> On May 1, 2017, at 12:10 AM, Xiaodi Wu <[email protected]> wrote: > > You may wish to read the rationale behind the current error handling design: > > https://github.com/apple/swift/blob/master/docs/ErrorHandlingRationale.rst > <https://github.com/apple/swift/blob/master/docs/ErrorHandlingRationale.rst> > > A Result type like you suggest has been considered and rejected in favor of > the current design. Briefly, optionals and throwing errors are distinct > because they are considered superior ways for handling distinct types of > error. > > In the case of a simple domain error, there is only one way to fail; > therefore, optional return values are considered the best way to model that > error. > > In the case of a recoverable error, the document above describes why marked > propagation (the current implementation in Swift) is considered superior to > typed propagation (your suggestion).
My proposal is not about replacing Optionals and throwing functions with a Result type, it’s about separating the representation of an error from its propagation. Optionals and throwing functions solve two different problems, but they are not dealing with two different types of error. Optionals are for storing and representing a value that might not exist (most commonly due to an unambiguous error). Error handling is for propagating an error. Returning an optional is essentially the same as returning a non-optional and throwing a dedicated “something went wrong” error, because due to the optional unwrapping mechanic, you cannot avoid dealing with the fact that there might have been an error. Optionals only allow you to delay the inevitable error handling, not avoid it. The use cases where the exact reason for an error is no important have nothing to do with whether or not that error should be available. The optional chaining, if-let statements and all other ways one might try to handle an optional value do not fundamentally require lack of error information. The error handling mechanism, on the other hand, does not concern itself with representing the error, but only propagating it. Even an optional value with a general-purpose .none case has different levels of importance in different cases. More often than not, when propagating an optional value to a non-optional target, you’ll be stuck with dealing with the error immediately, which is exactly what throwing functions force you to do. I suggest we enhance the current error representation and propagation mechanisms to be able to seamlessly handle cases where an erroneous value need to be stored as-is (along with its error) or unpacked and propagated (by throwing the error), not just representing a general “error”. The general use case is to be able to catch a throwing call into an enum that stores the value or the error and then being able to unpack it in a throwing context (unpack it or throw the error). This use case is a strict superset of the current Optional mechanic (catching a potentially missing value) and error handling (always manually throwing an error after manual checks). The aforementioned suggestion about how to do that is indeed faulty, as pointed out by Robert Widmann, but the problem is still valid, in my opinion. > On Sun, Apr 30, 2017 at 13:51 Gor Gyolchanyan via swift-evolution > <[email protected] <mailto:[email protected]>> wrote: > > > On Apr 30, 2017, at 9:29 PM, Robert Widmann <[email protected] > > <mailto:[email protected]>> wrote: > > > > > >> On Apr 30, 2017, at 1:43 PM, Gor Gyolchanyan <[email protected] > >> <mailto:[email protected]>> wrote: > >> > >> It doesn’t have to be a massive source-break, since this pitch is supposed > >> to be a strict superset of what Optional and throwing is currently. > >> The only thing that I can think of at this moment that would break is this > >> syntax: > >> > >> let foo: Int? = .none // Error: Can’t convert (Error) -> Int? to Int? > >> > > > > Except it’s not a strict superset if you break every use of this case as an > > RValue. Especially when so much of Swift’s syntax and major patterns > > revolve around the manipulation of optionals. > > > >> The ExpressibleByNilLiteral, the try/throw syntax, all of those things > >> would work as they are right now. > >> Error handling as it is currently, is essentially a hidden `error` out > >> parameter and a whole bunch of codegen. > >> Even the semantical changes described earlier would be purely additive. > > > > Don’t get me wrong, I think you’ve identified the problem space well, I > > just disagree with the solution. > > Yeah, you’re right. It would take some next-level fixits to deal with the > consequences of changing the most fundamental data type of Swift I can think > of. > I’d really appreciate it if you’d offer an alternative solution to this > problem. > The problem, as I understand it, is as follows: > > A lot of Swift’s logic revolves around the notion that some values might be > missing for whatever reason and some functions might fail for whatever reason. > Any function’s effect can be summed up as the union of its return value and > the global state that it changes (that includes captured closure scopes). > This could be boiled down to the statement that “Values that a function sets > and returns completely express the purpose of the function”. > The optional gives an extremely convenient way of representing values that > might not exist (which, when returned from a function often means “failed for > an unknown reason”). > The fact that Optional is a type, rather then a function attribute allows us > to store and imperatively manipulate the outcome of logically failable > functions, but unfortunately, it doesn’t allow us to reason about the cause > of the failure. > On the other hand, throwing functions captures the logic of dealing with > specific failures very well, but does not allow us to store and manipulate > them easily, leaving us with workarounds like wrapping errors in enums with > values and re-throwing the errors on their way out of the generic pipeline. > I’d like to come up with a solution that would unify the optionals and the > throwing functions into a single mechanism for dealing with the concept of > failure, taking the best of both worlds and getting the benefits of the new > synergies. > This pitch was a first rough idea about the direction in which we could go in > trying to find a solution. > I chose to enhance Optional instead of introducing a new type like Failable, > so that we could make do with minimal language changes and migration > procedures. > > This problem is kinda similar to the variadic parameter problem, which makes > it impossible to forward calls to variadic functions simply because that > feature is too magical and does not provide a way to store and propagate its > logic. > > Another way I could think of solving it would be to allow overloading the > postfix `!