Option-less pattern matching

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

Dotty supports a superset of scalac's extractors.

Extractors

Extractors are objects that expose a method unapply or unapplySeq:

def unapply[A](x: T)(implicit x: B): U
def unapplySeq[A](x: T)(implicit x: B): U

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

Fixed-Arity Extractors

Fixed-arity extractors expose the following signature:

def unapply[A](x: T)(implicit x: B): 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.

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 = Some[T] (for Scala2 compatibility)
• U <: R and U <: { def isEmpty: false }

Variadic Extractors

Variadic extractors expose the following signature:

def unapplySeq[A](x: T)(implicit x: B): U

The type U conforms to one of the following matches:

• sequence match
• product-sequence match

Or U conforms to the type R:

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

and S conforms to one of the two matches above.

The former form of unapplySeq has higher priority, and sequence match has higher precedence over product-sequence match.

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

• the extractor is used directly as a sequence match or product-sequence match
• U = Some[T] (for Scala2 compatibility)
• U <: R and U <: { def isEmpty: 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 (parameterless def or val) _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

• If there is exactly 1 pattern, pattern-matching on 1 pattern with type U

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

• N > 1 is the maximum number of consecutive (parameterless def or val) _1: P1 ... _N: PN members in U
• Pattern-matching on exactly N patterns with types P1, P2, ..., PN

object ProdEmpty {
  def _1: Int = ???
  def _2: String = ???
  def isEmpty = true
  def unapply(s: String): this.type = this
  def get = this
}

"" match {
  case ProdEmpty(_, _) => ???
  case _ => ()
}

Sequence Match

• U <: X, T2 and T3 conform to T1

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]
}

• 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.

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

• U <: Product
• N > 0 is the maximum number of consecutive (parameterless def or val) _1: P1 ... _N: PN members in U
• 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.

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, ns : _*) =>
  case Foo(name, x, y, ns : _*) =>
}

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