[swift-users] Generics with variable argument lists

[Rien]

Ah!, yes that would be perfect!

Many thanks!

Regards,
Rien

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> On 09 Mar 2017, at 12:36, Ole Begemann <ole at oleb.net> wrote:
> 
> On 09/03/2017 11:05, Rien via swift-users wrote:
>> I am trying to achieve the following:
>> 
>> enum FunctionResult<T> {
>>  case success(T)
>>  case error(String)
>> }
>> 
>> func tester<T>(test: (…) -> FunctionResult<T>, onError: (String) -> T) -> T {
>>   …
>> }
>> 
>> The problem is of course the (…) that simply does not work.
>> 
>> I would like to use this generic with a variety of different signatures, i.e.:
>> 
>> let result1 = tester(test: myfunc1(param: 26) -> FunctionResult<Bool>, onError: { … handle the error ... })
>> let result2 = tester(test: myfunc2(param: “A String") -> FunctionResult<Bool>, onError: { … handle the error ... })
>> let result3 = tester(test: myfunc3(param: 26, param2: “String") -> FunctionResult<Int>, onError: { … handle the error ... })
>> 
>> Is this at all possible?
> 
> This should do it:
> 
>    func tester<T>(test: @autoclosure () -> FunctionResult<T>,
>        onError: (String) -> T) -> T {
>        switch test() {
>        case .success(let value): return value
>        case .error(let error): return onError(error)
>        }
>    }
> 
> The insight is that you don't really want to pass a function in the first parameter, but only the _result_ of that function call. The @autoclosure attribute then makes sure that the expression you pass to tester is only evaluated inside tester. You can leave it out if you want (if you do, replace `switch test()` with `switch test`).