Scala New Added 26 September 2026

Scala: missing parameter type for expanded function

The compiler reached a placeholder or a lambda before it knew the expected type, so there is nothing to infer the parameter from. The cause is inference order rather than the lambda itself: a value with no declared type, an overloaded method, or a chain whose receiver type is still being worked out.

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

// Nothing to infer from: there is no expected type here
// val f = _ + 1

// Say what the function is, either side of the equals sign
val f: Int => Int = _ + 1
val g = (x: Int) => x + 1

// In a chain, annotate the value that lost its type, not the lambda
val names: List[String] = load()
names.map(_.toUpperCase)

// Overloads defeat inference, so make the choice explicit
def handle(f: Int => Int): Unit = ()
def handle(f: String => String): Unit = ()
handle((s: String) => s.trim)

// Separate parameter lists let the first inform the second
def fold[A, B](xs: List[A])(z: B)(f: (B, A) => B): B = xs.foldLeft(z)(f)
fold(List(1, 2, 3))(0)(_ + _)     // fine, B is known by the time f is typed

// Underscore inside a larger expression expands wider than people expect:
// xs.map(_ + 1) works, xs.map(f(_) + 1) often needs the parameter named

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

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