Scala 3/Reference/Error codes/E023: Wrong Number Of Type Args

E023: Wrong Number Of Type Args

This error is emitted when a type constructor is applied with the wrong number of type arguments.

Each generic type has a specific number of type parameters. For example:

List[A] takes exactly one type parameter
Map[K, V] takes exactly two type parameters
Either[L, R] takes exactly two type parameters

You must provide exactly the number of type arguments that the type expects.

Example

val x: List[Int, String] = List()

Error

-- [E023] Syntax Error: example.scala:1:7 --------------------------------------
1 |val x: List[Int, String] = List()
  |       ^^^^^^^^^^^^^^^^^
  |       Too many type arguments for List[A]
  |       expected: [A]
  |       actual:   [Int, String]
  |-----------------------------------------------------------------------------
  | Explanation (enabled by `-explain`)
  |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
  | You have supplied too many type parameters
  |
  | For example List takes a single type parameter (List[A])
  | If you need to hold more types in a list then you need to combine them
  | into another data type that can contain the number of types you need,
  | In this example one solution would be to use a Tuple:
  |
  | val tuple2: (Int, String) = (1, "one")
  | val list: List[(Int, String)] = List(tuple2)
   -----------------------------------------------------------------------------

Solution

// Use the correct number of type arguments
val x: List[Int] = List()

// For multiple types, use a tuple or a different container
val x: List[(Int, String)] = List()

// Or use a type that takes multiple parameters
val x: Map[Int, String] = Map()

// Or use Either for two alternatives
val x: Either[Int, String] = Right("hello")

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