Guard-Let-Catch (and If-Let-Catch) to avoid long (nested) do-blocks

[DandyLyons]

This is a fantastic pitch and this language problem should've been solved a long time ago. This pitch tackles this problem in a way that is simple, readable, and uses concepts and mechanisms that are already familiar in the language.

[DandyLyons]

@ksemianov

The current syntax allows for this:

func doWork() -> Int? {
    let a, b: Int

    do {
        a = try getA()
    } catch {
        print("Failed to get a: \(error)")
        return nil
    }

    do {
        b = try getB()
    } catch {
        print("Failed to get b: \(error)")
        return nil
    }

    return a + b
}

This code is precisely the problem with the current syntax. This code is filled with extra noise.

[tera]

@Jeehut

do let (data, response) = try await URLSession.shared.data(for: request) catch {
   throw RequestError.failedToLoadData(error)
}

Looking at this from slightly different angle... Once we have 1) do expressions implemented, and on top of that 2) allow omitting braces when do block has only one statement / expression → we may end up with something that looks very similar to what you are asking:

  let (data, response) = do try await URLSession.shared.data(for: request) catch {
     throw RequestError.failedToLoadData(error)
  }

[Nobody1707]

  let (data, response) = do { try await URLSession.shared.data(for: request) } catch {
     throw RequestError.failedToLoadData(error)
  }

This doesn't seem onerous enough to me that the braces should ever be made optional.

[GreatApe (Gustaf Kugelberg)]

I really like this idea, in some form or another. Slightly off-topic: A related issue I often have with throwing functions is when I want to do something like

• try x()
• if x throws, try y()
  etc

Currently it seems that I have to put the backup calls in increasingly nested catch blocks, and as much as I like this pitch, it doesn't seem to help this particular inconvenience. I wonder if there is a common solution to both problems?

[pyrtsa (Pyry Jahkola)]

I don't think those two features alone are quite enough.

On top of do expressions and omitted braces, being able to throw from the catch branch would still additionally require the pitched Never as bottom type including the mentioned future direction where throw statements would count as Never-valued expressions.

[maartene (Maarten Engels)]

Any news about do expressions? Those seem to solve the problem quite well and are very close syntax we already have (if-expressions and switch-expressions).

[Pippin (Ethan Pippin)]

I don't think do expressions alone will get any traction unless it somehow gets separated from the multi-line-expression debate.

[DandyLyons]

With respect, I think the repeated suggestions for do expressions in this thread is off topic. It's already been said that do expressions are a good idea:

@Jeehut

@anon9791410

Maybe you're looking for do expressions ?

I would love to see do expressions. It would be consistent with the if-switch expressions. But that doesn't mean I don't additionally want guard-catch clauses. Which would be consistent with guard-else clauses.

These things are not the same. They express slightly differing intents and I would love to see both.

do expressions are a good idea which deserve their own Swift pitch and proposal.

But I do not see how it eliminates the value or need of guard-let-catch and if-let-catch.