Match Types
A match type reduces to one of its righthand sides, depending on the type of its scrutinee. For example:
type Elem[X] = X match
case String => Char
case Array[t] => t
case Iterable[t] => t
This defines a type that reduces as follows:
Elem[String] =:= Char
Elem[Array[Int]] =:= Int
Elem[List[Float]] =:= Float
Elem[Nil.type] =:= Nothing
Here =:=
is understood to mean that left and righthand sides are mutually subtypes of each other.
In general, a match type is of the form
S match { P1 => T1 ... Pn => Tn }
where S
, T1
, ..., Tn
are types and P1
, ..., Pn
are type patterns. Type variables in patterns start with a lower case letter, as usual.
Match types can form part of recursive type definitions. Example:
type LeafElem[X] = X match
case String => Char
case Array[t] => LeafElem[t]
case Iterable[t] => LeafElem[t]
case AnyVal => X
Recursive match type definitions can also be given an upper bound, like this:
type Concat[Xs <: Tuple, +Ys <: Tuple] <: Tuple = Xs match
case EmptyTuple => Ys
case x *: xs => x *: Concat[xs, Ys]
In this definition, every instance of Concat[A, B]
, whether reducible or not, is known to be a subtype of Tuple
. This is necessary to make the recursive invocation x *: Concat[xs, Ys]
type check, since *:
demands a Tuple
as its right operand.
Dependent Typing
Match types can be used to define dependently typed methods. For instance, here is the value level counterpart to the LeafElem
type defined above (note the use of the match type as the return type):
def leafElem[X](x: X): LeafElem[X] = x match
case x: String => x.charAt(0)
case x: Array[t] => leafElem(x(0))
case x: Iterable[t] => leafElem(x.head)
case x: AnyVal => x
This special mode of typing for match expressions is only used when the following conditions are met:
 The match expression patterns do not have guards
 The match expression scrutinee's type is a subtype of the match type scrutinee's type
 The match expression and the match type have the same number of cases
 The match expression patterns are all Typed Patterns, and these types are
=:=
to their corresponding type patterns in the match type
So you know, while the case body will be expected to have the type on the righthand side of the corresponding match type case, that doesn't imply the match type argument is constrained. Using the example, the last case body must conform to X, but that doesn't constrain X to be AnyVal, and therefore a LeafElem[X] inside the body wouldn't reduce; it would remain stuck, and as such just an abstract type.
Representation of Match Types
The internal representation of a match type
S match { P1 => T1 ... Pn => Tn }
is Match(S, C1, ..., Cn) <: B
where each case Ci
is of the form
[Xs] =>> P => T
Here, [Xs]
is a type parameter clause of the variables bound in pattern Pi
. If there are no bound type variables in a case, the type parameter clause is omitted and only the function type P => T
is kept. So each case is either a unary function type or a type lambda over a unary function type.
B
is the declared upper bound of the match type, or Any
if no such bound is given. We will leave it out in places where it does not matter for the discussion. The scrutinee, bound, and pattern types must all be firstorder types.
Match Type Reduction
Match type reduction follows the semantics of match expressions, that is, a match type of the form S match { P1 => T1 ... Pn => Tn }
reduces to Ti
if and only if s: S match { _: P1 => T1 ... _: Pn => Tn }
evaluates to a value of type Ti
for all s: S
.
The compiler implements the following reduction algorithm:
 If the scrutinee type
S
is an empty set of values (such asNothing
orString & Int
), do not reduce.  Sequentially consider each pattern
Pi
 If
S <: Pi
reduce toTi
.  Otherwise, try constructing a proof that
S
andPi
are disjoint, or, in other words, that no values
of typeS
is also of typePi
.  If such proof is found, proceed to the next case (
Pi+1
), otherwise, do not reduce.
 If
Disjointness proofs rely on the following properties of Scala types:
 Single inheritance of classes
 Final classes cannot be extended
 Constant types with distinct values are nonintersecting
 Singleton paths to distinct values are nonintersecting, such as
object
definitions or singleton enum cases.
Type parameters in patterns are minimally instantiated when computing S <: Pi
. An instantiation Is
is minimal for Xs
if all type variables in Xs
that appear covariantly and nonvariantly in Is
are as small as possible and all type variables in Xs
that appear contravariantly in Is
are as large as possible. Here, "small" and "large" are understood with respect to <:
. But, type parameter will not be "large" if a pattern containing it is matched against lambda case in co or contravariant position.
For simplicity, we have omitted constraint handling so far. The full formulation of subtyping tests describes them as a function from a constraint and a pair of types to either success and a new constraint or failure. In the context of reduction, the subtyping test S <: [Xs := Is] P
is understood to leave the bounds of all variables in the input constraint unchanged, i.e. existing variables in the constraint cannot be instantiated by matching the scrutinee against the patterns.
Subtyping Rules for Match Types
The following rules apply to match types. For simplicity, we omit environments and constraints.

The first rule is a structural comparison between two match types:
S match { P1 => T1 ... Pm => Tm } <: T match { Q1 => U1 ... Qn => Un }
if
S =:= T, m >= n, Pi =:= Qi and Ti <: Ui for i in 1..n
I.e. scrutinees and patterns must be equal and the corresponding bodies must be subtypes. No case reordering is allowed, but the subtype can have more cases than the supertype.

The second rule states that a match type and its redux are mutual subtypes.
S match { P1 => T1 ... Pn => Tn } <: U U <: S match { P1 => T1 ... Pn => Tn }
if
S match { P1 => T1 ... Pn => Tn }
reduces toU

The third rule states that a match type conforms to its upper bound:
(S match { P1 => T1 ... Pn => Tn } <: B) <: B
Termination
Match type definitions can be recursive, which means that it's possible to run into an infinite loop while reducing match types.
Since reduction is linked to subtyping, we already have a cycle detection mechanism in place. As a result, the following will already give a reasonable error message:
type L[X] = X match
case Int => L[X]
def g[X]: L[X] = ???
 val x: Int = g[Int]
 ^
Recursion limit exceeded.
Maybe there is an illegal cyclic reference?
If that's not the case, you could also try to
increase the stacksize using the Xss JVM option.
A recurring operation is (inner to outer):

 subtype LazyRef(Test.L[Int]) <:< Int
Internally, the Scala compiler detects these cycles by turning selected stack overflows into type errors. If there is a stack overflow during subtyping, the exception will be caught and turned into a compiletime error that indicates a trace of the subtype tests that caused the overflow without showing a full stack trace.
Match Types Variance
All type positions in a match type (scrutinee, patterns, bodies) are considered invariant.