> On 12 Mar 2017, at 08:21, Xiaodi Wu <[email protected]> wrote:
>
> Sorry, I'm confused. The following works:
>
> ```
> protocol Promise {
> associatedtype Result
> }
>
> protocol Scanner {
> associatedtype ScannerPromise : Promise
> func frobnicate<T>(_: T) -> ScannerPromise
> where ScannerPromise.Result == T
> }
> ```
>
> Why does it matter if `ScannerPromise` is generic or not?
>
That’s some pretty strange syntax. I admit I didn’t even try that.
ScannerPromise would necessarily have to be generic in this context, because I
want to bind one of its associated types to a generic parameter from a
function. There is no way a non-generic type could do that.
That said, even though the compiler accepts your code, it doesn’t seem to
actually work:
protocol Promise {
associatedtype Result
func await() -> Result
}
protocol Scanner {
associatedtype ScannerPromise : Promise
func frobnicate<T>(_: T) -> ScannerPromise
where ScannerPromise.Result == T
}
func use<S: Scanner, T>(_ s: S, _ t: T) -> T {
return s.frobnicate(t).await()
}
3.0.2: Segfault
3.1:
error: repl.swift:13:14: error: cannot invoke 'frobnicate' with an argument
list of type '(T)'
return s.frobnicate(t).await()
^
repl.swift:13:14: note: expected an argument list of type '(T)'
return s.frobnicate(t).await()
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