How to do the equivalent of T[Any] for higher kinded types in scala-CodePudding

context: this is using scala-3

final case class TableSchema[T[_[_]] <: Table]( /* ... */ )

// None of these work...
final case class Schema(tableSchemas: List[TableSchema[_ <: Table]])
final case class Schema(tableSchemas: List[[t[_[_]] <: Table] => TableSchema[t]])
final case class Schema(tableSchemas: List[[t[_[_]] <: Table] =>> TableSchema[t]])
final case class Schema(tableSchemas: List[TableSchema[_[_[_]] <: Table]]) // isnt even valid syntax?

CodePudding user response：

I guess, correct is

final case class Schema(tableSchemas: List[TableSchema[_ <: ([_[_]] =>> Table)]])

or without extra brackets

final case class Schema(tableSchemas: List[TableSchema[_ <: [_[_]] =>> Table]])

i.e. unknown argument _ of existential type TableSchema[_] is a subtype of the constant type Table.

A quote from Type Lambdas - More Details / Relationship with Parameterized Type Definitions:

A parameterized abstract type

type T[X] >: L <: U

is regarded as shorthand for an unparameterized abstract type with type lambdas as bounds.

type T >: ([X] =>> L) <: ([X] =>> U)

Tested in Scala 3.1.3.