[swift-users] Which is the more idiomatic approach for pinning down the type of a generic parameter?

[zhaoxin肇鑫]

Then I think the function with NSButton.self is the best. It makes least
efforts for a programmer to get no mistakes.

zhaoxin

On Wed, Jan 20, 2016 at 7:25 PM, Marco Masser <lists at duckcode.com> wrote:

>
> On 2016-01-20, at 11:25, zhaoxin肇鑫 <owenzx at gmail.com> wrote:
>
> What about dropping T, just using Element, and converting Element to the
> type in your predicate?
>
>
> As I wrote in my original message, I’m writing a whole set of findFirst() methods
> and one of them simply takes a predicate and returns an Element:
>
> @warn_unused_result
> func findFirst(@noescape predicate: Self.Generator.Element throws -> Bool)
> rethrows -> Self.Generator.Element? {
>     for element in self where try predicate(element) {
>         return element
>     }
>     return nil
> }
>
> If I understand correctly, you propose to check the type of the passed-in
> Element in the predicate, like this:
>
> let result1 = containerView.subviews.findFirst { ($0 as? NSButton)?.state
> == NSOnState }
>
> But that is cumbersome and has a very significant drawback: The type of
> result1 is now NSView, not NSButton.
> That’s the whole point for the generic version of the method in my
> original message: the predicate doesn’t have to check the type, and the
> result of findFirst() has the type the caller specifies.
> So to get the same result as with my generic method (but uglier and slower
> due to more type casts) you’d have to do this:
>
> let result2 = containerView.subviews.findFirst { ($0 as? NSButton)?.state
> == NSOnState } as? NSButton
>
> I think that’s not very nice compared to how the two generic versions in
> question look:
>
> let result3 = containerView.subviews.findFirst(NSButton.self, matching: {
> $0.state == NSOnState })
> or:
> let result4: NSButton? = containerView.subviews.findFirst { $0.state ==
> NSOnState }
>
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