Scala New Added 19 September 2026

Scala: recursive value x needs type

A definition refers to its own name on the right hand side, so the compiler cannot infer the type it is in the middle of inferring. It is almost never real recursion: the usual cause is a val that shadows something with the same name, such as a constructor parameter reused as a field.

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

Quick fix

// Shadowing: the right hand side resolves to the val being defined, not the parameter
def make(config: Config): Server = {
  val config = config.withDefaults()   // recursive value config needs type
  new Server(config)
}

// Rename, which is the fix in most cases
def make(config: Config): Server = {
  val resolved = config.withDefaults()
  new Server(resolved)
}

// Genuinely recursive definitions need an explicit type annotation
lazy val stream: LazyList[Int] = 1 #:: stream.map(_ + 1)

// Same rule for mutually recursive methods: annotate at least one
def isEven(n: Int): Boolean = n == 0 || isOdd(n - 1)
def isOdd(n: Int): Boolean  = n != 0 && isEven(n - 1)

How to diagnose Scala errors

Scala's most distinctive error class is binary incompatibility: libraries are published per Scala major version, and mixing artifacts built for different versions produces confusing "not a member of" or NoSuchMethodError failures. The %% operator in sbt exists precisely to append the right suffix, and most unresolved-dependency errors are a % where %% was needed.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

Check the Scala version suffix on every dependency. Use %% for Scala libraries and plain % only for Java ones.
Run sbt evicted to list dependencies that were evicted by version conflict. A frequent source of runtime NoSuchMethodError.
Use sbt dependencyTree to see the resolved graph and locate the transitive dependency pulling in an old version.
For inference failures, add explicit type annotations at the boundary rather than at the error site; inference failures usually propagate from further up.
Clear stale state with sbt clean and remove ~/.ivy2/cache or ~/.cache/coursier entries only for the specific failing artifact.

Tools worth reaching for

sbt evicted
sbt dependencyTree
coursier resolve
scalac -explain
MiMa (binary compat checks)

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

docs.scala-lang.org docs.scala-lang.org

Related Scala errors

See all 10 Scala errors →

Browse other categories

Something missing or wrong?

This entry is maintained by hand. If the fix is out of date, incomplete, or you have a better one, email a correction and it will be reviewed.