<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">I deliberately avoid making any concrete suggestions, because you guys know a lot more about internals of Swift then me. I know how to use it (I know how to abuse it), but to really come up with the best way to compliment the error handling system to make the entire language fluidly work with it in generic and protocol contexts would take a real team effort.<div class=""><br class=""><div><blockquote type="cite" class=""><div class="">On May 1, 2017, at 7:41 PM, Gor Gyolchanyan <<a href="mailto:gor@gyolchanyan.com" class="">gor@gyolchanyan.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><meta http-equiv="Content-Type" content="text/html charset=utf-8" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class="">An alternative would be to make throwing error more widely available then just functions.</div><div class="">What if property getters and setters could throw? That would mean that throwing properties could be passed as parameters that would require the `try` keyword.</div><div class="">Stored properties could have a new type attribute available (just like `lazy var`, we could have `throws var` or something), which would allow storing the result of a throwing call without `try` and the value of the variable would be only accessible by assigning to another variable with the same attribute or with a `try` keyword.</div><div class="">I really really like how Swift handles error propagation and the problem is really not that a Result type is missing, but a proper way of spreading the error handling mechanism to a wider range of use cases.</div><div class=""><br class=""></div><br class=""><div class=""><blockquote type="cite" class=""><div class="">On May 1, 2017, at 7:34 PM, John McCall <<a href="mailto:rjmccall@apple.com" class="">rjmccall@apple.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><blockquote type="cite" class=""><div class=""><br class="Apple-interchange-newline">On May 1, 2017, at 9:01 AM, Rod Brown via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="auto" class=""><div class="">On 1 May 2017, at 8:16 pm, Gor Gyolchanyan <<a href="mailto:gor@gyolchanyan.com" class="">gor@gyolchanyan.com</a>> wrote:<br class=""><br class=""></div><blockquote type="cite" class=""><div class=""><div class="">Yeah, you’re absolutely right. the “value-or-nil” and “value-or-reason-why-not-value” are two different things and the former is used too liberally in place of the latter because of lack of support.</div></div></blockquote><blockquote type="cite" class=""><div class=""><div class="">In that case, the Result type should not replace the error handling, but augment it. The error handling mechanism is extremely well though-out and convenient for its purpose, but it makes it difficult to capture and store a union of (value; flying error).</div></div></blockquote><div class=""><br class=""></div><div class=""><span class="" style="background-color: rgba(255, 255, 255, 0);">I agree that the key problem with the current architecture that you're alluding to is it can't be easily</span><span class="" style="background-color: rgba(255, 255, 255, 0);"> </span><i class="" style="background-color: rgba(255, 255, 255, 0);">stored and transferred.<span class="Apple-converted-space"> </span></i><span class="" style="background-color: rgba(255, 255, 255, 0);">Swift errors are great for live action but holding and passing after the throwing event is problematic, and this is an elegant solution. </span><span class="" style="background-color: rgba(255, 255, 255, 0);">The storage issue is when holding it as a property, and </span><span class="" style="background-color: rgba(255, 255, 255, 0);">the transferring issue is when passing it to a closure as a results of an asynchronous operation etc. These are both definitely cases where storage of the type-or-error makes perfect sense.</span></div><div class=""><span class="" style="background-color: rgba(255, 255, 255, 0);"><br class=""></span></div><div class=""><span class="" style="background-color: rgba(255, 255, 255, 0);">I think the key problem getting this accepted by the Swift Team will be that it doesn't currently have any specific use in the standard library. As a low level set of types, errors are generated by the lower levels but rarely stored, so the Standard library doesn't need the storage. Generally the only place we have to do that is in end user code. And currently the standard library doesn't have to support asynchronous operations natively, so there's nothing inside the kit that would require it to do completion handlers with errors.</span></div></div></div></blockquote><div class=""><br class=""></div>We've definitely considered including a Result type, but our sense was that in an ideal world almost no code would be using it. It's hard to imagine an ordinary API that ought to be returning a Result rather than throwing, and once you've defined that away, the major remaining use case is just to shift computation around, like with a completion handler. That explicit computation-shifting pattern is something we're hoping to largely define away with something like C#'s async/await, which would leave Result as mostly just an implementation detail of such APIs. We didn't want to spend a great deal of time designing a type that would end up being so marginal, especially if the changing role would lead us into different directions on the design itself. We also didn't want to design a type that would become an obstacle to potential future language changes like, say, typed throws.</div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><br class=""></div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class="">The downside, of course, is that as long as we lack that async/await design, computation-shifting isn't real great.