问题
In Dotty given the following:
object Domain {
final case class Create(name: String) extends BaseCreate[Create] {
override type Model = Domain
override def service[F[_]](client: KeystoneClient[F]): CrudService[F, Domain, Create] = client.domains
}
}
case class Domain(id: String)
class CrudService[F[_], Model, Create]
final class Domains[F[_]] extends CrudService[F, Domain, Domain.Create]
class KeystoneClient[F[_]] {
val domains = new Domains[F]
}
trait BaseCreate[Create <: BaseCreate[Create]] {
type Model
def service[F[_]](client: KeystoneClient[F]): CrudService[F, Model, Create]
}
I want to "simplify" BaseCreate
such that I can implement Create
like this:
final case class Create(name: String) extends BaseCreate.Aux[Domain, Create](_.domains)
I've tried:
object BaseCreate {
abstract class Aux[M, Create](f: [F[_]] =>> KeystoneClient[F] => CrudService[F, M, Create]) extends BaseCreate[Create] {
type Model = M
def service[F[_]](client: KeystoneClient[F]): CrudService[F, Model, Create] = f(client)
}
}
But the compiler complains with:
Missing type parameter for [F[_$5]] =>> KeystoneClient[F] => CrudService[F, M, Create]
Which doesn't make sense to me. Whats the right syntax, if there is one?
Link to scastie with this code
PS: I don't want to introduce any more type parameters to BaseCreate
. Especially the F
since that would mean the class Domain.Create
would need to be final case class Create[F[_]]
which is not at all the intended solution.
回答1:
I guess you confused type lambdas with polymorphic functions.
A type lambda maps types to types, a polymorhic function maps values to values just type of its argument can vary.
For type lambdas you should use =>>
, for polymorhic functions you should use ordinary =>
(twice i.e. [A] => (a: A) => f(a)
).
So it's supposed to be
object BaseCreate {
abstract class Aux[M, Create](f: [F[_]] => KeystoneClient[F] => CrudService[F, M, Create]) extends BaseCreate[Create] {
type Model = M
def service[F[_]](client: KeystoneClient[F]): CrudService[F, Model, Create] = f(client)
}
}
final case class Create(name: String) extends BaseCreate.Aux[Domain, Create](
[F[_]] => (client: KeystoneClient[F]) => client.domains
)
but it doesn't compile in Dotty 0.28.0-bin-20200920-e99793e-NIGHTLY with error
Found: Object with PolyFunction {...}
Required: PolyFunction{
apply:
[F[_$6]]
(x$1: App.KeystoneClient[F]):
App.CrudService[F, App.Domain, App.Create]
}
[F[_]] => (client: KeystoneClient[F]) => client.domains
Seems to be a bug.
来源:https://stackoverflow.com/questions/63994512/type-lambda-with-higher-kind