Is it possible to "curry" higher-kinded types in Scala?

Question

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?

This compiles, although I have absolutely no idea if this is what you want: `type QuxApplied[A] = Qux[A, _]; trait Baz[A] extends Turkle[QuxApplied]` — huynhjl, Jun 06 '11 at 05:08
@huynhjl - My guess is that it is supposed to be the same `A`, which is not the case in your example. — Rex Kerr, Jun 06 '11 at 05:15

score 24 · Accepted Answer · answered Jun 06 '11 at 06:02

24

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]]

answered Jun 06 '11 at 06:02

Lachlan

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Thanks. It's nice to have an "inline" solution. – Scott Morrison Jun 09 '11 at 00:48

Kim Stebel · Answer 2 · 2011-06-06T05:26:17.853

4

You mean something like this?

trait QuxWithString[A] extends Qux[A, String]
new Turkle[QuxWithString]{}

This is the analog to partial application for types.

edited Jun 06 '11 at 05:26

answered Jun 06 '11 at 05:19

Kim Stebel

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score 2 · Answer 3 · answered Jun 06 '11 at 05:09

2

trait Turkle[C[_]]
trait Qux[A,B]
trait Wraps[A] {
  type Jkz[X] = Qux[A,X]
  trait Baz extends Turkle[Jkz]
}

answered Jun 06 '11 at 05:09

Rex Kerr

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This is actually pretty good. In my use cases it's not so unusual to have nested traits anyway, so there's little feeling of writing boilerplate. – Scott Morrison Jun 09 '11 at 00:49

score 1 · Answer 4 · answered Aug 06 '15 at 07:38

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,?]]

Mario Galic · Answer 5 · 2021-01-23T13:35:32.470

1

Scala 3 is proposing SIP: Underscore Syntax for Type Lambdas #5379 such that the following might be possible

trait Baz[A] extends Turkle[Qux[A, _]]

Scala 3 natively provides type lambdas using [X] =>> F[X] syntax so try

trait Baz[A] extends Turkle[[B] =>> Qux[A, B]]

edited Jan 23 '21 at 13:35

answered Apr 05 '20 at 14:30

Mario Galic

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Is it possible to "curry" higher-kinded types in Scala?

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