</div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><br class=""></div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class="">John.</div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><br class=""><blockquote type="cite" class=""><div class=""><div dir="auto" class=""><div class=""><span class="" style="background-color: rgba(255, 255, 255, 0);"><br class=""></span></div><div class=""><span class="" style="background-color: rgba(255, 255, 255, 0);">This would therefore be an element in the standard library purely so we don't have 50,000 different libraries with 50,000 different result types. I'd love to see this standardised so frameworks were more compatible. I'm just not sure whether the Core Team would see it as pressing to try and officiate a certain type that they themselves don't use.</span></div></div></div></blockquote><blockquote type="cite" class=""><div class=""><div dir="auto" class=""><div class=""><span class="" style="background-color: rgba(255, 255, 255, 0);"><br class=""></span></div><div class=""><br class=""></div><blockquote type="cite" class=""><div class=""><div class="">A built-in Failable enum with syntactic support to losslessly catch it from a throwing expression and unpack it into a throwing scope would be a very useful feature.</div><div class="">Optionals are extremely convenient, but in cases where the Optional is used as “value-or-error” rather then “value-or-nil” it falls a bit short and the programmer has to choose between extreme convenience of Optionals with the downside of lack of error information or the expressive power of throwing an error with the downside of a lot of boilerpate and poor integration with generics.</div></div></blockquote><blockquote type="cite" class=""><div class="">Here’s an example pseudo-swift that illustrates this:</div><div class=""><div class=""><br class=""></div><div class="">enum Failable<Wrapped> {</div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span></div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>case success(Wrapped)</div><div class=""><br class=""></div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>case failure(Error)</div><div class=""><br class=""></div><div class="">}</div><div class=""><br class=""></div><div class="">func foo() throws -> Int {</div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>guard myCondition else {</div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>throw EmbarressingError.oops</div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>}</div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>return 42</div><div class="">}</div><div class=""><br class=""></div><div class="">let failable = catch foo() // Failable<Int></div><div class=""><br class=""></div><div class="">func bar() throws -> Int {</div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>throw failable</div><div class=""><br class=""></div></div></blockquote><blockquote type="cite" class=""><div class=""><div class=""><br class=""></div><br class=""><div class=""><blockquote type="cite" class=""><div class="">On May 1, 2017, at 11:17 AM, Rod Brown <<a href="mailto:rodney.brown6@icloud.com" class="">rodney.brown6@icloud.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class="">The problem I see with your argument is that the core reason why the optional cast failed is actually there: It was an optional value, and you forced it to unwrap without checking. This is a correct description of the error.</div><div class=""><br class=""></div><div class="">If we plumbed our code with a tonne of errors saying “why this is optional, and why it is null” then we are practically making every optional an error in the case of nil, which is completely illogical considering that nil could be a completely legitimate case (especially in the case of not-implicitly-unwrapped optionals).</div><div class=""><br class=""></div><div class="">Optional is a wrapper for "value-or-null", not "value-or-reason-not-value".</div><div class=""><br class=""></div><div class="">The type you are talking about is a result/sum type as has been mentioned, which is fine, and is completely valid (I use them a lot too) but they are definitely not the same thing as an optional, and I think you’re conflating the two ideas.</div><div class=""><br class=""></div><div class="">- Rod</div><div class=""><br class=""></div><br class=""><div class=""><blockquote type="cite" class=""><div class="">On 1 May 2017, at 5:58 pm, Gor Gyolchanyan via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class="">I have read those documents before, but It’s worth re-reading them to see if I missed something, but I’l still explain my motivation and seek arguments against the postulated problem (rather then a specific solution).</div><div class=""><br class=""></div><div class=""><div class="">(a) There are different types of error.</div></div><div class=""><br class=""></div><div class="">Yes, there are different types of error in Swift, which require different reactions from the programmer.</div><div class="">If I’m not missing something, the three main types of error in Swift are:</div><div class=""> - Simple encapsulatable errors that are expected to be treated as normal values until the time comes for someone to take care of them by unpacking the content.</div><div class=""> - Automatic propagatable errors that require the programmer to either handle the error immediately or propagate it by delegating to its own caller.</div><div class="">- Fatal errors, which represent logic errors and broken invariants and preconditions, which are purely a programmer error and should not be dealt with dynamically, hence the terminated process with a message.</div><div class=""><br class=""></div><div class="">(b) The programmer is expected to react differently to different types of error.