Swift error handling - determining the error type

Question

In Swift error handling - how do you know what error type is being thrown without looking at the implementation and how do you handle the case where ErrorType-derived error and NSError can be thrown by the method?

e.g.

Code does not show what type of error will be thrown.

public func decode(jwt: String) throws -> JWT {
    return try DecodedJWT(jwt: jwt)
}

at least for the part of your question, here could be an answer: stackoverflow.com/a/30720807/5189361 — SalDev
– SalDev, Commented Feb 20, 2016 at 17:38

dfrib · Accepted Answer · 2016-02-20 18:33:30Z

4

You can catch the thrown error to a variable and do runtime analysis of the variable. E.g., for some unknown implementation:

/* ---------------------- */
/* unknown implementation */
enum HiddenError: ErrorType {
    case SomeError
}

class AnotherError : NSError { }

func foo() throws -> Int {
    let foo = arc4random_uniform(3);
    if foo == 0 {
        throw HiddenError.SomeError
    }
    else if foo == 1 {
        throw AnotherError(domain: "foo", code: 0, userInfo: [:])
    }
    else if foo == 2 {
        throw NSError(domain: "foo", code: 0, userInfo: [:])
    }
    else {
        return Int(foo)
    }
}
/* ---------------------- */

Investigate the error as:

/* "External" investigation */
func bar() throws -> Int {
    return try foo()
}

func fuzz() {
    do {
        let buzz = try bar()
        print("Success: got \(buzz)")
    } catch let unknownError {
        print("Error: \(unknownError)")
        print("Error type: \(unknownError.dynamicType)")
        if let dispStyle = Mirror(reflecting: unknownError).displayStyle {
          print("Error type displaystyle: \(dispStyle)")
        }
    }
}

fuzz()
/* Output examples:

   Error: SomeError
   Error type: HiddenError
   Error type displaystyle: Enum

   //

   Error: Error Domain=foo Code=0 "(null)"
   Error type: AnotherError
   Error type displaystyle: Class

   //

   Error: Error Domain=foo Code=0 "(null)"
   Error type: NSError
   Error type displaystyle: Class             */

edited Feb 20, 2016 at 18:33

answered Feb 20, 2016 at 17:44

dfrib

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2 Comments

Boon Over a year ago

So there is no way to know what error type will be thrown ahead of time?

dfrib Over a year ago

If the throwing method you're calling is a black box (and no docs), you're left with runtime introspection to investigate what kind of errors the method possibly throws. Generally, however, this shouldn't be the case, and you can knit your catch clauses to the specific errors you expext to be thrown. Note also that all the errors above all conform to ErrorType (and that an error thrown in Swift must be a type conforming to this).

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