Scala How to use pattern matching with a non generic LazyList?

Question

In Scala 2.13 I have encountered a problem with pattern matching using the operator #::, which displays error Cannot resolve method #::.unapply when used as follows:

def exampleFunction(lazyList: LazyList[Int]):Unit =
  lazyList match {
    case LazyList() => println("End")
    case head #:: tail => println(head); exampleFunction(tail) // Cannot resolve method #::.unapply
  }
exampleFunction(LazyList(1,2,3,4,5,6,7,8))

When the LazyList is generic, the operator does work as expected:

def exampleFunction[A](lazyList: LazyList[A]):Unit =
  lazyList match {
    case LazyList() => println("End")
    case head #:: tail => println(head); exampleFunction(tail)
  }
exampleFunction(LazyList(1,2,3,4,5,6,7,8)) // output: 1 2 3 4 5 6 7 8 End

Why does this problem occur and is there a way to fix it?

Your first snippet version appears to compile fine, see https://scastie.scala-lang.org/iab8q5EdS0qV20xLwY4J0Q — Marth, Nov 24 '19 at 21:39
I cannot seem to replicate the issue on my machine. Try with `import scala.collection.immutable.LazyList.#::` — Mario Galic, Nov 24 '19 at 22:05
Adding `import scala.collection.immutable.LazyList.#::` fixed the problem completely and everything now works, thank you! — kamov, Nov 24 '19 at 22:19
@kamnov . I also faced the same issue and adding the import solved the issue. How ever I am not sure why we need to add the import explicitly in case of LazyList, when this is not applicable in case of List. Any answer will be appreciated. — Shantiswarup Tunga, Apr 27 '20 at 21:02
@MarioGalic you should put an answer with your comment, it also fixed this issue for me. I also don't understand why this is needed. — jrod, May 03 '20 at 15:20

score 3 · Accepted Answer · answered May 03 '20 at 16:01

In case you are using IntelliJ, this might be due to in-editor error highlighting bug SCL-15834: Highlighting error on case matching using operator #:: In other words, this is a false positive where the code compiles successfully however IntelliJ's custom error highlighting process incorrectly identifies a problem. Providing explicit import

import scala.collection.immutable.LazyList.#::

seems to make the editor error highlighting happy, however the import should not be necessary. Few other suggestion to try

File | Invlidate Caches
From the root of the project rm -fr .idea and then re-import project
Update to bleeding edge Scala Plugin version: Preferences | Languages & Frameworks | Scala | Updates | Update channel | Nightly Builds
Within Registry... enable experimental flag scala.highlighting.compiler.errors.in.editor

Scala How to use pattern matching with a non generic LazyList?

1 Answers1