What is a good example to differentiate between fileprivate and private in Swift3

Question

This article has been helpful in understanding the new access specifiers in Swift 3. It also gives some examples of different usages of fileprivate and private.

My question is - isn't using fileprivate on a function that is going to be used only in this file the same as using private?

Answer 1

fileprivate is now what private used to be in earlier Swift releases: accessible from the same source file. A declaration marked as private can now only be accessed within the lexical scope it is declared in. So private is more restrictive than fileprivate.

As of Swift 4, private declarations inside a type are accessible to extensions of the same type if the extension is defined in the same source file.

Example (all in one source file):

class A {
    private func foo() {}
    fileprivate func bar() {}

    func baz() {
        foo()
        bar()
    }
}

extension A {
    func test() {
        foo() // Swift 3: error: use of unresolved identifier 'foo'
              // Swift 4: no error because extension is in same source file
        bar()
    }
}

let a = A()
a.foo() // error: 'foo' is inaccessible due to 'private' protection level
a.bar()

• The private foo method is accessible only within the scope of the class A { ... } definition. It is not even accessible from an extension to the type (in Swift 3, see the second note below for changes in Swift 4).

• The file-private bar method is accessible from the same source file.

Notes:

1. The proposal SE-0159 – Fix Private Access Levels suggested to revert to the Swift 2 semantics in Swift 4. After a lengthy and controversial discussion on the swift-evolution mailing list, the proposal was rejected.

2. The proposal SE-0169 – Improve Interaction Between private Declarations and Extensions suggests to make private declarations inside a type accessible to extensions of the same type if the extension is defined in the same source file. This proposal was accepted and implemented in Swift 4.