Swift Protocol Compile Time Check

I always thought that if you just use {get} on a protocol variable, then you can still set it i.e. it doesn’t matter if you give it a setter or not. That’s not true. It really depends on which compiler checks come in to place. Compiler checks are different depending on the type you want a variable to be.

Can you guess which of the two snippets will compile?

Snippet A

protocol Person {
    var age: Int { get }
}

class Adult: Person { 
    var age = 20
}

let a = Adult()
a.age = 20

Snippet B

protocol Person {
    var age: Int { get }
}

class Adult: Person { 
    var age = 10
}

let a: Person = Adult()
a.age = 20

Only snippet A compiles.

Snippet B gives the following error:

Cannot assign to property: ‘age’ is a get-only property

Why?

In snippet A, when we write down let a = Adult() the compiler looks at the definition of Adult.age and sees it’s mutable so it allows it.

Where in snippet B we have let a: Person = Adult(), the compiler looks at the definition of Person.age and sees it’s an immutable property. So it disallows that modification.

Summary

If the type that the compiler has to check against is the Protocol type itsef, then the getter/setter used on a protocol must be adhered exactly…

The compiler requires that you satisfy protocol requirements. As long as the class that adopts a protocol, conforms to its requirements then the compiler is happy. Yet the compiler will not assume the type of a variable to be its assignment type. It just so happens that the ‘assignment type’ satisfies the ‘declared type’.

To understand the jargon used:

var x: SomeType = SomeClass()
/*          |              | 
      declared type   assignment type
*/

Credits

Special thanks to Suyash Srijan who helped me figure this out.