Scala 3.8.2 is now available!

Release highlights

Warning for for with many vals and overloaded map (#25090)

Scala 3.8’s betterFors (available since 3.7 under -preview) changes for-comprehension desugaring and removes an intermediate map used for consecutive val bindings.

The following code snippet behaves differently at runtime depending on Scala version used for compilation. In Scala 3.7.x and earlier it produces List((43,29), (43,30), (43,31)), but starting with 3.8 it results in Map(43 -> 31).

def f(x: Int): (Int, Int) = (1, x)

val result: Iterable[(Int, Int)] =
  for // warning: For comprehension with multiple val assignments may change result type
    (k, v) <- Map(1 -> 1, 2 -> 1, 3 -> 1)
    x      = k + v
    (a, b) = f(x)
    (y, z) <- Map(42 -> 27)
  yield (a + y, b + z)

• Before (3.7): A synthetic tuple-producing map was inserted; that step could make Map select a generic map overload and become an Iterable (e.g. List)
• In 3.8: That synthetic step is removed, so the conversion no longer happens and a different map/flatMap path can be selected.

The previous runtime behaviour can be achived by explicitly converting first Map to Iterable type or by compilation with -source:3.7 settings.

The new warning highlights code where this migration risk exists.

Notable changes

• Support :dep ... to add library dependencies in the Scala REPL #24131

• Upgrade to Scala.js 1.20.2 #24898

• Bump Scala CLI to v1.12.2 (was v1.11.0) #25217: New aliases for RC and nightly Scala versions. See the Scala CLI release notes for additional details: v1.12.0, v1.12.1 and v1.12.2

For the complete list of changes and contributor credits, see the release notes on GitHub.