could not find implicit value for evidence parameter of type scalaz.Applicative
I'm trying to reduce this code (scalaz 7.0.x, scala 2.10.x):
type Error[+A] = /[String, A]
type Result[+A] = OptionT[Error, A]
into this:
type Result[+A] = OptionT[({ type λ[+α] = String / α })#λ, A]
And I got error "could not find implicit value for evidence parameter of type scalaz.Applicative[Main.Result]" for:
val result: Result[Int] = 42.point[Result]
Why reduced code doesn't look like the first example for scalac?
Implicit resolution of lambda types seems broken. Apparently the compiler first desugars the the type and then mismatches on the number of type parameters.
A 'simplified' example:
Defining a monad and two traits. One is similar to Either . Two is similar to EitherT
trait Monad[F[_]]
trait One[A, B]
object One {
implicit def m[A]: Monad[({ type T[x] = One[A, x] })#T] = ???
}
trait Two[F[_], A]
object Two {
implicit def m[F[_]]: Monad[({ type T[x] = Two[F, x] })#T] = ???
}
Defining a type alias and a case class to partially apply One with String as it's first parameter. The case class version can be used as a workaround.
type OneX[A] = One[String, A]
case class OneY[A](value: OneX[A])
object OneY {
implicit def m(implicit ev: Monad[OneX]): Monad[OneY] = ???
}
Implicit resolution of all 'simple' types works.
implicitly[Monad[OneX]]
implicitly[Monad[({ type T[x] = One[String, x] })#T]]
implicitly[Monad[OneY]]
Defining several type aliases that partially apply Two
type TwoX[A] = Two[OneX, A]
type TwoY[A] = Two[({ type T[x] = One[String, x] })#T, A]
type TwoZ[A] = Two[OneY, A]
Here we see that the one using the lambda type fails.
implicitly[Monad[TwoX]]
implicitly[Monad[TwoY]] // fails
implicitly[Monad[TwoZ]]
Here we see that all lambda types that use a type alias fail. Only the one that actually refers to a stable type with a single parameter succeeds.
implicitly[Monad[({ type T[x] = Two[OneX, x] })#T]] // fails
implicitly[Monad[({ type T[x] = Two[OneY, x] })#T]]
implicitly[Monad[({ type T[x] = Two[({ type T[x] = One[String, x] })#T, x] })#T]] //fails
My knowledge about the compiler is fairly limited and this might be related to the bug @TravisBrown points to.
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