问题
Let's suppose I have a trait with two type parameters, e.g.
trait Qux[A, B]
and another trait with a higher-kinded type parameter, e.g.
trait Turkle[C[_]]
I'd like to be able to substitute a fixed value for one of the type parameters for Qux
, so that it can be used to parametrize Turkle
.
Here's an example (of code that doesn't make sense in Scala!):
trait Baz[A] extends Turkle[Qux[A, _]]
Anyone have any ideas how to achieve this effect?
回答1:
Jason Zaugg came up with the most succinct way to do this:
trait Baz[A] extends Turkle[({type x[a]=Qux[A, a]})#x]
IntelliJ's Scala plugin will optionally collapse this to:
trait Baz[A] extends Turkle[x[a]=Qux[A, a]]
回答2:
You mean something like this?
trait QuxWithString[A] extends Qux[A, String]
new Turkle[QuxWithString]{}
This is the analog to partial application for types.
回答3:
trait Turkle[C[_]]
trait Qux[A,B]
trait Wraps[A] {
type Jkz[X] = Qux[A,X]
trait Baz extends Turkle[Jkz]
}
回答4:
The compiler plugin kind projector allows this as well:
// Explicit lambda, greek letters
trait Baz[A] extends Turkle[λ[α=>Qux[A,α]]]
// Explicit lambda, normal letters
trait Baz[A] extends Turkle[Lambda[a=>Qux[A,a]]]
// No explicit lambda, ? placeholder
trait Baz[A] extends Turkle[Qux[A,?]]
来源:https://stackoverflow.com/questions/6247817/is-it-possible-to-curry-higher-kinded-types-in-scala