E198: Unused Symbol

This warning is emitted when a symbol (variable, parameter, import, etc.) is declared but never used.

This warning helps identify dead code and potential mistakes where a value was intended to be used but was forgotten. The warning can identify various types of unused symbols:

• Unused imports
• Unused local definitions (vals, vars, defs)
• Unused parameters (explicit and implicit)
• Unused private members
• Pattern variables that are not used

This warning requires the -Wunused compiler flag with appropriate sub-options (e.g., -Wunused:all).

Example

//> using options -Wunused:all

def example(): Unit =
  val unused = 42
  println("hello")

Error

-- [E198] Unused Symbol Warning: example.scala:4:6 -----------------------------
4 |  val unused = 42
  |      ^^^^^^
  |      unused local definition

Solution

Remove the unused symbol:

//> using options -Wunused:all

def example(): Unit =
  println("hello")

Or use the symbol:

//> using options -Wunused:all

def example(): Unit =
  val value = 42
  println(s"The value is $value")

Or suppress the warning by using val with underscore identifier

//> using options -Wunused:all

def example(): Unit =
  val _ = 42
  println("hello")