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Swift Academy · Lesson

Choosing between some, any, and generics

Pick generics for homogeneous static typing, some P to hide a fixed concrete type, and any P for heterogeneous polymorphism.

Lesson 3 of 38 steps

Choosing between some, any, and generics is a free Swift Academy lesson on CoddyKit. This is lesson 3 of 3. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the Swift Academy learning path, and your progress syncs across the web and the CoddyKit app. The Swift Academy course includes 3 lessons in total.

When to use which?

Rule of thumb:

  • Generics: same concrete type per call; best compile-time checks & performance.
  • some P: hide the concrete return type, fixed per function.
  • any P: mix conformers (heterogeneous), dynamic dispatch.

Generics: homogeneous

Generics are ideal for homogeneous data and optimization (static dispatch, inlining).

protocol Shape { func area() -> Double }

struct Circle: Shape { let r: Double; func area() -> Double { .pi * r * r } }
struct Square: Shape { let s: Double; func area() -> Double { s * s } }

// Generic: one concrete Shape type per call site (homogeneous)
func totalAreaGeneric<S: Sequence, T: Shape>(_ xs: S) -> Double where S.Element == T {
    xs.reduce(0) { $0 + $1.area() }
}
print(totalAreaGeneric([Circle(r:1), Circle(r:2)]))  // OK
// totalAreaGeneric([Circle(r:1), Square(s:2)])      // ❌ different types

Opaque: hide but fixed

some P returns a fixed but hidden type. Great when you want encapsulation without losing static performance.

// Opaque factory hides the concrete type while guaranteeing Shape
func unitShape(flag: Bool) -> some Shape {
    // Must return the SAME underlying type on all paths; pick one.
    Circle(r: 1)
}
let s = unitShape(flag: true)
print(String(format: "%.2f", s.area()))  // 3.14

any P: heterogeneous

any P allows mixed types in one collection or variable; methods dispatch dynamically via the protocol interface.

// Existential: store mixed conformers
let mixed: [any Shape] = [Circle(r: 1), Square(s: 2)]
let sum = mixed.reduce(0) { $0 + $1.area() }   // dynamic dispatch
print(String(format: "%.2f", sum))

API shapes compared

Pick the surface:

  • Generic: caller controls concrete type.
  • Opaque: you hide the type, fixed per function.
  • Existential: you may vary concrete types dynamically.
// Three API shapes for the same idea: "make a Shape"

// 1) Generic return — exposes concrete type to caller
func makeGeneric<T: Shape>(_ t: T) -> T { t }

// 2) Opaque return — hides concrete type but fixed
func makeOpaqueCircle() -> some Shape { Circle(r: 1) }

// 3) Existential return — can return any conformer
func makeExistential(_ big: Bool) -> any Shape { big ? Square(s: 3) : Circle(r: 1) }

let g: Circle = makeGeneric(Circle(r: 2))      // caller knows concrete type
let o = makeOpaqueCircle()                      // caller sees only Shape
let e = makeExistential(true)                   // could be Circle or Square at runtime
print(g.area(), o.area(), e.area())

Cheat sheet

Guidelines:

  • Prefer generics for performance and strong typing when data is homogeneous.
  • Use some P to hide return types and keep APIs stable.
  • Use any P for plugin-like or mixed collections.
  • Avoid any P for protocols with associated types/Self unless you type-erase.

Decision check: choose some/any/generics

Quick check: Which option hides a fixed concrete return type behind protocol P?

Answer it in the interactive lesson

Recap

Recap:

  • Generics = homogeneous, fastest, most static.
  • some P = hide concrete type, fixed per function.
  • any P = heterogeneous, dynamic dispatch.
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Is the “Choosing between some, any, and generics” lesson free?

Yes — any 3 lessons of the Swift Academy learning path, including “Choosing between some, any, and generics”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The Swift Academy course includes 3 lessons in total.

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Pick generics for homogeneous static typing, some P to hide a fixed concrete type, and any P for heterogeneous polymorphism. You practise Swift Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

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All lessons in this course

  1. some P (opaque result types): hiding concrete types
  2. any P (existential): trade-offs & dynamic dispatch
  3. Choosing between some, any, and generics
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