Typeclass derivation doesn't work for type literals

[keuhdall]

Hi, I've been trying to use kittens to derive Show on one of my case class, however it seems to fail because of the type literals.

My case class looks like this:

case class BearerToken(
    access_token: String,
    token_type: "bearer",
    expires_in: Long,
    refresh_token: String,
    scope: "public",
    created_at: Long
) derives Show

When compiling, I get the following error message:

[error]    |No given instance of type cats.derived.DerivedShow[types.BearerToken] was found.
[error]    |I found:
[error]    |
[error]    |    cats.derived.DerivedShow.given_DerivedShow_A[types.BearerToken](
[error]    |      {
[error]    |        val gen$proxy1:
[error]    |
[error]    |            deriving.Mirror.Product{
[error]    |              type Kind = shapeless3.deriving².K0.type;
[error]    |                type MirroredType = types.BearerToken;
[error]    |                type MirroredMonoType = types.BearerToken;
[error]    |                type MirroredElemTypes <: Tuple
[error]    |            }
[error]    |           &
[error]    |            scala.deriving.Mirror.Product{
[error]    |              type MirroredMonoType = types.BearerToken;
[error]    |                type MirroredType = types.BearerToken;
[error]    |                type MirroredLabel = ("BearerToken" : String);
[error]    |                type MirroredElemTypes = (String, ("bearer" : String), Long,
[error]    |                  String, ("public" : String), Long);
[error]    |                type MirroredElemLabels = (("access_token" : String),
[error]    |                  ("token_type" : String), ("expires_in" : String),
[error]    |                  ("refresh_token" : String), ("scope" : String),
[error]    |                  ("created_at" : String))
[error]    |            }
[error]    |
[error]    |         =
[error]    |          types.BearerToken.$asInstanceOf[
[error]    |
[error]    |              deriving.Mirror.Product{
[error]    |                type Kind = shapeless3.deriving².K0.type;
[error]    |                  type MirroredType = types.BearerToken;
[error]    |                  type MirroredMonoType = types.BearerToken;
[error]    |                  type MirroredElemTypes <: Tuple
[error]    |              }
[error]    |             &
[error]    |              scala.deriving.Mirror.Product{
[error]    |                type MirroredMonoType = types.BearerToken;
[error]    |                  type MirroredType = types.BearerToken;
[error]    |                  type MirroredLabel = ("BearerToken" : String);
[error]    |                  type MirroredElemTypes = (String, ("bearer" : String), Long,
[error]    |                    String, ("public" : String), Long);
[error]    |                  type MirroredElemLabels = (("access_token" : String),
[error]    |                    ("token_type" : String), ("expires_in" : String),
[error]    |                    ("refresh_token" : String), ("scope" : String),
[error]    |                    ("created_at" : String))
[error]    |              }
[error]    |
[error]    |          ]
[error]    |        shapeless3.deriving².internals.ErasedProductInstancesN.apply[
[error]    |          shapeless3.deriving².K0.type,
[error]    |          cats.derived.Derived.Or[cats.Show[types.BearerToken]]](gen$proxy1,
[error]    |          {
[error]    |            val arr$proxy1: Array[Any] = new Array[Any](6)
[error]    |            {
[error]    |              arr$proxy1.update(0,
[error]    |                {
[error]    |                  given val instance: cats.Show[String] =
[error]    |                    cats.implicits.catsStdShowForString
[error]    |                  cats.derived.Derived.Or.apply²[cats.Show[String]](instance)
[error]    |                }:cats.derived.Derived.Or[cats.Show[String]]
[error]    |              )
[error]    |              {
[error]    |                arr$proxy1.update(1,
[error]    |                  compiletime.summonInline[
[error]    |                    cats.derived.Derived.Or[cats.Show[("bearer" : String)]]]
[error]    |                )
[error]    |                shapeless3.deriving².summonAsArray0[
[error]    |                  (cats.derived.Derived.Or[cats.Show[Long]],
[error]    |                    cats.derived.Derived.Or[cats.Show[String]],
[error]    |                    cats.derived.Derived.Or[cats.Show[("public" : String)]],
[error]    |                    cats.derived.Derived.Or[cats.Show[Long]])
[error]    |                ](2, arr$proxy1)
[error]    |              }:Array[Any]
[error]    |            }:Array[Any]
[error]    |          }:Array[Any]
[error]    |        ):
[error]    |          shapeless3.deriving².internals.ErasedProductInstances[
[error]    |            shapeless3.deriving².K0.type,
[error]    |            cats.derived.Derived.Or[cats.Show[types.BearerToken]]]
[error]    |        :
[error]    |          shapeless3.deriving².K0.ProductInstances[
[error]    |            [A] =>> cats.derived.Derived.Or[cats.Show[A]], types.BearerToken]
[error]    |      },
[error]    |    ???)
[error]    |
[error]    |But given instance mkProductInstances in object K0 does not match type shapeless3.deriving².K0.ProductInstances[cats.derived.DerivedShow.Or²,
[error]    |  types.BearerToken]
[error]    |
[error]    |where:    Or        is a type in object Derived with bounds <: [A] =>> Any
[error]    |          Or²       is a type in object DerivedShow which is an alias of [A] =>> cats.derived.Derived.Or[cats.Show[A]]
[error]    |          apply     is a method in object ErasedProductInstancesN
[error]    |          apply²    is a method in object Or
[error]    |          deriving  is a package in package scala
[error]    |          deriving² is a package in package shapeless3
[error]    |.
[error]    |----------------------------------------------------------------------------
[error]    |Inline stack trace
[error]    |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
[error]    |This location contains code that was inlined from DerivedShow.scala:20
[error]    |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
[error]    |This location contains code that was inlined from DerivedShow.scala:20
[error]     ----------------------------------------------------------------------------

