Could Be Sequence

kotlin.could_be_sequence

Severity

low

Remediation Complexity

trivial

Remediation Risk

low

Remediation Effort

low

Resource

Efficiency

Language

Kotlin

Description

Reports long chains of eager collection operators applied to a Collection. Operators such as filter, map, flatMap, take, drop, distinct, and sortedBy each allocate a fresh List holding their result. A chain of N such operators allocates N intermediate lists; lifting the receiver into a Sequence defers evaluation and runs the entire pipeline in a single pass with no intermediate allocations.

The threshold is configurable via properties.minChainLength (default: 4).

Rationale

// FLAW — four chained collection operators allocate three intermediate Lists
xs.filter { it > 0 }
  .map { it + 1 }
  .filter { it < 10 }
  .distinct()

// OK — single pass, no intermediate Lists
xs.asSequence()
  .filter { it > 0 }
  .map { it + 1 }
  .filter { it < 10 }
  .distinct()
  .toList()

For short chains the allocation overhead is negligible; the rule deliberately only fires beyond minChainLength operators where the saving becomes measurable.

Remediation

Insert .asSequence() between the collection and the first operator, and materialise the result with .toList() (or another terminal operator) at the end of the chain.

References

Kotlin - Sequences