Swift type-variable to generic-type-variable

Question

The error I get is unsurprising, but how would I accomplish something like this in Swift?

let stringType = String.self
let stringArrayType = Array<String>.self
let stringArrayTypeFromVariable = Array<stringType>.self // Error :Use of undeclared type 'stringType'

My final goal is to build a function like this.

print( getTypeAsArray( Int.self ) ) // prints "Array<Int>.Type"
print( getTypeAsArray( String.self ) ) // prints "Array<String>.Type"

Here's my current attempt. It doesn't compile, but I think maybe this is solvable with generics like this.

// this version doesn't compile...
func getTypeAsArray<T>(_ value: T) -> [T].Type {
    return []
}
getTypeAsArray(String.self)

// this version compiles, but doesn't give me the functionality I want...The expected output was Array<String>, not Array<String.Type>
func getTypeAsArray<T>(_ value: T) -> [T].Type {
    return type(of: [])
}
getTypeAsArray(String.self) // -> Array<String.Type>

Your final goal seems kind of pointless. Is there a final final goal? Are you going to pass in types unknown at compile time into getTypeAsArray? — Sweeper
– Sweeper, Commented Jan 3, 2020 at 0:47
Okay, I don't think this is possible then. What would the function signature look like? It would accept a Any.Type parameter, right? What type would it return? — Sweeper
– Sweeper, Commented Jan 3, 2020 at 0:51
Generics are bound at compile-time. If you don't know the type at compile time, generics won't help. Continuing on with my previous argument, the method would need to accept a parameter t of type Any.Type. This is so that at runtime, t could store any meta type object. But what would the method return? It can't return [Any].Type, because metatypes are not covariant. — Sweeper
– Sweeper, Commented Jan 3, 2020 at 1:00
I see your point. There not way to have it return [T].Type and have T determined dynamically because T must be determined at compile time — joehinkle11
– joehinkle11, Commented Jan 3, 2020 at 1:01
I see that you are even struggling with a "compile-time only" version of getTypeAsArray. You probably meant func getTypeAsArray<T>(_ value: T.Type) -> [T].Type { return [T].self }. — Sweeper
– Sweeper, Commented Jan 3, 2020 at 1:03

Alexander · Accepted Answer · 2020-01-03 02:15:35Z

1

You can achieve your final goal by doing something like this:

func getTypeAsArray<T>(_ value: T.Type) -> [T].Type { 
    return [T].self 
}

But this won't work in situations like this:

let t: UIView.Type = UITableView.self
print(getTypeAsArray(t))

You'd expect it to print Array<UITableView>, but it prints Array<UIView>.

Unfortunately, I don't think the you can make the above case work as well. Because such a method must accept an Any.Type, but what would its return type be? It can't return a [Any].Type because although metatypes are covariant, metatype arrays are not:

let t: [UIView].Type = [UITableView].self

error: cannot convert value of type [UIView].Type to specified type [UITableView].Type

edited Jan 3, 2020 at 2:15

Alexander

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answered Jan 3, 2020 at 1:11

Sweeper

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1 Comment

joehinkle11 Over a year ago

This works for me because in my project I'm already downcasting before passing it through. Thank you!

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