On Sat, Dec 30, 2017 at 11:35 PM, Nevin Brackett-Rozinsky via swift-users <
swift-users@swift.org> wrote:
> So, I know this doesn’t actually address your issue, but I think it is
> important to clarify that “rethrows” does not guarantee anything about the
> *type* of error thrown by a function. What “rethrows” implies is that the
> function *will not throw* unless at least one of its arguments throws.
>
> In particular, if the argument throws, the outer function can throw *any
> error or none at all*. For example,
>
> enum CatError: Error { case hairball }
> enum DogError: Error { case chasedSkunk }
>
> func foo(_ f: () throws -> Void) rethrows -> Void {
> do { try f() }
> catch { throw CatError.hairball }
> }
>
> do {
> try foo{ throw DogError.chasedSkunk }
> } catch {
> print(error) // hairball
> }
>
> Inside foo’s catch block, it is legal to throw any error, or not throw an
> error at all. But *outside* that catch block foo cannot throw, which is
> causing you consternation.
>
> • • •
>
> I don’t have a good solution for you, but in attempting to find one I
> *did* uncover something which compiles that probably shouldn’t. It seems
> that a “rethrows” function is currently allowed to throw if a *local*
> function throws:
>
> func rethrowing(_ f: () throws -> Void) rethrows -> Void {
> func localThrowing() throws -> Void { throw CatError.hairball }
> return try localThrowing()
> }
>
Yikes, that looks bad; worth filing a bug at bugs.swift.org if there isn't
one already.
>
> do {
> try rethrowing{ throw DogError.chasedSkunk }
> } catch {
> print(error) // hairball
> }
>
> I wouldn’t count on this functionality as it is most likely a bug. Indeed,
> if we pass in a non-throwing argument then we get a runtime error:
>
> rethrowing{ return } // EXC_BAD_ACCESS (code=1, address=0x0)
>
> Although, if we change “localThrowing” to use do/catch on a call to “f”
> and throw only in the catch block, or even use your “var caught: Error?”
> trick, then it appears to work as intended with no problems at runtime.
>
> • • •
>
> In the unlikely scenario that the above local-function behavior is valid
> and intended, the following function will, technically speaking, let you
> work around the issue you’re having:
>
> func withPredicateErrors <Element, Return>
> (_ predicate: (Element) throws -> Bool,
> do body: @escaping ((Element) -> Bool) -> Return
> ) rethrows -> Return
> {
> func bodyWrapper(_ f: (Element) throws -> Bool) throws -> Return {
> var caught: Error?
> let value = body{ elem in
> do {
> return try f(elem)
> } catch {
> caught = error
> return true
> }
> }
> if let caught = caught { throw caught }
> return value
> }
>
> return try bodyWrapper(predicate)
> }
>
> It is not pretty, and it probably relies on a compiler bug, but at the
> present time, against all odds, it look like this operates as you intend.
>
> Nevin
>
>
>
> On Sat, Dec 30, 2017 at 11:15 PM, Brent Royal-Gordon via swift-users <
> swift-users@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@swift.org
>> https://lists.swift.org/mailman/listinfo/swift-users
>>
>
>
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