docs/_docs/reference/error-codes/E024.md
This error is emitted when a variable binding is used in a pattern alternative (|). Variables are not allowed in alternative patterns.
When you use the |
operator to combine patterns, each alternative must match independently, and
Scala cannot guarantee that a variable bound in one alternative would have the
same value or even be bound in another alternative.
def test(pair: (Int, Int)): Int = pair match
case (1, n) | (n, 1) => n
case _ => 0
-- [E024] Syntax Error: example.scala:2:11 -------------------------------------
2 | case (1, n) | (n, 1) => n
| ^
| Illegal variable n in pattern alternative
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| Variables are not allowed within alternate pattern matches. You can workaround
| this issue by adding additional cases for each alternative. For example, the
| illegal function:
|
| def g(pair: (Int,Int)): Int = pair match {
| case (1, n) | (n, 1) => n
| case _ => 0
| }
| could be implemented by moving each alternative into a separate case:
|
| def g(pair: (Int,Int)): Int = pair match {
| case (1, n) => n
| case (n, 1) => n
| case _ => 0
| }
-----------------------------------------------------------------------------
-- [E024] Syntax Error: example.scala:2:17 -------------------------------------
2 | case (1, n) | (n, 1) => n
| ^
| Illegal variable n in pattern alternative
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| Variables are not allowed within alternate pattern matches. You can workaround
| this issue by adding additional cases for each alternative. For example, the
| illegal function:
|
| def g(pair: (Int,Int)): Int = pair match {
| case (1, n) | (n, 1) => n
| case _ => 0
| }
| could be implemented by moving each alternative into a separate case:
|
| def g(pair: (Int,Int)): Int = pair match {
| case (1, n) => n
| case (n, 1) => n
| case _ => 0
| }
-----------------------------------------------------------------------------
// Split into separate cases
def test(pair: (Int, Int)): Int = pair match
case (1, n) => n
case (n, 1) => n
case _ => 0
// Use wildcards if you don't need the value
def isEdge(pair: (Int, Int)): Boolean = pair match
case (1, _) | (_, 1) => true
case _ => false
// Use a guard for more complex conditions
def test(pair: (Int, Int)): Int = pair match
case (a, b) if a == 1 || b == 1 => if a == 1 then b else a
case _ => 0