Status check: Typed throws

Although that does then raise the question of what should happen if the Objective-C code violates the contract at runtime? Do you throw yet another exception (that's also the wrong type)? Seems pointless. Just crash? Seems severe, although well-precedented with e.g. how nil tends to get handled.

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My point wasn’t to argue that it’s simpler to implement. It was to highlight that the demand for “typed throws” comes from people who want to be able to exhaustively enumerate all possible errors a function may emit, whereas @Douglas_Gregor’s motivation for starting this implementation was to implement throws in a runtime without existentials. The existence of opaque types means solving the latter does not require allowing the former.

It might make a lot of people in this thread mad, but if the motivation for hacking on this really is embedded Swift, then I believe the plan should be to only support some Error.

But then there's no way for callers to handle the error based on its actual type, as they'd only have available to them what's in the protocol (basically nothing). Normally this is accomplished with dynamic casting, which I assume would also be unavailable in embedded Swift. The other option would be to have some compile-time type specialization available to callers, but in my opinion this would be brittle and probably end up being typed errors with extra steps.

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I’m not sure this is actually such a fatal situation. Embedded programs are far more likely to rely on integer error codes than dynamic constructs such as exception hierarchies, for multiple reasons: one being the historical biases of C, but another being the extremely tight bounds on memory usage and algorithmic complexity guaranteed by such a simple scheme.

Besides, error introspection is not the only place where dynamic casts are currently used and would be unavailable in an embedded environment.

Instead of allowing the programmer to dynamically switch over error types, why not elevate the domain and code properties from NSError to the Error protocol, and vend a concrete type (GenericError, perhaps?) from the stdlib that library authors can use in embedded or full-fledged environments?

In practice, we’re talking about case .myErrorDomain rather than if let _ = error as? MyError. This doesn’t introduce a prohibition on extended error information, but it also doesn’t immediately encourage library authors to make the mistake of trying to fully specify their error domains in the type system.

This is what this proposal tried to do before we stopped it because of missing typed throws:

Typed throws are just as an important carriers of static type information as things like classes. What you are saying is equivalent to saying "class instances should not be allowed to have a specific class type in resilient libraries and should always be AnyObject". This is obviously very wrong. The possibility of accidentally locking oneself into a specific error type and then regretting it after a major version release is not a language problem, it's an engineering incompetence problem. The author should just take the precaution to design their error type with future expansion in mind (e.g. a struct with optional metadata instead of a bare enum).

While we're on the topic:

Same goes for anonymous union types (A | B) which are nothing more than syntactic sugar around certain generic enums (e.g. Either<A, B>). This is nothing new and Swift is already perfectly capable of expressing this kind of type, so the "commonly rejected proposal due to compiler complexity" that I keep hearing is also obviously very wrong.


The type system must be as expressive as the engineer sees fit to make their code expressive. Just because someone can't think of a reason to retain static type information (be that by using a specific error type or using an anonymous union type), doesn't mean there is no reason.


I don’t know how to state my position more clearly than “Swift made the correct decision a decade ago to regard typed errors as a mistake, and the language team should not accidentally undo that correct decision for the sake of avoiding existential errors in embedded environments.”


I still keep hearing baseless opinions without any rational justification and counter-argumentation. An answer like "because said so" is not an answer and will not be taken seriously by anyone worth listening to.

This is the classical "argument from authority" logical fallacy.

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The justifications for rejecting typed errors were thoroughly discussed and debated on the Swift mailing list at the time of the original discussions of Swift’s error system.

The typeless error design makes Swift a rather unique language among its peers. Since the beginning, there has been a large contingent of folks who have wanted a typed error system instead. The biggest reason they lost the argument at the time is because of ABI. The Swift team’s combined decades of lived experience made it clear that errors are only categorizable in trivial cases. Most functions call other functions, and once you compose functions you must compose their errors. This quickly scales beyond tractability.

Furthermore, Swift not only lacked a stable ABI at the time, the final shape of the ABI was still a ways off. Any type that appears in a type signature has ABI impact. It was known that resilience was going to be a critical part of the Swift ABI story, and a single universal error type is far easier to design resilience for than a type parameter.

These are not arguments from authority. They might just be arguments you weren’t around to witness.


I agree, the nature of this particular throwing function (along with many others like this) necessitates handling any possible type of error.

But that doesn't necessarily preclude adding predetermined information to that error.
I can't imagine this being any less correct than simply throwing any Error:

func decodeSomething() throws(NewDecoringError) -> Something {

in this example NewDecodingError is essentially equivalent to DecodingError.Context.

The error is still capable of representing any specific error type, but it also always tell you where in the object graph the error happened.

It's also very easy to modify the decoder to catch the error during every step of the decoding process and wrap it into this error type while adding the coding path.

