// In the defining module. func foo_impl(a: Int, b bval: Int?, c val: Float?) { let b = bval ?? 42 let c = cval ?? 3.14 ... } // On the caller side. foo_impl(a: 192, b: nil, c: nil) // In the defining module. func foo_impl(a: Int, b bval: Int?, c val: Float?) { let b = bval ?? 42 let c = cval ?? 3.14 ... } // On the caller side. foo_impl(a: 192, b: nil, c: nil)