Neat extension for array delta

Array+ReduceDelta.swift

extension Array {
    public func reduceDelta<Result>(_ initialResult: Result, _ nextPartialResult: (Result, Element, Element) throws -> Result) rethrows -> Result {
        guard var previous = first else {
            return initialResult
        }

        return try dropFirst().reduce(initialResult, { result, element in
            let nextResult = try nextPartialResult(result, previous, element)
            previous = element
            return nextResult
        })
    }
}

// TESTS
class ArrayTests: XCTestCase {
    func testReduceDelta() {
        let testArray = [1, 2, 4, 10]
        let delta = testArray.reduceDelta(0, { result, previous, current in
            return result + current - previous
        })
        XCTAssertEqual(delta, 9)
    }
}