</div><div class=""><br class=""></div><div class="">Yes, and the three main ways a programmer is expected to react to the an error are:</div><div class=""> - If it’s an optional, they’re encouraged to store and pass it around freely until someone down the line decides to unpack it and deal with the possibility that it isn’t there.</div><div class=""> - If it’s an error, they’re encouraged to either handle it on the spot or declare themselves throwing and delegate the responsibility to the caller.</div><div class=""> - Look at the standard output and figure out why the fatal error occurred, perhaps with the help of the debugger.</div><div class=""><br class=""></div><div class="">(c) The language is a tool to help the programmer react.</div><div class=""><br class=""></div><div class="">Yes, that comes in the form of three language constructs:</div><div class=""> - Optionals, which allow storing a union of a value and its absence (for an undefined and hopefully obvious reason).</div><div class=""> - Throwing functions, which allow making sure that the error will be handled as soon as possible.</div><div class=""> - Fatal errors, which allow the programmer to mark points in code which should never be reached in a correct system in order to keep the logic from going AWOL in case the programmer screwed up somewhere.</div><div class=""><br class=""></div><div class="">(d) Optionals and errors are not unified, and unification is a non-goal, because they are designed to help the programmer react differently to different types of error.</div><div class=""><br class=""></div><div class="">Yes, and those different types of error with different reactions are all valid and shouldn’t be unified.</div><div class="">My point is that the language should make it easy for a programmer to transition from one type of error to another, because the same error has different severity in different contexts.</div><div class="">For instance, a “file not found” error when trying to open a file handler is not critical at all in the context of the file opening function, because it’s a perfectly expected outcome of the operation.</div><div class="">However, for a module that handles loading critical data from disk (like encryption keys needed to decrypt the manipulated content) it is a critical error that cannot be dealt with.</div><div class="">In this case it deserves to be a fatal error, because the programmer didn’t bother to implement a logic for creating the missing file or properly notifying the user of the lacking permissions to do so.</div><div class="">Conversely, some errors start as being urgent (like a JSON parser that throws an error when it encounters invalid syntax), but become less urgent for the client (a JSON editor that simply displays the error message).</div><div class=""><br class=""></div><div class="">As for my use case:</div><div class=""><br class=""></div><div class="">I have a JSON parser that may throw, and I have a a JSON Editor class that allows editing JSON files as well as displaying the parsing errors.</div><div class="">I have a malformed JSON file that I open in the editor. The JSON parser throws an error, which should be caught and stored somewhere for the editor to display.</div><div class="">I have file reader that reads a file in some encoding and returns an optional string with the file contents (nil means file couldn’t be read or the encoding is wrong).</div><div class=""><br class=""></div><div class="">For the JSON parser, a malformed JSON file is an obvious error, but for the editor, it’s a perfectly valid and expected condition, which doesn’t deserve to be an error.</div><div class="">Therefore, the thrown error of the JSON parse has to be caught and encapsulated indiscriminately to demote it from an error to a return value.</div><div class="">Conversely, the returned nil form the file reader is perfectly valid and expected condition, but for the editor, it’s an error.</div><div class="">Therefore, the returned nil should be checked and converted to an error that will be thrown to promote it to a full-fledged error.</div><div class=""><br class=""></div><div class="">I would want to have a way to easily promote/demote different types of errors to accommodate the changing perception of their urgency.</div><div class="">For instance, by being able to throw an optional, thus introducing a new way of unpacking it (by promoting it to an error). Currently, it is by manually unpacking the optional, deciding what error to throw and throwing it manually.</div><div class="">Or, being able to catch an error into an optional, thus introducing a new way of handling it (by demoting it to an optional). There is a way to do that currently in the form of `try?` and `try!`, but their downside is that they are lossy (losing the error itself).</div><div class=""><br class=""></div><div class="">All I want is for the language to help me losslessly catch errors into something like an optional, except with the error intact with the possibility of easily re-throwing it in the future.</div><div class="">This would also solve the problem of multiple throwing calls having different urgency to them and being forced to write a lot of boilerplate to catch their errors individually and deal with them separetely.