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

```
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 Scala 2 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 conditions holds:

- the extractor is used directly as a sequence match or product-sequence match
`U = Some[T]`

(for Scala 2 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, 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