ArrayScan.swift

extension Array {
    
    func scan(forEachNeighbors action: (Element, Element) -> Void) {
        guard count >= 2 else {
            return
        }
        var indexA = 0
        var indexB = 1
        
        while indexB <= count - 1 {
            let elementA = self[indexA]
            let elementB = self[indexB]
            action(elementA, elementB)
            indexA += 1
            indexB += 1
        }
    }
    
}

Or what if

import Foundation

extension Array {
    func adjacent_pairs() -> AnySequence<ArraySlice<Element>> {
        var idx = self.startIndex
        return AnySequence {
            return AnyIterator {
                if idx >= self.endIndex { return nil }
                let nextEnd = self.index(idx, offsetBy: 2)
                if nextEnd > self.endIndex { return nil }

                defer { idx = self.index(after: idx) }
                return self[idx ..< nextEnd]
            }
        }
    }
}

for i in [1,2,3,4,5,6,7].adjacent_pairs() {
    print(i)
}

@michaelrockhold Hmm... That doesn't print the (6, 7) pair.

It wouldn't be an original Rock code if it didn't have an off-by-one bug (but I fixed it)

And of course this is more in the spirit of your scan() function:

[1,2,3,4,5,6,7].adjacent_pairs().forEach { i in
    print(i)
}

I gotta ask: Snake case in Swift? Surely you need more boundaries in your life.