I am trying to learn how to nest monads using MonadTransformers in cats library.
So I am trying to build a data type for Either[String, Option[A]]
This is the code which I have written
import cats.data.OptionT
import cats.instances.list._
import cats.syntax.applicative._
object Ex11 extends App {
type ErrorEither[A] = Either[String, A]
type ErrorOrOption[A] = OptionT[ErrorEither, A]
val x = 42.pure[ErrorOrOption]
println(x)
}
But I get an error
[error] Ex11.scala:13: could not find implicit value for parameter F: cats.Applicative[Ex11.ErrorOrOption]
[error] val x = 42.pure[ErrorOrOption]
[error] ^
I took this from a sample which was using Xor but I guess the latest cats library removed Xor in favor of Either.
I suspect that you are missing an import of cats instance:
import cats.instances.either._
(Also the import of import cats.instances.list._ seems to be superfluous here.)
The following should compile:
import cats.data.OptionT
import cats.instances.either._
import cats.syntax.applicative._
object Ex11 extends App {
type ErrorEither[A] = Either[String, A]
type ErrorOrOption[A] = OptionT[ErrorEither, A]
val x = 42.pure[ErrorOrOption]
println(x)
}
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