` and `?` operators (which would currently only be defined for > Optionals), which would allow us to define the Failable enum type with some > error handling syntax integration and make it feel more at home in the midst > of Optionals. > > Or better yet, make an OptionalProtocol and move the current magical logic to > it, leaving the existing Optional perfectly intact and allowing userspace > implementations. > This would also greatly benefit numerous use cases of “invalidatable” types > (like file handlers that can be closed) that would no longer have to either > fatalError or use unwieldy wrappers that operate on Optionals. > > > ~Robert Widmann > > > >> > >>> On Apr 30, 2017, at 8:35 PM, Robert Widmann <[email protected] > >>> <mailto:[email protected]>> wrote: > >>> > >>> This "revamp" is isomorphic to adding a Sum type to stdlib and plumbing > >>> error handling syntax through. I'd much rather see that than the massive > >>> source-break this would entail. > >>> > >>> ~Robert Widmann > >>> > >>> 2017/04/30 13:11、Gor Gyolchanyan via swift-evolution > >>> <[email protected] <mailto:[email protected]>> のメッセージ: > >>> > >>>> I’d like to suggest a bit of redesigning the Optional type and throwing > >>>> functions to provide a single powerful and flexible mechanism for > >>>> dealing with unexpected situations. > >>>> > >>>> In short, The Optional would have an associated value of type Error > >>>> added to its `none` case, which would describe the reason why the > >>>> wrapped value is missing. > >>>> > >>>> public enum Optional<Wrapped> { > >>>> > >>>> case .some(Wrapped) > >>>> > >>>> case .none(Error) > >>>> > >>>> } > >>>> > >>>> The Optional's ExpressibleByNilLiteral would initialize it with an error > >>>> that corresponds to what is currently fatalError-ed as "unexpectedly > >>>> found nil while unwrapping an Optional value". > >>>> > >>>> The forced unwrapping operator (postfix `!`) would behave the same way > >>>> as it does now, except in case of a fatal error it would print out the > >>>> underlying error, instead of the aforementioned hard-coded string. > >>>> > >>>> The optional chaining operator (postfix `?`) would behave the same way > >>>> as it does now, except when it stops evaluating and returns the > >>>> Optional, it would contain the error, returned by the sub-expression > >>>> that failed to evaluate. > >>>> > >>>> Any throwing function would be equivalent to a function that returns an > >>>> Optional. If the function is declared as throwing and returning an > >>>> Optional at the same time, it would be equivalent to a function > >>>> returning an Optional Optional. > >>>> > >>>> The if-let statement would bind the `let` variable to the wrapped value > >>>> inside the "then" block and would bind it to the error in the "else" > >>>> block. Chained else-if blocks would all be considered part of the > >>>> overarching "else" block, so all of them would be able to access the > >>>> error bound to the if-let name. > >>>> > >>>> The guard-let and case-let statements are essentially just rewrites of > >>>> if-let with some added logic. > >>>> > >>>> The `try` keyword, applied to an optional would behave like this: > >>>> > >>>> public func try<T>(_ optional: T?) throws -> T { > >>>> guard let wrapped = optional else { > >>>> throw wrapped // Remember, if-let, guard-let and case-let statements > >>>> bind the let name to the error in case of a failure. > >>>> } > >>>> return wrapped > >>>> } > >>>> > >>>> Multiple let bindings in a single if-let statement are essentially > >>>> rewrites of a nested chain of if-let statements. > >>>> > >>>> The `try` keyword applied to an optional would unwrap the value or throw > >>>> the error. > >>>> The `try?` keyword applied to a throwing function call would cause any > >>>> thrown errors to be caught and put into the returned Optional, instead > >>>> of simply ignored. > >>>> The `try!` keyword applied to a throwing function call would behave as > >>>> you'd expect: just like `try?` except immediately force-unwrapped. > >>>> > >>>> A throwing function would be convertible to a non-throwing > >>>> optional-returning function and vice versa. > >>>> This would allow making use of throwing functions when dealing with > >>>> generics or protocols that allow arbitrary return types, without having > >>>> to sacrifice the convenience of error-handling logic. Conversely, it > >>>> would allow to write generic code that deals with any type of function > >>>> without having to implement special cases for throwing functions. This > >>>> means that the two function types would be interchangeable and one would > >>>> be able to satisfy protocol requirements of the other. The `rethrows` > >>>> idiom would then become a natural consequence of writing generic > >>>> functions that may return optional and non-optional results just as well. > >>>> > >>>> _______________________________________________ > >>>> swift-evolution mailing list > >>>> [email protected] <mailto:[email protected]> > >>>> https://lists.swift.org/mailman/listinfo/swift-evolution > >>>> <https://lists.swift.org/mailman/listinfo/swift-evolution> > >> > > > > _______________________________________________ > swift-evolution mailing list > [email protected] <mailto:[email protected]> > https://lists.swift.org/mailman/listinfo/swift-evolution > <https://lists.swift.org/mailman/listinfo/swift-evolution>
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