In previous articles, I’ve talked about accumulating errors with Validated, and for that we said that we needed a Semigroup on the left side that will be used to accumulate the errors (We used a NonEmptyList[A] as an example). Today well talk about Chain and NonEmptyChain[A] and how it’s different from a List.

Appending to a List requires iterating over the entire collection (linear time). So using ValidatedNel[A,B] will make us incur a heavy performance penalty specially if we traverse it.

In comes Chain. Chain is very similar to List, but supports O(1) time append and prepend. This makes it a much better fit to use with Validated, Ior or Writer (article on this pretty soon).

Cats also offers type aliases like ValidatedNec or IorNec as well as helper functions like groupByNec or Validated.invalidNec.

Here are some benchmarks from the typelevel cats documentation

[info] Benchmark                                  Mode  Cnt   Score   Error  Units
[info] CollectionMonoidBench.accumulateChain     thrpt   20  51.911 ± 7.453  ops/s
[info] CollectionMonoidBench.accumulateList      thrpt   20   6.973 ± 0.781  ops/s
[info] CollectionMonoidBench.accumulateVector    thrpt   20   6.304 ± 0.129  ops/s

and some function calls:

[info] Benchmark                           Mode  Cnt          Score         Error  Units
[info] ChainBench.foldLeftLargeChain      thrpt   20        117.267 ±       1.815  ops/s
[info] ChainBench.foldLeftLargeList       thrpt   20        135.954 ±       3.340  ops/s
[info] ChainBench.foldLeftLargeVector     thrpt   20         61.613 ±       1.326  ops/s
[info] ChainBench.mapLargeChain           thrpt   20         59.379 ±       0.866  ops/s
[info] ChainBench.mapLargeList            thrpt   20         66.729 ±       7.165  ops/s
[info] ChainBench.mapLargeVector          thrpt   20         61.374 ±       2.004  ops/s


Similarly to NonEmptyList NonEmptyChain has a semigroup but not a monoid (obviously 😛).

Cats doc shows a couple examples on how to create them or transform NonEmptyList into a NonEmptyChain:

// import

NonEmptyChain(1, 2, 3, 4)
// res0:[Int] = Chain(1, 2, 3, 4)

NonEmptyChain.fromNonEmptyList(NonEmptyList(1, List(2, 3)))
// res1:[Int] = Chain(1, 2, 3)

NonEmptyChain.fromNonEmptyVector(NonEmptyVector(1, Vector(2, 3)))
// res2:[Int] = Chain(1, 2, 3)
// res3:[Int] = Chain(1)