</div><div class=""><br class=""></div><div class=""><blockquote type="cite" class=""><div class="">On May 1, 2017, at 1:44 AM, Xiaodi Wu <<a href="mailto:xiaodi.wu@gmail.com" class="">xiaodi.wu@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;">On Sun, Apr 30, 2017 at 5:05 PM, Gor Gyolchanyan<span class="Apple-converted-space"> </span><span dir="ltr" class=""><<a href="mailto:gor@gyolchanyan.com" target="_blank" class="">gor@gyolchanyan.com</a>></span><span class="Apple-converted-space"> </span>wrote:<br class=""><div class="gmail_extra"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-style: solid; border-left-color: rgb(204, 204, 204); padding-left: 1ex;"><div class="" style="word-wrap: break-word;"><div class=""><br class=""></div><div class=""><span class="gmail-"><blockquote type="cite" class=""><div class="">On May 1, 2017, at 12:10 AM, Xiaodi Wu <<a href="mailto:xiaodi.wu@gmail.com" target="_blank" class="">xiaodi.wu@gmail.com</a>> wrote:</div><br class="gmail-m_5116877186566974179Apple-interchange-newline"><div class="">You may wish to read the rationale behind the current error handling design:<br class=""><br class=""><a href="https://github.com/apple/swift/blob/master/docs/ErrorHandlingRationale.rst" target="_blank" class="">https://github.com/apple/<wbr class="">swift/blob/master/docs/<wbr class="">ErrorHandlingRationale.rst</a><br class=""><br class="">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.<br class=""><br class="">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.<br class=""><br class="">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).<br class=""></div></blockquote></span><div class=""><div class=""><br class=""></div><div class="">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.</div><div class="">Optionals and throwing functions solve two different problems, but they are not dealing with two different types of error.</div></div></div></div></blockquote><div class=""><br class=""></div><div class="">The basic premise of Swift error handling design is that there exist different types of error. From the document:</div><div class=""><br class=""></div><div class="">> What is an error? There may be many different possible error conditions in a program, but they can be categorized into several kinds based on how programmers should be expected to react to them. Since the programmer is expected to react differently, and since the language is the tool of the programmer's reaction, it makes sense for each group to be treated differently in the language.</div><div class=""><br class=""></div><blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-style: solid; border-left-color: rgb(204, 204, 204); padding-left: 1ex;"><div class="" style="word-wrap: break-word;"><div class=""><div class=""><div class="">Optionals are for storing and representing a value that might not exist (most commonly due to an unambiguous error).</div><div class="">Error handling is for propagating an error.</div><div class="">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.</div><div class="">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.</div><div class="">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”.</div><div class="">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).</div><div class="">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).</div><div class="">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.</div></div></div></div></blockquote><div class=""><br class=""></div><div class="">I'd highly recommend taking some time to digest the existing rationale. You're basing your argument on contradicting the fundamental premise of the existing design, which begins with this: (a) there are different types of error; (b) the programmer is expected to react differently to different types of error; (c) the language is a tool to help the programmer react; (d) optionals and errors are not unified, and unification is a non-goal, because they are designed to help the programmer react differently to different types of error.</div><div class=""><br class=""></div><div class="">Do you have a specific use case in mind that is not well accommodated by optionals or by throwing functions? What is it? Into what category does that use case fall, in terms of the types of error enumerated in the error handling rationale document?</div><div class=""><br class=""></div><blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-style: solid; border-left-color: rgb(204, 204, 204); padding-left: 1ex;"><div class="" style="word-wrap: break-word;"><div class=""><div class="gmail-h5"><div class=""><blockquote type="cite" class=""><div class=""><div class="gmail_quote"><div dir="ltr" class="">On Sun, Apr 30, 2017 at 13:51 Gor Gyolchanyan via swift-evolution <<a href="mailto:swift-evolution@swift.org" target="_blank" class="">swift-evolution@swift.org</a>> wrote:<br class=""></div><blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-style: solid; border-left-color: rgb(204, 204, 204); padding-left: 1ex;"><br class="">> On Apr 30, 2017, at 9:29 PM, Robert Widmann <<a href="mailto:devteam.codafi@gmail.com" target="_blank" class="">devteam.codafi@gmail.com</a>> wrote:<br class="">><br class="">><br class="">>> On Apr 30, 2017, at 1:43 PM, Gor Gyolchanyan <<a href="mailto:gor@gyolchanyan.com" target="_blank" class="">gor@gyolchanyan.com</a>> wrote:<br class="">>><br class="">>> 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.<br class="">>> The only thing that I can think of at this moment that would break is this syntax:<br class="">>><br class="">>> let foo: Int? = .none // Error: Can’t convert (Error) -> Int? to Int?<br class="">>><br class="">><br class="">> 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.<br class="">><br class="">>> The ExpressibleByNilLiteral, the try/throw syntax, all of those things would work as they are right now.<br class="">>> Error handling as it is currently, is essentially a hidden `error` out parameter and a whole bunch of codegen.<br class="">>> Even the semantical changes described earlier would be purely additive.<br class="">><br class="">> Don’t get me wrong, I think you’ve identified the problem space well, I just disagree with the solution.<br class=""><br class="">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.<br class="">I’d really appreciate it if you’d offer an alternative solution to this problem.<br class="">The problem, as I understand it, is as follows:<br class=""><br class="">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.