Forgive me if this has already come up, but since we’re talking about fixing
generics, I wonder if there is any solution in the pipeline for this problem:
--
protocol P { func foo() }
struct S: P { func foo() { print("foo") } }
func doSomething<C: CollectionType where C.Generator.Element: P>(c: C) {
for each in c {
each.foo()
}
}
let arr: [P] = [S()]
doSomething(arr) // error: cannot invoke 'doSomething' with an argument list of
type '([P])’
--
Why is this an error? The whole definition of [P] is basically an array of
things that conform to P. Isn’t that exactly what “where Element: P” is asking
for?
Changing Element: P to Element == P solves this particular issue, of course,
but then passing something like [S] to the array will fail. The result is that
you need to write two functions, and either have one eat the performance cost
of constructing a new array that has the correct static type to pass to the
other (since, unlike arrays, I can’t figure out a way to convert “Collection
where Element: P” into “Collection where Element == P” with a simple cast), or
just fill it with the dreaded copy-paste code. Neither seems ideal.
Charles
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