[swift-users] Rethrows issue

[Kenny Leung]

How about this?

func withPredicateErrors<Element, Return>(
    _ predicate: @escaping (Element) throws -> Bool,
    do body: ((Element) -> Bool) -> Return
    ) rethrows
    -> Return
{
    var caught: Error?
    var element: Element?
    
    let value = body { elem in
        element = elem
        do {
            return try predicate(elem)
        }
        catch {
            caught = error
            return true     // Terminate search
        }
    }
    
    if let _ = caught,
        let element = element {
        try _ = predicate(element)
    }

    return value
}

-Kenny


> On Dec 30, 2017, at 8:15 PM, Brent Royal-Gordon via swift-users <swift-users at swift.org> wrote:
> 
> I need to do something like this:
> 
> 	func withPredicateErrors<Element, Return>(_ predicate: (Element) throws -> Bool, do body: ((Element) -> Bool) -> Return) rethrows -> Return {
> 	  var caught: Error?
> 	  let value = body { elem in 
> 	    do {
> 	      return try predicate(elem)
> 	    }
> 	    catch {
> 	      caught = error
> 	      return true     // Terminate search
> 	    }
> 	  }
> 	  
> 	  if let caught = caught {
> 	    throw caught
> 	  }
> 	  else {
> 	    return value
> 	  }
> 	}
> 
> The problem is, the Swift compiler doesn't allow the explicit `throw` statement; even though it can only throw errors originally thrown by `predicate`, the compiler is not smart enough to prove that to itself. I cannot make `body` a `throws` function.
> 
> Is there any way to do this? Either to override the compiler's safety check, or to rewrite this function to avoid it?
> 
> -- 
> Brent Royal-Gordon
> Architechies
> 
> _______________________________________________
> swift-users mailing list
> swift-users at swift.org
> https://lists.swift.org/mailman/listinfo/swift-users

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