However, when switching types of token_type and scope back to String, everything works just fine.

I'm using kittens 3.0.0 and Scala 3.3.0

[joroKr21]

Hi, it looks like there was an oversight while implementing Show - we should be using ContravariantShow. In the meantime you can work around this problem by defining given [A <: String]: Show[A] = _.toString.

[keuhdall]

The fix works indeed, thanks ! Out of curiosity I tried deriving Eq, and I get the same issue, so it affects probably other typeclasses too.

[joroKr21]

Out of curiosity I tried deriving Eq, and I get the same issue, so it affects probably other typeclasses too.

True, but there is no ContravariantEq typeclass. Now that I thought of it some more maybe this should be fixed in Cats proper by adding those instances 🤔 - about Eq it could even just return true for singleton types.

[keuhdall]

Sorry for the late reply, should I open an issue on Cats related to this one ?

[joroKr21]

Sorry for the late reply, should I open an issue on Cats related to this one ?

That would be helpful, yes 👍

[keuhdall]

Will do ! I took some time to test most of the typeclasses supported by kittens (I used the same case class as above for my tests), here's a recap:

Typeclass	Status
Eq	❌
PartialOrder	❌
Order	❌
Hash	❌
Show	❌
Semigroup	❌
CommutativeSemigroup	❌
Monoid	❌
CommutativeMonoid	❌
Functor	✅
Contravariant	✅
Invariant	✅
Pure *	❌
Apply	❌
Applicative	❌
Foldable	✅
Reducible	❌
Traverse	✅
Empty *	❌
NonEmptyTraverse	❌

[joroKr21]

Here's my take on it:

Typeclass	Status
Eq	We can probably implement it here
PartialOrder	We can probably implement it here
Order	We can probably implement it here
Hash	Theoretically it should work, but it's tricky
Show	We can use ContravariantShow
Semigroup	Can't work - how do you combine singletons?
CommutativeSemigroup	Same as Semigroup
Monoid	Same as Semigroup
CommutativeMonoid	Same as Semigroup
Functor	✅
Contravariant	✅
Invariant	✅
Pure *	We can probably make it work
Apply	Can't work because no Semigroup
Applicative	Can't work because no Semigroup
Foldable	✅
Reducible	That's surprising, it should work
Traverse	✅
Empty *	I don't think we should allow this
NonEmptyTraverse	That's surprising, it should work

[joroKr21]

@keuhdall I've implemented my suggestion - lmk what you think