Nested do catch swift 3.0

Question

I'd like to use consecutive try statements. If one returns an error I'd like to proceed to the next one, otherwise return the value. The code below seems to work fine, however I'll end up with a big nested do catch pyramid. Is there a smarter/better way to do it in Swift 3.0?

do {
    return try firstThing()
} catch {
    do {
        return try secondThing()
    } catch {
        return try thirdThing()
    }
}

do all your `try` statements inside your `do` and catch any exception in your `catch`. No need to have them nested at all. — Pancho, Aug 14 '17 at 10:35
This won't work @Pancho if the OP only wants to run second thing if firstThing fails and run thirdThing if secondThing fails. — Abizern, Aug 14 '17 at 10:38
Thanks @Pancho, but since I'm returning the value(or error) any code the first return won't be executed. — Mymodo, Aug 14 '17 at 10:40
@Abizern this is true. In such case the do-catch have to be replaced by if-else or switch statement which is not going to reduce the code or make it pretty. — Pancho, Aug 14 '17 at 10:46

Martin R · Accepted Answer · 2017-08-14T11:11:04.750

17

If the actual errors thrown from those function calls are not needed then you can use try? to convert the result to an optional, and chain the calls with the nil-coalescing operator ??.

For example:

if let result = (try? firstThing()) ?? (try? secondThing()) ?? (try? thirdThing()) {
    return result
} else {
    // everything failed ...
}

Or, if the error from the last method should be thrown if everything fails, use try? for all but the last method call:

return (try? firstThing()) ?? (try? secondThing()) ?? (try thirdThing())

edited Aug 14 '17 at 11:11

answered Aug 14 '17 at 11:01

Martin R

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I only need the last error, so this should do the job. Thanks. – Mymodo Aug 14 '17 at 11:09
Can this be adapted for void functions? – Declan McKenna May 21 '18 at 13:40
1

@Deco: Sure, `(try? firstThing()) ?? (try? secondThing()) ?? (try thirdThing())` calls the functions in turn until one does not throw. – Martin R May 21 '18 at 14:16

score 17 · Answer 2 · answered Aug 14 '17 at 11:09

17

If Martin's answer is too terse for your taste you can just go with individual catch blocks.

do {
    return try firstThing()
} catch {}

do {
    return try secondThing()
} catch {}

do {
    return try thirdThing()
} catch {}

return defaultThing()

As each throwing function's result is immediately returned no nesting is necessary.

answered Aug 14 '17 at 11:09

Nikolai Ruhe

81,520
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Cool, this solves it without converting to optionals. Thanks Nikolai. – Mymodo Aug 14 '17 at 11:17

Yannick · Answer 3 · 2017-08-14T12:41:38.620

Another way to do this is to write a function that takes all your throwing functions as an argument. It returns the first one that was successfully executed or nil.

func first<T>(_ values: (() throws -> T)...) -> T? {
    return values.lazy.flatMap({ (throwingFunc) -> T? in
        return try? throwingFunc()
    }).first
}

The lazy ensures that the values are only called until it finds the first match. Doing it this way, you can also add a lot of cases very quickly.

You can use the function like this

return first(firstThing, secondThing, thirdThing) ?? "Default"

I also included the code I used to test this in playground:

enum ThingError: Error {
    case zero
}

func firstThing() throws -> String {
    print("0")
    throw ThingError.zero
    return "0"
}

func secondThing() throws -> String {
    print("1")
    return "1"
}

func thirdThing() throws -> String {
    print("2")
    return "B"
}

func first<T>(_ values: (() throws -> T)...) -> T? {
    return values.lazy.flatMap({ (throwingFunc) -> T? in
        return try? throwingFunc()
    }).first
}

func tryThings() -> String {
    return first(firstThing, secondThing, thirdThing) ?? "Default"
}

tryThings() // prints "0" and "1"

Nested do catch swift 3.0

3 Answers3