[swift-users] Is it possible to specify error type thrown in a protocol method

[Tim Wang]

Thanks Howard, it's a good workaround.

Do you think it would be better to be part of swift language feature? I
wish swift team could consider this :)

On Thu, Jul 6, 2017 at 11:04 AM Howard Lovatt <howard.lovatt at gmail.com>
wrote:

> You could use a result type, e.g.:
>
> https://github.com/antitypical/Result
>
> Or roll your own result type.
>
> I think the reason that Swift doesn't support what you want is because of
> rethrows. When you declare a method as rethrows it can throw anything
> because the closure it is re-throwing can throw anything. They could have
> fully typed the rethrows but obviously decided that was not worth it. At
> present rethrows is light weight; the compiler generates two versions of
> the method, one that throws and one that doesn't. If it was typed then it
> would be like a generic method and a version of the method would be
> required for each type combination that was actually used.
>
>   -- Howard.
>
> On 6 July 2017 at 10:38, Tim Wang via swift-users <swift-users at swift.org>
> wrote:
>
>> Hi Swifters,
>>
>> I am wondering if it is possible to specify error types thrown in a
>> protocol method. By allowing us to do it and letting the compiler check all
>> the implementations of the protocol to make sure only they would only throw
>> the specified error types, we only need to catch our error types when
>> calling these methods.
>>
>> For the code below:
>>
>> enum MyError: Error {
>>
>>     case justError
>>
>> }
>>
>> protocol MethodWillThrow {
>>
>>     func testMethod() throws MyError
>>
>> }
>>
>>
>> extension MethodThrow {
>>
>>     func testMethod() throws {
>>
>>         throw MyError.justError
>>
>>     }
>>
>> }
>>
>> class TestClass: MethodThrow {
>>
>>     func testMethod() throws {
>>
>>         throw MyError.justError
>>
>>     }
>>
>>     func anotherMethod() {
>>
>>         do {
>>
>>             try testMethod()
>>
>>         } catch MyError.justError {
>>
>>             print("my error")
>>
>>         } *catch {*
>>
>> *            print("other error")*
>>
>> *        }*
>>
>>     }
>>
>> }
>>
>> Now we need add this extra default catch to make it compile and work and
>> I really want to remove this catch.
>>
>> Please let me know if there is a way to do it and thanks for help in
>> advance.
>>
>> Tim Wang
>>
>> _______________________________________________
>> swift-users mailing list
>> swift-users at swift.org
>> https://lists.swift.org/mailman/listinfo/swift-users
>>
>>
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