On the contrary, it is this exact error wrapping process that was discussed heavily during the addition of try.

Answering in order of appearance:

  • Swift has evolved way beyond what it was years ago. What used to be reasonable to assume back then isn't necessarily so now. The very fact that this thread exists and is being actively discussed by the core team is a testament that to that.
  • The ABI stabilization has not only brought a stable ABI at that point, but has made ABI stability an ongoing concern that every evolution proposal must address. This is specifically to enable features like this to be possible.
  • Making swift "a unique language" is a non-goal. The goal is making Swift a practical language that is capable of solving practical problems. This is not a "language of the year" contest.
  • Making features "far easier" is also a non-goal. It would've been far easier to just keep pure object-oriented approach that Objective-C has provided, add new syntax to it and call it a day. There are specific problems that need to be solved. It's not a matter of "too hard", it's only a matter of "not enough time right now" at best.
  • For those chunks of code that have to be resilient, they can either revert to using any Error or a technique that I've outlined in my previous post in this thread (e.g. wrapping any Error into a struct with some optional meta-information).
  • I've started leaning swift right after WWDC 2014 and have been keeping in touch with its development ever since. I'm plenty aware of its history.

Again, it's not the language's business to dictate how specific API's should be designed. This is not a matter of memory safety or thread safety that can only be deterministically guaranteed by heavy language support and limitation. Even @Douglas_Gregor considered fully specific decoding error type a "bad idea", instead of something like "categorically unacceptable". The programmer should always be assumed to know better what they're doing than the programming language, when the programmer specifically requests the functionality.
any Error is still the default, which is perfect, because at the very least, if the programmer doesn't care or doesn't know - the language will choose the "safest" option.

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The fact that error type polymorphism is essential is beyond doubt. My argument is simply that there should be a choice between static polymorphism and dynamic polymorphism.

Swift has both enums with associated values and protocols. They both exist because choosing between static polymorphism and dynamic polymorphism is a necessity.

Why wouldn't the possibility of that choice be equally valid here as well? It's still very much possible by way of a dedicated union type enum.


It occurs to me that we could allow typed throw without preventing evolution of the error type. For instance, this declares a typed throw:

func fetch() throws(NetworkError) { ... }

And because of this, fetch can only throw errors of this type. But that doesn't necessarily imply that at the call site catching NetworkError is enough. If we assume the type can change at a later time, we still need a catch-all clause to make sure no other errors have been thrown to handle them:

do {
   try fetch()
} catch let error as NetworkError {
} catch let error {

This solves the problem of making the ABI around thrown errors more efficient without changing the language model. That said, it'd be nice to know this last catch is a fallback that isn't expected to be triggered, so we could add a mechanism similar to switching on non-frozen enums, with @unknown in the last catch:

do {
   try fetch()
} catch let error as NetworkError {
} @unknown catch let error {

Here @unknown catch would emit a warning for any declared typed throws not handled by the previous catch clauses. It's a warning though, it won't stop code from building if the typed throw becomes untyped or changes to another type.

In other words, with @unknown catch you can be exhaustive at the call site, but you don't have to (just don't use @unknown).

This could be extended more generally by allowing more types in typed throws, even when it has no ABI or performance benefit:

func fetch() throws(NetworkError, JSONError, *) { ... }
// Here the * denotes that the function is able to throw anything,
// but the two first types are "worth" checking for.

And if you're using @unknown catch the compiler would warn you about any types comming from typed throws unhandled by previous catch clauses:

do {
   try fetch()
} catch let error as NetworkError {
} @unknown catch let error { // warning: missing catch for JSONError

So the benefits are:

  • At the ABI level, single-type typed throws can skip the existential box (except at the boundary of resilient libraries because evolution is allowed to change the type).
  • At the call site, you can use @unknown catch and let the compiler tell you about the error types "worth" checking. There's no pretence those are the only types however, hence the trailing @unknown catch for handling less expected errors.

But if you don't use @unknown catch, you are free to ignore that typed throw is a thing that exists.

Future direction: I suppose we could add "frozen" typed throws, like frozen enums, which could allow exhaustive catching without the need for @unknown catch at the end. That could be a liability however, so I don't know.


Isn't this what the default behavior is for?
Unless anyone specifically wants to have a concrete error type, they can just write throws and not care. Any specific error type that are thrown out of their body should get automatically promoted to any Error, so even if the dependencies are using this feature, the client code still doesn't have to.

That makes sense, but that's what documentation is for. In a similar way, nothing in the language specifically says "don't use classes unless you have a good reason to". A lot of people (actually, most of the people I worked with, unfortunately) will just go with what they know, instead of using value types instead.

