should
> have to be *two* explicit switches that you flip instead of one.
> No. One does not add a protocol conformance by whim. One adds a protocol
> conformance by need. So the conformance to the protocol is a *given* in our
> analysis of the consequence of code synthesis. You can not say "just don't
> adopt it".
> As soon as I type the protocol name, I get synthesis. That's the reason
> why the synthesized code is implicit. The synthesis is explicitly written
> in the protocol documentation, if you want. But not in the programmer's
> code.
> I did use "non-avoidable" badly, you're right: one can avoid it, by
> providing its custom implementation.
> So the code synthesis out of a mere protocol adoption *is* implicit.
> Let's imagine a pie. The whole pie is the set of all Swift types. Some
> slice of that pie is the subset of those types that satisfy the conditions
> that allow one of our protocols to be synthesized. Now that slice of pie
> can be sliced again, into the subset of types where (1) the synthesized
> implementation is correct both in terms of strict value and of business
> logic, and (2) the subset where it is correct in terms of strict value but
> is not the right business logic because of something like transient data.
> Yes.
> What we have to consider is, how large is slice (2) relative to the whole
> pie, *and* what is the likelihood that developers are going to mistakenly
> conform to the protocol without providing their own implementation, *and*
> is the added complexity worth protecting against this case?
> That's quite a difficult job: do you think you can evaluate this
> likelihood?
> Explicit synthesis has big advantage: it avoids this question entirely.
> Remember that the main problem with slide (2) is that developers can not
> *learn* to avoid it.
> For each type is slide (2) there is a probability that it comes into
> existence with a forgotten explicit protocol adoption. And this probability
> will not go down as people learn Swift and discover the existence of slide
> (2). Why? because this probability is driven by unavoidable human behaviors:
> - developer doesn't see the problem (a programmer mistake)
> - the developper plans to add explicit conformance later and happens to
> forget (carelessness)
> - a developper extends an existing type with a transient property, and
> doesn't add the explicit protocol conformance that has become required.
> Case 2 and 3 bite even experienced developers. And they can't be improved
> by learning.
> Looks like the problem is better defined as an ergonomics issue, now.
> If someone can show me something that points to accidental synthesized
> implementations being a significant barrier to smooth development in Swift,
> I'm more than happy to consider that evidence. But right now, this all
> seems hypothetical ("I'm worried that...") and what's being proposed is
> adding complexity to the language (an entirely new axis of protocol
> conformance) that would (1) solve a problem that may not exist to any great
> degree, and (2) does not address the fact that if that problem does indeed
> exist, then the same problem just as likely exists with certain
> non-synthesized default implementations.
> 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.

> Gwendal
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