CODEWITHSUNDEEP
swiftexception
noamtm
noamtm

Reputation: 12983

Swift return or throw

The following pattern is quite common in certain places in my code, but it doesn't feel Swifty:

if let res = calcSomething() {
    return res
} else {
    throw err.invalidSomething
}

In English: return the calue of calcSomething() if it returned a non-nil, throw an exception otherwise.

I was hoping for a structure that looks like the following:

return calcSomething() ?? throw err.invalidSomething

But so far didn't find anything similar. Am I missing something?

Upvotes: 3

Views: 3773

Answers (2)

Ahmad F
Ahmad F

Reputation: 31645

For such a case, I would prefer to use the guard statement instead, so it would be similar to:

func myFunction() throws {
    guard let res = calcSomething() else {
        throw err.invalidSomething
    }

    // keep going now...

    // if something went wrong, you might want to throw another error:
    throw err.invalidSomething

    // if everything is ok:
    return res
}

The guard would bounce the calling of the function, by throwing the desired error directly if calcSomething() is nil.

About return calcSomething() ?? throw err.invalidSomething:

It seems to be an obvious goal when implementing such a function, however as some point it is not so logical to the language because the nil-coalescing operator (??):

The nil-coalescing operator (a ?? b) unwraps an optional a if it contains a value, or returns a default value b if a is nil. The expression a is always of an optional type. The expression b must match the type that is stored inside a.

The nil-coalescing operator is shorthand for the code below:

a != nil ? a! : b

which means that it returns a value as a result; throw err.invalidSomething does not a value to be returned, instead it is an exceptional case for something went wrong that throws an error.

Upvotes: 1

Malik
Malik

Reputation: 3802

I don't think you can use null coalesce in a return statement. However, you can do something like this

guard let res = calcSomething() else { throw err.invalidSomething }
return res

Upvotes: 2

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