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Kotlin Academy · Lesson

Chaining Pipelines and Avoiding Intermediate Lists with Sequence

Build multi-step pipelines and use Sequence for lazy evaluation.

Lesson 4 of 413 steps

Chaining Pipelines and Avoiding Intermediate Lists with Sequence is a free Kotlin Academy lesson on CoddyKit. This is lesson 4 of 4. 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 Kotlin Academy learning path, and your progress syncs across the web and the CoddyKit app. The Kotlin Academy course includes 4 lessons in total.

The Problem: Intermediate Lists

Each collection operation (filter, map, etc.) creates a new list. For large collections or long chains, this wastes memory and time.

Eager vs Lazy Evaluation

Collection operations are eager: each step fully processes the list before the next. Sequence operations are lazy: processing one element through all steps before moving to the next.

Converting to Sequence

Call asSequence() to switch from eager (list) to lazy (sequence) processing.
val result = (1..1_000_000)
    .asSequence()
    .filter { it % 2 == 0 }
    .map { it * it }
    .take(5)
    .toList()
println(result)  // [4, 16, 36, 64, 100]

Sequence Is Lazy: No Work Until Terminal

No computation happens until you call a terminal operator (toList, first, sum, etc.).
val seq = generateSequence(1) { it + 1 }  // infinite!
    .filter { it % 3 == 0 }
    .map { it * it }
val first5 = seq.take(5).toList()
println(first5)  // [9, 36, 81, 144, 225]

generateSequence: Infinite Sequences

generateSequence creates lazy infinite sequences.
val fibonacci = generateSequence(Pair(0, 1)) { (a, b) -> Pair(b, a + b) }
    .map { it.first }
val first10 = fibonacci.take(10).toList()
println(first10)  // [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]

sequence Builder

The sequence { } builder with yield creates custom lazy sequences.
val evens = sequence {
    var n = 0
    while (true) {
        yield(n)
        n += 2
    }
}
println(evens.take(5).toList())  // [0, 2, 4, 6, 8]

When to Use Sequence

Use Sequence when: the collection is large (>1000 elements), you use multiple chained operations, and/or you only need a small subset of results.

When NOT to Use Sequence

For small collections (<100 elements), sequences add overhead. The iterator-based mechanism is slower for short chains.
// Overkill for small list:
listOf(1, 2, 3).asSequence().map { it * 2 }.toList()
// Just use:
listOf(1, 2, 3).map { it * 2 }

Sequence Terminal Operations

Terminal operations trigger processing. Common ones: toList(), first(), last(), count(), sum(), any(), all(), none().
val s = (1..100).asSequence().filter { it % 7 == 0 }
println(s.first())          // 7
println(s.count())          // 14
println(s.sum())            // 728

Stateful vs Stateless Operations

Some sequence ops are stateful (sort, distinct) — they must see all elements before producing output. They break lazy evaluation.
val s = (1..10).asSequence()
    .filter { it > 3 }   // stateless: lazy
    .sorted()            // stateful: must see all elements
    .take(3).toList()
println(s)  // [4, 5, 6]

constrainOnce: Single-Iteration Safety

Use constrainOnce() to make a sequence throw if iterated multiple times — useful for streams.
val seq = generateSequence(1) { if (it < 5) it + 1 else null }
    .constrainOnce()
val first = seq.toList()   // OK
// val second = seq.toList()  // IllegalStateException!

Quick Check

When does a Sequence start processing elements?
Answer it in the interactive lesson

Recap

Sequences are lazy — no work until a terminal op. Use asSequence() for large collections with multiple chained operations. generateSequence and sequence { } create infinite lazy streams. Terminal ops trigger processing.
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Frequently Asked Questions

Is the “Chaining Pipelines and Avoiding Intermediate Lists with Sequence” lesson free?

Yes — any 3 lessons of the Kotlin Academy learning path, including “Chaining Pipelines and Avoiding Intermediate Lists with Sequence”, 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 Kotlin Academy course includes 4 lessons in total.

What will I learn in “Chaining Pipelines and Avoiding Intermediate Lists with Sequence”?

Build multi-step pipelines and use Sequence for lazy evaluation. You practise Kotlin 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 Kotlin Academy?

No prior experience is required. Kotlin 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 4 of 4.

How long does the “Chaining Pipelines and Avoiding Intermediate Lists with Sequence” 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 Kotlin Academy lesson?

Yes. Every Kotlin 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

  1. map and flatMap: Transforming Every Element
  2. filter, filterNot, and partition
  3. fold, reduce, and runningFold
  4. Chaining Pipelines and Avoiding Intermediate Lists with Sequence
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