Scala compiler plugin for making type lambdas and polymorphic lambdas easier to write
Scala compiler plugin for making type lambdas (type projections) and polymorphic lambdas easier to write https://github.com/typelevel/kind-projector
Questions tagged [kind-projector]
27 questions
24
votes
5 answers
Is it possible to "curry" higher-kinded types in Scala?
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…
Scott Morrison
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7
votes
1 answer
Partially applied type lambda in Scala with kind projector
Consider the following type definition:
trait LiftF[F[_], G[_]] {
def liftF[A](fa: F[A]): G[A]
}
When providing a requirement for an implicit of this type in context bounds (using kind projector plugin) we have to write it like this:
def func[A,…
a7emenov
- 162
- 8
7
votes
2 answers
Functor Instance for Type Constructor with Two Parameters in Scala
I have a class Foo with two parameters, and I am trying to write a Functor instance for Foo with the first parameter fixed, as follows:
object Scratchpad {
trait Functor[F[_]] {
def fmap[A, B](f: A => B): F[A] => F[B]
}
case class Foo[X,…
Fried Brice
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7
votes
2 answers
What is a kind projector
I've been digging into FP and everything that surrounds it, and I found the concept of kind projector written somewhere, without details nor explanations.
The only thing I found was this github project, and I'm starting to think if it was referring…
pedrorijo91
- 7,635
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6
votes
1 answer
Understanding Type Projection
Taken from typelevel/kind-projector, what's the distinction between:
// partially-applied type named "IntOrA"
type IntOrA[A] = Either[Int, A]
and
// type projection implementing the same type anonymously (without a name).
({type L[A] = Either[Int,…
Kevin Meredith
- 41,036
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5
votes
2 answers
Kind compiler plugin λ not found
I have enabled the kind compiler plugin addCompilerPlugin("org.spire-math" % "kind-projector" % "0.9.6") and I can now use the ? symbol e.g. Map[String, ?] however Lambda and λ are not resolved.
val f: Id ~> Future = λ[Id ~> Future](...)
produces…
user404345
3
votes
1 answer
Difference between * (star) and _ (underscore) in type parameter
Here someone says that star is underscore from scala 3, but I've seen some code like this in scala 2.13:
def make[F[_]: ContextShift: MonadError[*[_], Throwable]: Effect: Logging](): ...
Does it have a same meaning and just specify that type in *…
ibanezn04
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3
votes
1 answer
kind-projector returns strange results
I have these types:
SomeTypeClass
A higher kinded type which has one type parameter of kind * => * => *
trait SomeTypeClass[P[_, _]] {
def test[F[_], S, T, A, B](f: (A => F[B]) => S => F[T])
(pab: P[A, B])
…
Sagi
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- 3
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3
votes
1 answer
scala generic function `not found: type ?`
When I use scala to create a function like this,It told me that not found: type ?
scala
def save[
K: SpatialComponent: TypeTag,
V <: CellGrid: TypeTag: ? => TileMergeMethods[V]: ? => TilePrototypeMethods[V]
](id: LayerId, rdd:…
wsf1990
- 195
- 2
- 12
3
votes
2 answers
How do I translate this type lambda into Kind-Projector syntax?
Given type parameters of F[_] and A[_] how do I turn the following type lambda into the more pleasant Kind-Projector syntax?
({type λ[α] = F[A[α]]})#λ
I would have imagined it would be something like F[A[?_]], but the compiler complains about…
Noel M
- 15,812
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3
votes
1 answer
Extend NaturalTransformation of Coproducts
I have
F ~> H
G ~> H
Where ~> is cats.NaturalTransformation.
I'm able to construct a
λ[A => F[A] :+: G[A] :+: CNil] ~> H
Using the kind-projector syntax for readability
Here's how I'm doing it
def or[G[_]](g: G ~> H): λ[A => F[A] :+: G[A] :+:…
Gabriele Petronella
- 106,943
- 21
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3
votes
1 answer
What does λ[α =>F] mean?
I'm learning Scalaz recently. I would like to know how λ[α =>F] works?
scala> Applicative[λ[α => Int]].point(10)
res45: Int = 0
scala> Applicative[λ[α => String]].point(10)
res46: String = ""
I can understand λ means some type here, but I could…
Binzi Cao
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2
votes
1 answer
Polymorphic method works with type lambda, but not with type wildcard in Scala 3
In Scala 3 I can define a functor for state using type lambda:
given stateFunctor[S]: Functor[[A] =>> State[S, A]] with
override def map[A, B](a: State[S, A])(fx: A => B): State[S, B] = State(a.run.andThen { case (s, a) => (s, fx(a)) })
I…
Andronicus
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2
votes
1 answer
Clarifying cross version behavior in Scala
I wonder what is the difference between the two here:
addCompilerPlugin("org.typelevel" %% "kind-projector" % "0.11.3" cross CrossVersion.full)
// if your project uses multiple Scala versions, use this for cross…
MaatDeamon
- 9,532
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2
votes
1 answer
Generalized constraints within type parameter clause?
SLS specifies syntax of type parameter clause as
TypeParamClause ::= ‘[’ VariantTypeParam {‘,’ VariantTypeParam} ‘]’
FunTypeParamClause::= ‘[’ TypeParam {‘,’ TypeParam} ‘]’
VariantTypeParam ::= {Annotation} [‘+’ | ‘-’] TypeParam
TypeParam …
Mario Galic
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