Scala New Added 12 September 2026

Scala: type mismatch; found: Unit, required: String

The last expression in the block is an assignment or a `for` loop, both of which evaluate to Unit, so the block returns nothing useful. It reads as a compiler mistake because the value the author meant to return is two lines up, discarded by whatever came after it.

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

// Returns Unit: the assignment is the last expression
def name(user: User): String = {
  val n = user.firstName + " " + user.lastName
  cachedName = n
}

// Return the value explicitly
def name(user: User): String = {
  val n = user.firstName + " " + user.lastName
  cachedName = n
  n
}

// A for loop is Unit as well: yield makes it a collection
val names = for (u <- users) yield u.firstName

# Turn the silent cases into errors while you are in the file
scalacOptions ++= Seq("-Wvalue-discard", "-Wnonunit-statement", "-Xfatal-warnings")

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

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