Feels like this is one of those cases where no language design could reasonably compensate for ignorance. We'd have to just communicate the dangers of over-specification of errors in documentation form (the swift book perhaps).

This looks amazing to me! It combines the runtime efficiency of statically-typed errors with the flexibility of untyped errors. With some extra thought, this could become the default. If the function body throws more than one type of error, the type under some would end up being any. This would keep the behavior exactly as it is now, but also open up optimization opportunities.

To reiterate, the choice between typed throws and untyped throws is not black and white. Swift provides both static polymorphism, type erasure, and type composition. All of those a very useful, while leaving the opportunity to still carry an untyped error as well.


At least it is not what I'd expect from union types: (A | A) should be A, whereas Either<A, A> can't be reduced.

I agree that A | A doesn't make sense and will have to be collapsed. If the two types are known to be equal (e.g. they're concrete types), it should be a compilation warning. When they are not known to be equal, (e.g. in a generic context), the union type would be preserved, but when looking at a specialization of the generic context (where both types happen to be the same), the exact type would be collapsed into just one.

This is a limitation on top of what an enum can do, so if anything, it's reducing type complexity, because a union of two types that are known to be equal would never exist.

The only complexity would be to implement the type collapsing behavior if and when the two types are known to be the same.

A union with more than two types is equivalent to a union of two types where one of those types is also a union, so it's a simple recursive type collapsing algorithm.

Another simplicity comes from the fact that these unions are unordered: A | B is exactly equivalent to B | A.

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It may seem like a complexity reduction, but unions make type checking harder, since if unions collapse, then every individual type T is also potentially a substitution for T | U | V | ... where the generic parameters are all equal, potentially needing an exponential search to attempt the unification. It's also not something that you can always assume is safe to do, since in a generic context working with T | U the provenance of the T and U may be semantically important even when T == U, but if the distinction is collapsed when the types are the same, then you can't test for it, and the reachability of the else in a construct like if x is T { } else { } would change depending on the generic parameters.


That's a very good point! Thank you for clarifying!

From what you described (and I agree), it seems like automatically collapsing a union into a single type is not desirable even if there was no exponential type checking problem.

If the distinction between T and U is useful even if T == U, then perhaps being able to refer to the generic type name itself would be a way of discriminating. Perhaps something like this:

    func getUnion<T, U>(_: T.Type, _: U.Type) -> T | U {
        // ...

    func useUnion() {
        /// Simple case
        switch getUnion(String.self, Int.self) {
            case x as String:
                // ...
            case x as Int:
                // ...
        // no need for default, the switch is known to be exhaustive

        // complex case (highly bikesheddable syntax)
        switch<T, U> getUnion(T.self, U.self) where T == String, U == String {
            case x as T:
                // ...
            case x as U:
                // ...

I know I've seen such local generic parameter syntax somewhere in the parameter pack discussion thread, so the concept of declaring local generic parameters for the purpose of type composition is not new. Another place where such an ad-hoc generic type declaration was mentioned was in parametrized extensions.

With this approach, the fact that the two types are the same gets abstracted away behind a local generic type declaration, which allows one to refer to two types that may be the same in local scope, but different in the scope of the callee (which in this case is significant to preserve).


With what I've been calling local generic parameter syntax (please, correct me if this technique already has a name), the union type doesn't need to be collapsed at all, which makes it a self-sufficient type, just like enums are. The type resolution becomes identical to that of enums and the whole point of such a union type would be reduced to essentially syntactic sugar for a dedicated variadic generic enum (with variadic number of cases) whose purpose is to facilitate ad-hoc static polymorphism. The fact that trivial decomposition of such a union type without clear type distinction (either by having distinct types or by using local generic parameter syntax) is impossible would not be a problem. Just like one can't refer to a member of an optional without unwrapping it first (using optional chaining, something like if let, or a switch on the optional itself), one also can't decompose a union type without being able to discriminate its component types (by either having them already distinct or introducing a distinction via the aforementioned syntax).


Even without this local generic parameter syntax, one can still pass the union type into another generic function that does specify two different generic parameters. It's a workaround, for sure, but it demonstrates that distinction can be introduced even if in a certain context, it might not be there.

func decompose<T, U>(_ union: T | U, onT: (T) -> Void, onU: (U) -> Void) {
    switch union {
        case x as T:
        case x as U:

In fact, such a function could be made variadic generic and part of the Swift standard library (as a necessity for now, and as a convenience for the future).


Such a union would relate to enums in the exact same way as a tuple relates to structs, as in: provide anonymous alternative to an explicitly defined type, which is useful for any situation where definition a whole type for this one use case would be suboptimal. Granted, tuples also serve the purpose of providing bridging with C structs, but unions would also have the potential to solve a similar use case.