Swift do-try-catch syntax

Question

I give it a try to understand new error handling thing in swift 2  Here is what I did  I first declared an error enum   enum SandwichError  ErrorType       case NotMe     case DoItYourself     And then I declared a method that throws an error  not an exception folks  It is an error    Here is that method   func makeMeSandwich names   String  String   throws - gt  String       guard let sandwich   names  sandwich   else           throw SandwichError NotMe            return sandwich     The problem is from the calling side  Here is the code that calls this method   let kitchen     sandwich    ready    breakfeast    not ready    do       let sandwich   try makeMeSandwich kitchen      print  i eat it   sandwich      catch SandwichError NotMe       print  Not me error     catch SandwichError DoItYourself       print  do it error       After the do line compiler says Errors thrown from here are not handled because the enclosing catch is not exhaustive  But in my opinion it is exhaustive because there is only two case in SandwichError enum    For regular switch statements swift can understands it is exhaustive when every case handled

User · Answer

I was also disappointed by the lack of type a function can throw, but I get it now thanks to @rickster and I'll summarize it like this: let's say we could specify the type a function throws, we would have something like this:

enum MyError: ErrorType { case ErrorA, ErrorB }

func myFunctionThatThrows() throws MyError { ...throw .ErrorA...throw .ErrorB... }

do {
    try myFunctionThatThrows()
}
case .ErrorA { ... }
case .ErrorB { ... }

The problem is that even if we don't change anything in myFunctionThatThrows, if we just add an error case to MyError:

enum MyError: ErrorType { case ErrorA, ErrorB, ErrorC }

we are screwed because our do/try/catch is no longer exhaustive, as well as any other place where we called functions that throw MyError

User · Answer

Create enum like this     Error Handling in swift enum spendingError   Error  case minus case limit     Create method like    func calculateSpending morningSpending Double eveningSpending Double  throws - gt Double  if morningSpending  lt  0    eveningSpending  lt  0      throw spendingError minus   if  morningSpending   eveningSpending   gt  100      throw spendingError limit   return morningSpending   eveningSpending     Now check error is there or not and handle it   do  try calculateSpending morningSpending  60  eveningSpending  50    catch spendingError minus  print  This is not possible        catch spendingError limit  print  Limit reached

User · Answer

Swift is worry that your case statement is not covering all cases  to fix it you need to create a default case   do       let sandwich   try makeMeSandwich kitchen      print  i eat it   sandwich      catch SandwichError NotMe       print  Not me error     catch SandwichError DoItYourself       print  do it error     catch Default       print  Another Error

User · Answer

I suspect this just hasn   t been implemented properly yet  The Swift Programming Guide definitely seems to imply that the compiler can infer exhaustive matches  like a switch statement   It doesn   t make any mention of needing a general catch in order to be exhaustive  You ll also notice that the error is on the try line  not the end of the block  i e  at some point the compiler will be able to pinpoint which try statement in the block has unhandled exception types  The documentation is a bit ambiguous though  I   ve skimmed through the    What   s new in Swift    video and couldn   t find any clues  I   ll keep trying  Update  We   re now up to Beta 3 with no hint of ErrorType inference  I now believe if this was ever planned  and I still think it was at some point   the dynamic dispatch on protocol extensions probably killed it off  Beta 4 Update  Xcode 7b4 added doc comment support for Throws   which    should be used to document what errors can be thrown and why     I guess this at least provides some mechanism to communicate errors to API consumers  Who needs a type system when you have documentation  Another update  After spending some time hoping for automatic ErrorType inference  and working out what the limitations would be of that model  I   ve changed my mind - this is what I hope Apple implements instead  Essentially     allow us to do this  func myFunction   throws - gt  Int     or this  func myFunction   throws CustomError - gt  Int     but not this  func myFunction   throws CustomErrorOne  CustomErrorTwo - gt  Int  Yet Another Update Apple   s error handling rationale is now available here  There have also been some interesting discussions on the swift-evolution mailing list  Essentially  John McCall is opposed to typed errors because he believes most libraries will end up including a generic error case anyway  and that typed errors are unlikely to add much to the code apart from boilerplate  he used the term  aspirational bluff    Chris Lattner said he   s open to typed errors in Swift 3 if it can work with the resilience model

User · Answer

There are two important points to the Swift 2 error handling model  exhaustiveness and resiliency  Together  they boil down to your do catch statement needing to catch every possible error  not just the ones you know you can throw   Notice that you don t declare what types of errors a function can throw  only whether it throws at all  It s a zero-one-infinity sort of problem  as someone defining a function for others  including your future self  to use  you don t want to have to make every client of your function adapt to every change in the implementation of your function  including what errors it can throw  You want code that calls your function to be resilient to such change   Because your function can t say what kind of errors it throws  or might throw in the future   the catch blocks that catch it errors don t know what types of errors it might throw  So  in addition to handling the error types you know about  you need to handle the ones you don t with a universal catch statement -- that way if your function changes the set of errors it throws in the future  callers will still catch its errors   do       let sandwich   try makeMeSandwich kitchen      print  i eat it   sandwich      catch SandwichError NotMe       print  Not me error     catch SandwichError DoItYourself       print  do it error     catch let error       print error localizedDescription      But let s not stop there  Think about this resilience idea some more  The way you ve designed your sandwich  you have to describe errors in every place where you use them  That means that whenever you change the set of error cases  you have to change every place that uses them    not very fun   The idea behind defining your own error types is to let you centralize things like that  You could define a description method for your errors   extension SandwichError  CustomStringConvertible       var description  String           switch self               case NotMe  return  Not me error              case DoItYourself  return  Try sudo                      And then your error handling code can ask your error type to describe itself -- now every place where you handle errors can use the same code  and handle possible future error cases  too   do       let sandwich   try makeMeSandwich kitchen      print  i eat it   sandwich      catch let error as SandwichError       print error description    catch       print  i dunno       This also paves the way for error types  or extensions on them  to support other ways of reporting errors -- for example  you could have an extension on your error type that knows how to present a UIAlertController for reporting the error to an iOS user

User · Answer

enum NumberError  Error     case NegativeNumber number  Int    case ZeroNumber   case OddNumber number  Int     extension NumberError  CustomStringConvertible            var description  String            switch self                case  NegativeNumber let number                    return  Negative number   number  is Passed                case  OddNumber let number                   return  Odd number   number  is Passed                case  ZeroNumber                  return  Zero is Passed                    func validateEvenNumber   number  Int  throws - gt Int        if number    0           throw NumberError ZeroNumber        else if number  lt  0           throw NumberError NegativeNumber number  number         else if number   2    1            throw NumberError OddNumber number  number             return number     Now Validate Number     do        let number   try validateEvenNumber 0       print  Valid Even Number    number        catch let error as NumberError        print error description

[swift] Swift do-try-catch syntax

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