Catching throw from initializer

Question

Neil Faiman OP

Created Jul ’20

Replies 6

Boosts 0

Participants 3

I have a struct that can throw an error.

If I just wanted to know if it had succeeded or not, I would use

Code Block swiftguard let try? thing = Thing()
else { terminate }

But I want to catch the error so that I can give a useful message before terminating, and I don't think I can do/catch in a guard statement.. The best I can come up with is

Code Block swiftlet thing: Thing
do
		try thing = Thing()
catch ... { 
		give useful message
		terminate
}

That isn't as attractive in several ways. Is there a better way?

Boost

Answer 1

Neil Faiman OP

Jul ’20

I meant to say that I have a struct whose initializer can throw an error.

0

Answer 2

OOPer OP

Jul ’20

That isn't as attractive in several ways. Is there a better way?

What do you think attractive? What do you think better? What is the reason you think it is not attractive?

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Answer 3

Neil Faiman OP

Jul ’20

What do you think attractive? What do you think better? What is the reason you think it is not attractive?

(1) Having to separate the declaration and the initialization of a let constant is sometimes necessary, but always feels like a hack. One of the big advantages of guard is that I can initialize a let constant with an optional binding, without having to wrap the entire body of a function in an if statement.

(2) A guard statement says immediately, “if his doesn't work, I’m out of here.” The do/catch requires a second look to determine that it’s really moral equivalent of a guard.

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Answer 4

OOPer OP

Jul ’20

always feels like a hack.

So, the main reason is you feel it like a hack.

As far as I know, there are no ways other than using do-catch.
But you may want to wait for someone who knows a real hack for this issue.

0

Answer 5

Claude31 OP

Jul ’20

Or you can suggest an evolution on Swift forum.

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Answer 6

OOPer OP

Jul ’20

A hacky workaround for people who thinks using guard is really a big issue.

Prepare some extension for Swift.Result.

Code Block extension Result {
		func getError() -> Error? {
				switch self {
				case .failure(let error):
						return error
				default:
						return nil
				}
		}
}

And then, you can write something like this:

Code Block let result = Result(catching: {try Thing()})
guard case .success(let thing) = result else {
		print(result.getError()!)
		exit(1)
}

I do not think it is any better than do-catch, but it actually is using guard.

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