CODEWITHSUNDEEP
swifttypestuplesselfabstraction
PipWasHere
PipWasHere

Reputation: 103

How can I determine the type of a tuple's elements directly from its type name in Swift?

I have been exploring different abstractions in Swift, and I found myself stumped on how to determine the type of a tuple's element from its own type. I know the compiler and Xcode are quick to provide such information to me, the coder, and I know how to use type(of:) on the elements of a tuple variable or constant, but I cannot seem to figure out how to do it from code using only the tuple type.

Here is my question using incomplete code:

// Somewhere else in the code...
typealias NamedTuple = (/*someunknowntype*/, /*someotherunknowntype*/)

// Later in the code...
let tupleType = NamedTuple.self // repetitive, I know... but I know how to get its own type
let tupleElement0Type = // <-- How do I get this from the NamedTuple alias?

I cannot seem to discover any documentation that mentions a way to access a tuple's elements from its type name or literal format, although the latter would be silly.

I already know how to work around such issues using generic solutions. I am just looking for something straight forward, like with .self on a type name or alias.

I can already get the element types of arrays (ArrayType.Element.type) and dictionaries (Dictionary.Key.self and Dictionary.Value.self), but those are obviously through their typealias properties.

Does anyone know how to type the elements of a type directly from its name or alias? Or, at a minimum, from collections types other than an Array or a Dictionary?

Upvotes: 6

Views: 472

Answers (1)

Cristik
Cristik

Reputation: 32853

For tuples without labels, this can be achieved via parameter packs:

func tupleTypes<each T>(_: (repeat (each T)).Type) -> (repeat (each T).Type) {
    ((repeat (each T).self))
}

typealias MyTuple = (Int, String, [Double])
print(tupleTypes(MyTuple.self))
// (Swift.Int, Swift.String, Swift.Array<Swift.Double>)

let types = tupleTypes(MyTuple.self)
print(types.0)
// Int

Upvotes: 2

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