Recursive constraints & higher-order generics
Write recursive constraints (e.g., nested sequences) and higher-order generic functions that take generic closures/functions.
Recursive constraints & higher-order generics is a free Swift Academy lesson on CoddyKit. This is lesson 2 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.
What & why
This lesson shows:
- Recursive constraints: constraints on nested generic members (e.g., Sequence of Sequence).
- Higher-order generics: generic functions that accept generic closures/functions.
Nested sequences (flatten)
A classic recursive constraint: we require C.Element: Sequence so we can iterate two levels deep.
// Works for any Collection whose elements are Sequences
func flatten<C: Collection>(_ c: C) -> [C.Element.Element]
where C.Element: Sequence {
var result: [C.Element.Element] = []
result.reserveCapacity(c.reduce(0) { $0 + ($1 as AnySequence<C.Element.Element>).underestimatedCount })
for inner in c {
for e in inner { result.append(e) }
}
return result
}
let nested = [[1,2], [3], [4,5]]
print(flatten(nested)) // [1,2,3,4,5]Map nested generically
We constrain twice: C: Collection and C.Element: Collection, letting us call map at each level.
// Map over a two-level structure without fixing concrete types
func mapNested<C: Collection, R>(_ c: C, _ transform: (C.Element.Element) -> R) -> [[R]]
where C.Element: Collection {
return c.map { inner in inner.map(transform) }
}
let doubled = mapNested([[1,2],[3]], { $0 * 2 })
print(doubled) // [[2,4],[6]]compose(f,g)
Higher-order generics: the function is generic over functions themselves, building reusable pipelines like compose.
// Compose two generic transforms that work on the same element type
func compose<A, B, C>(
_ f: @escaping (B) -> C,
_ g: @escaping (A) -> B
) -> (A) -> C {
{ a in f(g(a)) }
}
let toString: (Int) -> String = { "\\($0)" }
let length: (String) -> Int = { $0.count }
let intToLen = compose(length, toString)
print(intToLen(1234)) // 4Generic closure params
The closure f is a generic parameter too. We keep types flexible via where constraints.
// A generic "transform each" that accepts ANY transform (generic in T/U)
func transformEach<T, U, C: Collection>(_ c: C, using f: (T) -> U) -> [U]
where C.Element == T {
var out: [U] = []
out.reserveCapacity(c.count)
for x in c { out.append(f(x)) }
return out
}
let words = ["swift","kit"]
print(transformEach(words, using: { $0.uppercased() })) // ["SWIFT","KIT"]Guidelines
Tips:
- Write the smallest constraints you need (
SequencevsCollection). - Prefer where clauses for readability.
- Test with multiple concrete types to ensure generality.
- Document assumptions (e.g., element order, uniqueness).
Higher-order generics definition
Quick check: What makes a function a higher-order generic one?
Recap
Recap: Use recursive constraints (e.g., C.Element: Sequence) to work with nested generics; use higher-order generics to compose reusable function pipelines cleanly.
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Frequently Asked Questions
Is the “Recursive constraints & higher-order generics” lesson free?
Yes — any 3 lessons of the Swift Academy learning path, including “Recursive constraints & higher-order 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.
What will I learn in “Recursive constraints & higher-order generics”?
Write recursive constraints (e.g., nested sequences) and higher-order generic functions that take generic closures/functions. 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.
Do I need any experience to start Swift Academy?
No prior experience is required. Swift Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 2 of 3.
How long does the “Recursive constraints & higher-order generics” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Swift Academy lesson?
Yes. Every Swift Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Conditional conformances
- Recursive constraints & higher-order generics
- where clauses on extensions