Bound value in a conditional binding must be of Optional Type

Question

I have a protocol defined:

protocol Usable {
    func use()
}

and a class that conforms to that protocol

class Thing: Usabl         


        

Answer 1

As metioned in the Swift doc, the is operator is the guy you need for the job:

The is operator checks at runtime to see whether the expression is of the specified type. If so, it returns true; otherwise, it returns false.

The check must not be known to be true or false at compile time.

Therefore, the following test would normally be what you need:

if thing is Usable { 
    usableThing.use()
} else {
    println("can't use that")
}

However, as the doc specifies, Swift can detect at compile time that the expression is always true and declares an error to help the developer.

Answer 2

This works for me in the playground

protocol Usable {
    func use()
}

class Thing: Usable {
    func use () {
        println ("you use the thing")
    }
}

let thing = Thing()
let testThing : AnyObject = thing as AnyObject

if let otherThing = testThing as? Thing {
    otherThing.use()
} else {
    println("can't use that")
}

Answer 3

You can get it to compile by making the cast as Usable? instead of as Usable, like this:

// Check whether or not a class is useable
if let usableThing = thing as Usable? { // error here
    usableThing.use()
}
else {
    println("can't use that")
}

Answer 4

You are getting

Bound value in a conditional binding must be of Optional Type

because thing as Usable must return an optional type so making it as? should solved the problem. Unfortunately, the error still persisted for some odd reason. Anyway, a workaround I found to get it to work is to extract out the variable assignment inside the if statement

let thing = Thing()

let usableThing = thing as? Usable

if useableThing { 
    usableThing!.use()
}
else {
    println("can't use that")
}

Answer 5

swift protocols does not work in Playgrounds in the first beta, try to build a real project instead.