Support lifting polymorphic functions to FunctionK in Scala 3 #3895
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The CI failure seems to be GH actions playing up. |
| class FunctionKLiftSuite extends CatsSuite { | ||
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| test("lift a polymorphic function directly") { | ||
| val fHeadOption = FunctionK.lift[List, Option]([X] => (_: List[X]).headOption) |
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Note that in the future the compiler should support polymorphic sam, so you'll be able to write val fHeadOption: List ~> Option = [X] => (_: List[X]).headOption (or maybe even = _.headOption)
| * val headOptionK = FunctionK.lift[List, Option]([X] => (_: List[X]).headOption) | ||
| * }}} | ||
| */ | ||
| def lift[F[_], G[_]](f: [X] => F[X] => G[X]): FunctionK[F, G] = |
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This isn't a macro is it? Why the name?
I wonder if there could be a Conversion here so you could use a literal in scala 3 without having to call lift.
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I think it's convenient to put it in this file/class, because it was already present (the Scala 2 side uses it for macros). Ideally it could be renamed something more appropriate.
FTR I personally haven't had a good experiencing using Conversion on Scala 3, as it needs a feature import to enable implicit conversions. That makes me tend to stay away.
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Maybe I'm wrong but it shouldn't need a feature import if the conversion is on the companion object.
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Thanks, I tried slapping an implicit in front of this but actually it doesn't seem to work 🤔
| def lift[F[_], G[_]](f: [X] => F[X] => G[X]): FunctionK[F, G] = | |
| implicit def lift[F[_], G[_]](f: [X] => F[X] => G[X]): FunctionK[F, G] = |
[error] -- [E007] Type Mismatch Error: /workspace/cats/tests/shared/src/test/scala-3/cats/tests/FunctionKLiftSuite.scala:40:77
[error] 40 | val fHeadOption: FunctionK[List, Option] = [X] => (_: List[X]).headOption
[error] | ^
[error] | Found: [X] => (List[X]) => Option[X]
[error] | Required: cats.arrow.FunctionK[List, Option]
I also tried using given Conversion[...] but the compiler didn't like that with our current scalacOptions.
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Yeah, it seems this just doesn't work. Maybe worth opening a dotty issue?
//> using scala "3.1.2"
//> using lib "org.typelevel::cats-core::2.7.0"
import cats.arrow.FunctionK
def lift[F[_], G[_]](f: [X] => F[X] => G[X]): FunctionK[F, G] =
new FunctionK[F, G] {
def apply[A](fa: F[A]): G[A] = f(fa)
}
given [F[_], G[_]]: Conversion[[X] => F[X] => G[X], FunctionK[F, G]] = lift(_)
@main def main =
val fHeadOption: FunctionK[List, Option] = [X] => (_: List[X]).headOption
val a = List(1, 2, 3)
assert(fHeadOption(a) == a.headOption)[error] ./functionk.scala:14:76: Found: [X] => (List[X]) => Option[X]
[error] Required: cats.arrow.FunctionK[List, Option]
[error] val fHeadOption: FunctionK[List, Option] = [X] => (_: List[X]).headOption
[error]
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I think just type inference fails here:
scala> summon[Conversion[[X] => List[X] => Option[X], FunctionK[List, Option]]](using given_Conversion_X_FunctionK)
-- [E007] Type Mismatch Error: -------------------------------------------------
1 |summon[Conversion[[X] => List[X] => Option[X], FunctionK[List, Option]]](using given_Conversion_X_FunctionK)
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|Found: Conversion[[X] => (Nothing) => Any, FunctionK[F, G]]
|Required: Conversion[[X] => (List[X]) => Option[X], FunctionK[List, Option]]
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|where: F is a type variable with constraint <: [_] =>> Any
| G is a type variable with constraint <: [_] =>> Any
longer explanation available when compiling with `-explain`
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yeah, this seems like maybe it is a bug that could be filed with scala.
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I opened scala/scala3#15276.
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@bplommer do you mind fixing this up? My efforts to revive it have fallen short. |
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