implicit

问题 I was reading about type classes where implicit objects were mentioned: object Math { trait NumberLike[T] { def plus(x: T, y: T): T def divide(x: T, y: Int): T def minus(x: T, y: T): T } object NumberLike { implicit object NumberLikeDouble extends NumberLike[Double] { def plus(x: Double, y: Double): Double = x + y def divide(x: Double, y: Int): Double = x / y def minus(x: Double, y: Double): Double = x - y } implicit object NumberLikeInt extends NumberLike[Int] { def plus(x: Int, y: Int): Int

问题 I was reading about type classes where implicit objects were mentioned: object Math { trait NumberLike[T] { def plus(x: T, y: T): T def divide(x: T, y: Int): T def minus(x: T, y: T): T } object NumberLike { implicit object NumberLikeDouble extends NumberLike[Double] { def plus(x: Double, y: Double): Double = x + y def divide(x: Double, y: Int): Double = x / y def minus(x: Double, y: Double): Double = x - y } implicit object NumberLikeInt extends NumberLike[Int] { def plus(x: Int, y: Int): Int

问题 I have a method with implicits: def f(x: String)(implicit dispatcher: ExecutionContextExecutor, mat: ActorMaterializer) = ??? And I want to create a helper method like: def g1(y: String) = f("uri1" + y) def g2(y: String) = f("uri2" + y) Of course, this cannot compile as no implicits found for parameter ex: ExecutionContext for method g . I don't want to repeat the implicits declaration in g . So, what's the idiomatic solution for this case? 回答1: Idiomatic solution is to repeat implicit

问题 I am trying to diagnose a compiler error which is caused by diverging implicit expansion errors. Here is what I am seeing: ❯ scala Welcome to Scala 2.13.1 (OpenJDK 64-Bit Server VM, Java 1.8.0_252). Type in expressions for evaluation. Or try :help. scala> :settings -Xlog-implicits scala> :paste // Entering paste mode (ctrl-D to finish) trait Invariant[A] trait Covariant[+A] trait Foo implicit def invariantTuple1[T1: Invariant]: Invariant[Tuple1[T1]] = ??? implicit def invariantFoo: Invariant

问题 I am trying to diagnose a compiler error which is caused by diverging implicit expansion errors. Here is what I am seeing: ❯ scala Welcome to Scala 2.13.1 (OpenJDK 64-Bit Server VM, Java 1.8.0_252). Type in expressions for evaluation. Or try :help. scala> :settings -Xlog-implicits scala> :paste // Entering paste mode (ctrl-D to finish) trait Invariant[A] trait Covariant[+A] trait Foo implicit def invariantTuple1[T1: Invariant]: Invariant[Tuple1[T1]] = ??? implicit def invariantFoo: Invariant

问题 The following code compiles: class X[U, T <: U] object X { implicit def genericX[U, T <: U]: X[U, T] = new X[U, T] } implicitly[X[String, String]] // compiles When we make T covariant ( class X[U, +T <: U] ) it compile as well: class X[U, +T <: U] object X { implicit def genericX[U, T <: U]: X[U, T] = new X[U, T] } implicitly[X[String, String]] // compiles When we make T contravariant ( class X[U, -T <: U] ), compiler fails to materilize implicitly[X[String, String]] . Strangely enough, it is

问题 I'm trying to implement the Absurd typeclass (as seen in Haskell's Data.Boring library) in Scala. I'm able to define an Absurd instance for Nothing . Unfortunately, when I try to define an absurd instance for Either , I get a missing implicit error sealed trait Absurd[A] { def absurd[X](a: A): X } object Absurd { def apply[A: Absurd, B](a: A):B = implicitly[Absurd[A]].absurd[B](a) implicit val absurdForNothing: Absurd[Nothing] = new Absurd[Nothing]{ override def absurd[X](a: Nothing): X = a }

问题 I'm trying to implement the Absurd typeclass (as seen in Haskell's Data.Boring library) in Scala. I'm able to define an Absurd instance for Nothing . Unfortunately, when I try to define an absurd instance for Either , I get a missing implicit error sealed trait Absurd[A] { def absurd[X](a: A): X } object Absurd { def apply[A: Absurd, B](a: A):B = implicitly[Absurd[A]].absurd[B](a) implicit val absurdForNothing: Absurd[Nothing] = new Absurd[Nothing]{ override def absurd[X](a: Nothing): X = a }

问题 The following code does not compile on Scala 2.12 / 2.13. Why? class X[U, T] object X { implicit def genericX[U, T](implicit ev: T <:< U): X[U, T] = new X[U, T] } implicitly[X[AnyRef, String]] // compiles implicitly[X[String, Nothing]] // does not compile 回答1: Long story short, compiler doesn't like to infer Nothing in implicits. Why doesn't Scala's implicit class work when one of the type parameters should be Nothing? Implicit error when trying to implement the `Absurd` typeclass https://www

问题 The following code does not compile on Scala 2.12 / 2.13. Why? class X[U, T] object X { implicit def genericX[U, T](implicit ev: T <:< U): X[U, T] = new X[U, T] } implicitly[X[AnyRef, String]] // compiles implicitly[X[String, Nothing]] // does not compile 回答1: Long story short, compiler doesn't like to infer Nothing in implicits. Why doesn't Scala's implicit class work when one of the type parameters should be Nothing? Implicit error when trying to implement the `Absurd` typeclass https://www