Need to specify type explicitly when using match type with enum - Scala 3

user_12835890 0
Viewed 57

I'm trying to understand why I need to specify the specific enum type I'm using to be able to use the "match type" feature of scala 3.

val doesntCompile: String = testTypeMatch(FieldType.Text)
val compile: String = testTypeMatch[FieldType.Text.type](FieldType.Text)

def testTypeMatch[T <: FieldType](fieldType: T): Elem[T] =
  fieldType match
    case _: FieldType.Text.type   => "randomString"
    case _: FieldType.Number.type => 55

type Elem[x <: FieldType] = x match
  case FieldType.Text.type   => String
  case FieldType.Number.type => Int

enum FieldType {
  case Text
  case Number
}

Without hardcoding [FieldType.Text.type] I'm getting a compilation error.

I've tried the same with sealed trait and it worked fine, specifying the type was not needed

sealed trait FieldType
object FieldType{
  case object Text extends FieldType
  case object Number extends FieldType
}
match-types scala-3 enums scala types
1 Answers

The inferred type of FieldType.Text is FieldType, not FieldType.Text.type.

Analogously, for the enum

enum Foo:
  case Bar

the expression Foo.Bar will have the type Foo by default (try it out in the repl), similarly to how "hello" has the type String and not the singleton type "hello".

Demoting the FieldType to a marker trait solves the issue:

val compilesAsExpected: String = testTypeMatch(Text)

def testTypeMatch[T <: FieldType](fieldType: T): Elem[T] =
  fieldType match
    case _: Text.type   => "randomString"
    case _: Number.type => 55

type Elem[x <: FieldType] = x match
  case Text.type   => String
  case Number.type => Int

sealed trait FieldType
object Text extends FieldType
object Number extends FieldType

I must admit that I'm not so sure whether this is the most idiomatic way to do it, though; The enum keyword was supposed to replace sealed traits, but this seems to be one of those cases where sealed traits behave quite differently from the enums.

user_27077920
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