[Swift] - reduce

reduceDetail.swift

 /// Returns the result of combining the elements of the sequence using the
    /// given closure.
    ///
    /// Use the `reduce(_:_:)` method to produce a single value from the elements
    /// of an entire sequence. For example, you can use this method on an array
    /// of numbers to find their sum or product.
    ///
    /// The `nextPartialResult` closure is called sequentially with an
    /// accumulating value initialized to `initialResult` and each element of
    /// the sequence. This example shows how to find the sum of an array of
    /// numbers.
    ///
    ///     let numbers = [1, 2, 3, 4]
    ///     let numberSum = numbers.reduce(0, { x, y in
    ///         x + y
    ///     })
    ///     // numberSum == 10
    ///
    /// When `numbers.reduce(_:_:)` is called, the following steps occur:
    ///
    /// 1. The `nextPartialResult` closure is called with `initialResult`---`0`
    ///    in this case---and the first element of `numbers`, returning the sum:
    ///    `1`.
    /// 2. The closure is called again repeatedly with the previous call's return
    ///    value and each element of the sequence.
    /// 3. When the sequence is exhausted, the last value returned from the
    ///    closure is returned to the caller.
    ///
    /// If the sequence has no elements, `nextPartialResult` is never executed
    /// and `initialResult` is the result of the call to `reduce(_:_:)`.
    ///
    /// - Parameters:
    ///   - initialResult: The value to use as the initial accumulating value.
    ///     `initialResult` is passed to `nextPartialResult` the first time the
    ///     closure is executed.
    ///   - nextPartialResult: A closure that combines an accumulating value and
    ///     an element of the sequence into a new accumulating value, to be used
    ///     in the next call of the `nextPartialResult` closure or returned to
    ///     the caller.
    /// - Returns: The final accumulated value. If the sequence has no elements,
    ///   the result is `initialResult`.
    public func reduce<Result>(_ initialResult: Result, _ nextPartialResult: (Result, Element) throws -> Result) rethrows -> Result