<br class="">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).<br class="">This could be boiled down to the statement that “Values that a function sets and returns completely express the purpose of the function”.<br class="">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”).<br class="">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.<br class="">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.<br class="">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.<br class="">This pitch was a first rough idea about the direction in which we could go in trying to find a solution.<br class="">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.<br class=""><br class="">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.<br class=""><br class="">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.<br class=""><br class="">Or better yet, make an OptionalProtocol and move the current magical logic to it, leaving the existing Optional perfectly intact and allowing userspace implementations.<br class="">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.<br class=""><br class="">> ~Robert Widmann<br class="">><br class="">>><br class="">>>> On Apr 30, 2017, at 8:35 PM, Robert Widmann <<a href="mailto:devteam.codafi@gmail.com" target="_blank" class="">devteam.codafi@gmail.com</a>> wrote:<br class="">>>><br class="">>>> 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.<br class="">>>><br class="">>>> ~Robert Widmann<br class="">>>><br class="">>>> 2017/04/30 13:11、Gor Gyolchanyan via swift-evolution <<a href="mailto:swift-evolution@swift.org" target="_blank" class="">swift-evolution@swift.org</a>> のメッセージ:<br class="">>>><br class="">>>>> 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.<br class="">>>>><br class="">>>>> 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.<br class="">>>>><br class="">>>>> public enum Optional<Wrapped> {<br class="">>>>><br class="">>>>> case .some(Wrapped)<br class="">>>>><br class="">>>>> case .none(Error)<br class="">>>>><br class="">>>>> }<br class="">>>>><br class="">>>>> 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".<br class="">>>>><br class="">>>>> 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.<br class="">>>>><br class="">>>>> 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.<br class="">>>>><br class="">>>>> 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.<br class="">>>>><br class="">>>>> 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.<br class="">>>>><br class="">>>>> The guard-let and case-let statements are essentially just rewrites of if-let with some added logic.<br class="">>>>><br class="">>>>> The `try` keyword, applied to an optional would behave like this:<br class="">>>>><br class="">>>>> public func try<T>(_ optional: T?) throws -> T {<br class="">>>>> guard let wrapped = optional else {<br class="">>>>> throw wrapped // Remember, if-let, guard-let and case-let statements bind the let name to the error in case of a failure.<br class="">>>>> }<br class="">>>>> return wrapped<br class="">>>>> }<br class="">>>>><br class="">>>>> Multiple let bindings in a single if-let statement are essentially rewrites of a nested chain of if-let statements.<br class="">>>>><br class="">>>>> The `try` keyword applied to an optional would unwrap the value or throw the error.<br class="">>>>> 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.<br class="">>>>> The `try!` keyword applied to a throwing function call would behave as you'd expect: just like `try?` except immediately force-unwrapped.<br class="">>>>><br class="">>>>> A throwing function would be convertible to a non-throwing optional-returning function and vice versa.<br class="">>>>> 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.<br class="">>>>><br class="">>>>> ______________________________<wbr class="">_________________<br class="">>>>> swift-evolution mailing list<br class="">>>>><span class="Apple-converted-space"> </span><a href="mailto:swift-evolution@swift.org" target="_blank" class="">swift-evolution@swift.org</a><br class="">>>>><span class="Apple-converted-space"> </span><a href="https://lists.swift.org/mailman/listinfo/swift-evolution" rel="noreferrer" target="_blank" class="">https://lists.swift.org/<wbr class="">mailman/listinfo/swift-<wbr class="">evolution</a><br class="">>><br class="">><br class=""><br class="">______________________________<wbr class="">_________________<br class="">swift-evolution mailing list<br class=""><a href="mailto:swift-evolution@swift.org" target="_blank" class="">swift-evolution@swift.org</a><br class=""><a href="https://lists.swift.org/mailman/listinfo/swift-evolution" rel="noreferrer" target="_blank" class="">https://lists.swift.org/<wbr class="">mailman/listinfo/swift-<wbr class="">evolution</a></blockquote></div></div></blockquote></div></div></div></div></blockquote></div></div></div></div></blockquote></div><br class=""></div>_______________________________________________<br class="">swift-evolution mailing list<br class=""><a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a><br class=""><a href="https://lists.swift.org/mailman/listinfo/swift-evolution" class="">https://lists.swift.org/mailman/listinfo/swift-evolution</a><br class=""></div></blockquote></div><br class=""></div></div></blockquote></div><br class=""></div></blockquote></div>_______________________________________________<br class="">swift-evolution mailing list<br class=""><a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a><br class=""><a href="https://lists.swift.org/mailman/listinfo/swift-evolution" class="">https://lists.swift.org/mailman/listinfo/swift-evolution</a></div></blockquote></div></div></blockquote></div><br class=""></div></div></blockquote></div><br class=""></div></body></html>