[swift-evolution] Proposal: Add function SequenceType.find()

[Marco Masser]

> On 2016-02-18, at 23:13, Brent Royal-Gordon via swift-evolution <swift-evolution at swift.org> wrote:
> 
> A funny thing just happened—I was writing some Swift, needed something like `find`, remembered this thread, copied it in, andt ehn realized I wanted to find-and-cast at the same time. Here's what I ended up doing:
> 
> extension SequenceType {
>    func find<U>(@noescape finder: Generator.Element throws -> U?) rethrows -> U? {
>        for elem in self {
>            if let returnValue = try finder(elem) {
>                return returnValue
>            }
>        }
>        return nil
>    }
> 
>    func find(@noescape predicate: Generator.Element throws -> Bool) rethrows -> Generator.Element? {
>        return try find { try predicate($0) ? $0 : nil }
>    }
> }
> 
> Then the code that used it ended up being:
> 
>        var bitmap = representations.find { $0 as? NSBitmapImageRep } ?? makeNormalizedBitmapRep()
> 
> Which I think is about as nice as I could've hoped for.

If I’m not mistaken, this works OK as long as you only want to find something of a specific type. Maybe I’m missing something here, but to add some kind of predicate (e.g. checking for bitsPerPixel == 8), this would lead to code like this:

var bitmap: NSBitmapImageRep? = representations.find {
    if let bitmap = $0 as? NSBitmapImageRep where bitmap.bitsPerPixel == 8 {
        return bitmap
    }
    return nil
} ?? makeNormalizedBitmapRep()


… whereas my original version would look like this:

var bitmap = representations.find(NSBitmapImageRep.self) { $0.bitsPerPixel == 8 } ?? makeNormalizedBitmapRep()


The point of my proposed method is to filter for a *subtype* of Generator.Element and apply a given predicate only to those that match that subtype, then return the an instance of that subtype or nil.

Again, maybe I’m missing something here?
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