I may be missing your point, but there&#x27;s no special casing or special forms going on in Swift.<p>In Swift, + and += are just operators. They&#x27;re two separate operators which happen to do similar things. In Swift, an operator can be defined to mutate its left-hand operand, which += is defined to do. You can create your own operators that do that as well.<p>If you want to do it for methods, Swift has mutating methods which can change the thing they&#x27;re sent to. In this case, you&#x27;d mark your Xform func as &quot;mutating&quot; and then you can write someComplexThing.Xform(bar) and it will mutate someComplexThing in place.<p>Of course, if you want to support both forms, you end up with a bit of redundancy as you have to define stuff twice. But the built-in stuff is the same way, and you can at least define one in terms of the other to cut down on duplication.