MutatingForEach.swift

import Foundation
import XCTest

extension MutableCollection {
    
    /// Iterates through a mutable collection and mutates values based on the closure argument
    /// - Parameter closure: Takes and element and mutates it
    mutating func mutatingForEach(_ closure: (inout Element) -> Void) {
        indices.forEach {
            closure(&self[$0])
        }
    }
}

class MutatingForEachTests: XCTestCase {
    var sut = [1, 2, 3, 4, 5, 6, 7, 8]
    
    func testMutation() {
        let copy = sut
        sut.mutatingForEach { $0 *= 2 }
        
        sut.enumerated().forEach {
            XCTAssertNotEqual($0.element, copy[$0.offset])
            XCTAssertEqual($0.element / 2, copy[$0.offset])
        }
    }
}

MutatingForEachTests.defaultTestSuite.run()