forEachDispatch.swift

import Dispatch

/// Dispatch options for `forEach` loops
public enum ForEachClosureDispatch {
    case sequential
    case concurrent
}

extension Sequence {
    /// Calls the given closure on each element in the sequence in the same order
    /// as a `for`-`in` loop.
    ///
    /// The two loops in the following example produce the same output:
    ///
    ///     let numberWords = ["one", "two", "three"]
    ///     for word in numberWords {
    ///         print(word)
    ///     }
    ///     // Prints "one"
    ///     // Prints "two"
    ///     // Prints "three"
    ///
    ///     numberWords.forEach { word in
    ///         print(word)
    ///     }
    ///     // Same as above
    ///
    /// Using the `forEach` method is distinct from a `for`-`in` loop in two
    /// important ways:
    ///
    /// 1. You cannot use a `break` or `continue` statement to exit the current
    ///    call of the `body` closure or skip subsequent calls.
    /// 2. Using the `return` statement in the `body` closure will exit only from
    ///    the current call to `body`, not from any outer scope, and won't skip
    ///    subsequent calls.
    ///
    /// - Parameter approach: a flag that controls whether the
    ///   body should be dispatched asynchronously or performed sequentially
    /// - Parameter body: A closure that takes an element of the
    ///   sequence as a parameter.
    @_inlineable
    public func forEach(
        _ approach: ForEachClosureDispatch = .sequential,
        _ body: @escaping (Element) throws -> Void
        ) rethrows {
        for element in self {
            switch approach {
            case .sequential:
                try body(element)
            case .concurrent:
                DispatchQueue.global().async {
                    do {
                      try body(element)
                    } catch { fatalError("\(error)") }
                }
            }
        }
    }
}