swift - Using a Type Variable in a Generic

Question

I have this question except for Swift. How do I use a Type variable in a generic?

I tried this:

func intType() -> Int.Type {
    return Int.self
}

func test() {
    var t = self.intType()
    var arr = Array<t>() // Error: "'t' is not a type". Uh... yeah, it is.
}

This didn't work either:

var arr = Array<t.Type>() // Error: "'t' is not a type"
var arr = Array<t.self>() // Swift doesn't seem to even understand this syntax at all.

Is there a way to do this? I get the feeling that Swift just doesn't support it and is giving me somewhat ambiguous error messages.

Edit: Here's a more complex example where the problem can't be circumvented using a generic function header. Of course it doesn't make sense, but I have a sensible use for this kind of functionality somewhere in my code and would rather post a clean example instead of my actual code:

func someTypes() -> [Any.Type] {
    var ret = [Any.Type]()
    for (var i = 0; i<rand()%10; i++) {
        if (rand()%2 == 0){ ret.append(Int.self) }
        else {ret.append(String.self) }
    }
    return ret
}

func test() {
    var ts = self.someTypes()

    for t in ts {
        var arr = Array<t>()
    }
}

Answer 1

Swift's static typing means the type of a variable must be known at compile time.

In the context of a generic function func foo<T>() { ... }, T looks like a variable, but its type is actually known at compile time based on where the function is called from. The behavior of Array<T>() depends on T, but this information is known at compile time.

When using protocols, Swift employs dynamic dispatch, so you can write Array<MyProtocol>(), and the array simply stores references to things which implement MyProtocol — so when you get something out of the array, you have access to all functions/variables/typealiases required by MyProtocol.

But if t is actually a variable of kind Any.Type, Array<t>() is meaningless since its type is actually not known at compile time. (Since Array is a generic struct, the compiler needs know which type to use as the generic parameter, but this is not possible.)

I would recommend watching some videos from WWDC this year:

• Protocol-Oriented Programming in Swift
• Building Better Apps with Value Types in Swift

I found this slide particularly helpful for understanding protocols and dynamic dispatch: