turn foo.xInPlace(arg) into a foo.x(arg)...

modifiedCopy.swift

func modifiedCopy<Struct, Arg>(var start: Struct, @noescape mutator: (inout Struct) -> Arg -> (), arg: Arg) -> Struct {
    mutator(&start)(arg)
    return start
}

extension Array {
    func arrayByAppending(e: Element) -> Array {
        return modifiedCopy(self, mutator: Array.append, arg: e)
    }
}