Opaque types (some) vs existentials (any)

Swift language Mid-level Swift 5.7+ iOS 17+ Reviewed 31 Oct 2026 By Alok Singh

Pick the fix

A function always returns one Shape type, and callers should get static dispatch. Which return type?

Options-> some Shape-> any Shape-> Shapefunc make<T: Shape>() -> T

Hint 1 of 1

Who chooses the concrete type?

Choose an answer, then open Show answer below to check it.

Show answer

Answer: A -> some Shape

Correct. One hidden concrete type with no boxing.

Why not the others?

B -> any Shape. It works but boxes the value and uses dynamic dispatch without need.
C -> Shape. Swift 6 requires any for an existential, and that would box anyway.
D func make<T: Shape>() -> T. Here the caller picks T, but this function wants to pick the type itself.

30-second answer

some P is an opaque type: one specific concrete type that the compiler knows but hides from the caller, with static dispatch and no boxing. any P is an existential: a box that can hold any conforming type, chosen at runtime, with dynamic dispatch. Use some by default; use any when you really need different types in one place, like a heterogeneous array.

Practise saying it · 45 s. No recording, just a timer

Explain some vs any.

Say your answer out loud, then check which of these points you covered:

some is one hidden concrete type
any is a runtime box
Default to some

Deep dive

SwiftUI's some View needs one concrete type per body.

Since Swift 5.7, some P in parameters is a generic parameter.

Swift 6 requires writing any explicitly for existentials.

Existentials of protocols with associated types work, but hide the associated type unless it is constrained.

Common mistakes

Returning any View from body.
Using any everywhere and paying for boxing.

Senior take & follow-ups

Seniors explain the reverse-generic idea: the function, not the caller, picks the type.

Follow-up questions

Think of your answer first, then open the hint.

Can an opaque return type return different types in different branches?
Show hint
No; use @ViewBuilder or an enum.
When must you use any?
Show hint
Collections of mixed conforming types.

Suggest a fix if something here is wrong or out of date.