Scala 3/Reference/Error codes/E175: Value Discarding

E175: Value Discarding

This warning occurs when a non-Unit expression's value is discarded (not used). This often indicates a programming error where a return value is accidentally ignored.

This warning is enabled with the -Wvalue-discard compiler flag and helps catch bugs where important values like error results or updated collections are not handled.

Example

//> using options -Wvalue-discard

import scala.collection.mutable

def example: Unit = {
  mutable.Set.empty[String].remove("")
}

Error

-- [E175] Potential Issue Warning: example.scala:6:34 --------------------------
6 |  mutable.Set.empty[String].remove("")
  |  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |discarded non-Unit value of type Boolean. Add `: Unit` to discard silently.

Solution

//> using options -Wvalue-discard

import scala.collection.mutable

def example: Unit = {
  val set = mutable.Set.empty[String]
  // Use the return value
  val wasRemoved: Boolean = set.remove("")
  if wasRemoved then println("Removed")
}

//> using options -Wvalue-discard

import scala.collection.mutable

def example: Unit = {
  // Explicitly discard by adding : Unit
  mutable.Set.empty[String].remove(""): Unit
}

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