I have a couple of different items in my Swift wish list that would cover the feature you are pitching.
Currently you can't use the instance itself to construct the default value of an argument:
class Class {
var defaultFoo: String = "Default Foo"
func f(foo: String = defaultFoo) { // error: cannot use instance member 'defaultFoo' as a default parameter
print("f(\(foo)")
}
}
The manual workaround for this is:
class Class {
var defaultFoo: String = "Default Foo"
func f(foo: String? = nil) {
let foo = foo ?? defaultFoo
print("f(\(foo)")
}
}
I wish Swift implemented default arguments automatically like the above manual workaround, so that it would let me directly refer to the instance itself (and even non-defaulted argument values) in the value of the default argument. If Swift acted like that, default parameter overrides would also automatically work.
The other feature I wish Swift had is being able to make default arguments part of protocol contract. For example I wish protocols supported the following syntax:
protocol Protocol {
// Specify the argument should have a default, without specifying it:
func f(foo: String = default)
// The above would let me call `f()` on a protocol instance.
func g() -> String
// Specify the argument should have a default and define the default value:
func h(bar: String = g())
// Or constant:
func h2(bar: String = "Constant Default")
}
The above features provide a superset of what you are asking and I would pitch these features instead if I had time to follow it up.