Difference in the Applicative inferred for Seq[Seq[?]]#sequence between Scala 2 and 3
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Description
Minimized:
import cats.syntax.all._
Seq(Seq()).sequence
Infers Seq[Seq[ in Scala 2, but Int => Seq[ in Scala 3.
Original use case, encountered in #scala-users:
import cats.syntax.all._
for {
x <- Seq(0, 1, 2).permutations
y <- Seq(Seq(-1, 1), Seq(-1, 1), Seq(-1, 1)).sequence
} yield (x, y)
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start by reproducing the minimized Seq(Seq()).sequence example with cats.syntax.all._ under Scala 2 and Scala 3, then compare the inferred types. Trace the sequence syntax and Applicative/type inference behavior involved in the difference. Done means the discrepancy and the original permutations example have an agreed, verified outcome.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- scala
- Domain
- compilers
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 35/100