Option-less pattern matching

The implementation of pattern matching in Scala 3 was greatly simplified compared to Scala 2. From a user perspective, this means that Scala 3 generated patterns are a lot easier to debug, as variables all show up in debug modes and positions are correctly preserved.

Scala 3 supports a superset of Scala 2 extractors.

Extractors

Extractors are objects that expose a method unapply or unapplySeq:

def unapply(x: T): U
def unapplySeq(x: T): U

Where T is an arbitrary type, if it is a subtype of the scrutinee's type Scrut, a type test is performed before calling the method. U follows rules described in Fixed Arity Extractors and Variadic Extractors.

Note: U can be the type of the extractor object.

unapply and unapplySeq can actually have a more general signature, allowing for a leading type clause, as well as arbitrarily many using clauses, both before and after the regular term clause, and at most one implicit clause at the end, for example:

def unapply[A, B](using C)(using D)(x: T)(using E)(using F)(implicit y: G): U = ???

Extractors that expose the method unapply are called fixed-arity extractors, which work with patterns of fixed arity. Extractors that expose the method unapplySeq are called variadic extractors, which enables variadic patterns.

Fixed-Arity Extractors

Fixed-arity extractors expose the following signature (with potential type, using and implicit clauses):

def unapply(x: T): U

The type U conforms to one of the following matches:

• Boolean match
• Product match

Or U conforms to the type R:

type R = {
  def isEmpty: Boolean
  def get: S
}

and S conforms to one of the following matches:

• single match
• name-based match

The former form of unapply has higher precedence, and single match has higher precedence over name-based match.

Note: the S in R can be U.

A usage of a fixed-arity extractor is irrefutable if one of the following condition holds:

• U = true
• the extractor is used as a product match
• U <: R and U <: { def isEmpty: false }
• U = Some[T]

Note: The last rule is necessary because, for compatibility reasons, isEmpty on Some has return type Boolean rather than false, even though it always returns false.

Boolean Match

• U =:= Boolean
• Pattern-matching on exactly 0 patterns

For example:

object Even:
  def unapply(s: String): Boolean = s.size % 2 == 0

"even" match
  case s @ Even() => println(s"$s has an even number of characters")
  case s          => println(s"$s has an odd number of characters")

// even has an even number of characters

Product Match

• U <: Product
• N > 0 is the maximum number of consecutive (val or parameterless def) _1: P1 ... _N: PN members in U
• Pattern-matching on exactly N patterns with types P1, P2, ..., PN

For example:

class FirstChars(s: String) extends Product:
  def _1 = s.charAt(0)
  def _2 = s.charAt(1)

   // Not used by pattern matching: Product is only used as a marker trait.
  def canEqual(that: Any): Boolean = ???
  def productArity: Int = ???
  def productElement(n: Int): Any = ???

object FirstChars:
  def unapply(s: String): FirstChars = new FirstChars(s)

"Hi!" match
  case FirstChars(char1, char2) =>
    println(s"First: $char1; Second: $char2")

// First: H; Second: i

Single Match

• Pattern-matching on 1 pattern with type S

For example, where Nat <: R, S = Int:

class Nat(val x: Int):
  def get: Int = x
  def isEmpty = x < 0

object Nat:
  def unapply(x: Int): Nat = new Nat(x)

5 match
  case Nat(n) => println(s"$n is a natural number")
  case _      => ()

// 5 is a natural number

Name-based Match

• S has N > 1 members such that they are each vals or parameterless defs, and named from _1 with type P1 to _N with type PN
• S doesn't have N+1 members satisfying the previous point, i.e. N is maximal
• Pattern-matching on exactly N patterns with types P1, P2, ..., PN

For example, where U = AlwaysEmpty.type <: R, S = NameBased:

object MyPatternMatcher:
  def unapply(s: String) = AlwaysEmpty

object AlwaysEmpty:
  def isEmpty = true
  def get = NameBased

object NameBased:
  def _1: Int = ???
  def _2: String = ???

"" match
  case MyPatternMatcher(_, _) => ???
  case _ => ()

Variadic Extractors

Variadic extractors expose the following signature (with potential type, using and implicit clauses):

def unapplySeq(x: T): U

Where U has to fullfill the following:

1. Set V := U
2. V is valid if V conforms to one of the following matches:

• sequence match
• product-sequence match

3. Otherwise U has to conform to the type R:

type R = {
  def isEmpty: Boolean
  def get: S
}

4. Set V := S, and reattempt 2., if it fails U is not valid.

The V := U form of unapplySeq has higher priority, and sequence match has higher precedence over product-sequence match.

Note: This means isEmpty is disregarded if the V := U form is valid

A usage of a variadic extractor is irrefutable if one of the following conditions holds:

• the extractor is used directly as a sequence match or product-sequence match
• U <: R and U <: { def isEmpty: false }
• U = Some[T]

Note: The last rule is necessary because, for compatibility reasons, isEmpty on Some has return type Boolean rather than false, even though it always returns false.

Note: Be careful, by the first condition and the note above, it is possible to define an irrefutable extractor with a def isEmpty: true. This is strongly discouraged and, if found in the wild, is almost certainly a bug.

Sequence Match

• V <: X

type X = {
  def lengthCompare(len: Int): Int // or, `def length: Int`
  def apply(i: Int): T1
  def drop(n: Int): scala.Seq[T2]
  def toSeq: scala.Seq[T3]
}

• T2 and T3 conform to T1
• Pattern-matching on exactly N simple patterns with types T1, T1, ..., T1, where N is the runtime size of the sequence, or
• Pattern-matching on >= N simple patterns and a vararg pattern (e.g., xs: _*) with types T1, T1, ..., T1, Seq[T1], where N is the minimum size of the sequence.

For example, where V = S, U = Option[S] <: R, S = Seq[Char]

object CharList:
  def unapplySeq(s: String): Option[Seq[Char]] = Some(s.toList)

"example" match
  case CharList(c1, c2, c3, c4, _, _, _) =>
    println(s"$c1,$c2,$c3,$c4")
  case _ =>
    println("Expected *exactly* 7 characters!")

// e,x,a,m

Product-Sequence Match

• V <: Product
• N > 0 is the maximum number of consecutive (val or parameterless def) _1: P1 ... _N: PN members in V
• PN conforms to the signature X defined in Seq Pattern
• Pattern-matching on exactly >= N patterns, the first N - 1 patterns have types P1, P2, ... P(N-1), the type of the remaining patterns are determined as in Seq Pattern.

For example, where V = S, U = Option[S] <: R, S = (String, PN) <: Product, PN = Seq[Int]

class Foo(val name: String, val children: Int*)
object Foo:
  def unapplySeq(f: Foo): Option[(String, Seq[Int])] =
    Some((f.name, f.children))

def foo(f: Foo) = f match
  case Foo(name, x, y, ns*) => ">= two children."
  case Foo(name, ns*)       => "< two children."

There are plans for further simplification, in particular to factor out product match and name-based match into a single type of extractor.

Type testing

Abstract type testing with ClassTag is replaced with TypeTest or the alias Typeable.

• pattern _: X for an abstract type requires a TypeTest in scope
• pattern x @ X() for an unapply that takes an abstract type requires a TypeTest in scope

More details on TypeTest