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Scala 3 Language Reference

Vararg Splices

Language

The syntax of vararg splices in patterns and function arguments has changed. The new syntax uses a postfix *, analogously to how a vararg parameter is declared.

val arr = Array(0, 1, 2, 3)
val lst = List(arr*)                   // vararg splice argument
lst match
  case List(0, 1, xs*) => println(xs)  // binds xs to Seq(2, 3)
  case List(1, _*) =>                  // wildcard pattern

The old syntax for splice arguments will be phased out.

/*!*/ val lst = List(arr: _*)      // syntax error
      lst match
        case List(0, 1, xs @ _*)  // ok, equivalent to `xs*`

Syntax

ArgumentPatterns  ::=  ‘(’ [Patterns] ‘)’
                    |  ‘(’ [Patterns ‘,’] Pattern2 ‘*’ ‘)’

ParArgumentExprs  ::=  ‘(’ [‘using’] ExprsInParens ‘)’
                    |  ‘(’ [ExprsInParens ‘,’] PostfixExpr ‘*’ ‘)’

Compatibility considerations

To enable cross compilation between Scala 2 and Scala 3, the compiler will accept both the old and the new syntax. Under the -source future setting, an error will be emitted when the old syntax is encountered. An automatic rewrite from old to new syntax is offered under -source future-migration.

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