type-level-computation

问题 I'm trying to define some structure like this case class Transformer[From, To]( name: String, get: PaymentEvent => From, to: From => To I want to filter elements with names that are part of a Set class filterName(names: Set[String]) extends lowPriority { implicit def get[From, To] = at[Transformer[From, To]]{ trans => if (names.contains(trans.name)) trans :: HNil else HNil } } This is the concrete value: type HTransformer = Transformer[String, String] :: Transformer[Long, Long] :: HNil When I

问题 Consider this code: trait TypeOr[E, F] { type T } implicit def noneq2[E, F](implicit ev: E =!= F): TypeOr[E, F] = new TypeOr[E, F] { type T = (E, F) } sealed trait Error[+E, +A] case class Err[E, A](e: Error[E, A]) { def combine[B, F](f: A => Error[F, B])(implicit ev: TypeOr[E, F]): Error[ev.T, B] = ??? } val result = Err(null.asInstanceOf[Error[Int, Int]]).combine(_ => null.asInstanceOf[Error[String, String]]) So far so good. From the definitions above, I concluded, that the expanded type of

问题 When looking at the source of some Scala libraries, e.g. shapeless, I often find traits named LowPriorityImplicits . Can you please explain this pattern? What is the problem that is solved, and how does the pattern solve it? 回答1: That pattern allows you to have hierarchy of implicits avoiding ambiguity-related errors by the compiler and providing a way to prioritise them. As an example consider the following: trait MyTypeclass[T] { def foo: String } object MyTypeclass { implicit def

问题 The following code seems obvious enough to compile and run case class Pair(a: String, b: Int) val pairGen = Generic[Pair] object size extends Poly1 { implicit def caseInt = at[Int](x => 1) implicit def caseString = at[String](_.length) } def funrun(p: Pair) = { val hp: HList = pairGen.to(p) hp.map(size) } but the compiler says "could not find implicit value for parameter mapper". In my use case, I want to map over an HList to get and HList of String(s) and then convert the HList of String(s)

问题 The following code seems obvious enough to compile and run case class Pair(a: String, b: Int) val pairGen = Generic[Pair] object size extends Poly1 { implicit def caseInt = at[Int](x => 1) implicit def caseString = at[String](_.length) } def funrun(p: Pair) = { val hp: HList = pairGen.to(p) hp.map(size) } but the compiler says "could not find implicit value for parameter mapper". In my use case, I want to map over an HList to get and HList of String(s) and then convert the HList of String(s)

问题 I have a trait Foo[T, U] and a type-level algorithm that given an L <: HList and a target type U , tells me whether there exists T in L such that there is an implicit Foo[T, U] in scope. This is implemented using the following type class: trait Search[L <: HList, U] object Search { def apply[L <: HList, U](implicit s: Search[L, U]): U = null ... } and we have the following: object Test { type L = Int :: String :: HNil implicit val foo: Foo[String, Boolean] = null Search[L, Boolean] //compiles

问题 I'm trying to derive a tuple instance for a type class with dependent type. I'm using shapeless to create summon the type class for the tuple elements. I'm having trouble matching tuple instance types: import shapeless.the import simulacrum.typeclass @typeclass trait Identifiable[M] { type K def identify(id: M): K } object Identifiable{ implicit def identifiableTuple[K1: Identifiable, K2: Identifiable]: Identifiable[(K1,K2)] = new Identifiable[(K1,K2)]{ val b = the[Identifiable[K2]] val a =

来源： https://stackoverflow.com/questions/26723337/using-the-prolog-in-scala-to-find-available-type-class-instances

问题 I have a typeclass Search , which has an instance Search[A] if we have a TypeClass1[A] or a TypeClass2[A] instance. With preference given to the 1 instance. The following compiles: trait TypeClass1[A] trait TypeClass2[A] trait Search[A] object Search extends LPSearch { implicit def case1[A](implicit ev: TypeClass1[A]): Search[A] = null } trait LPSearch { implicit def case2[A](implicit ev: TypeClass2[A]): Search[A] = null } object Test { implicit val ev1: TypeClass1[Int] = null implicit val

问题 I have a typeclass Search , which has an instance Search[A] if we have a TypeClass1[A] or a TypeClass2[A] instance. With preference given to the 1 instance. The following compiles: trait TypeClass1[A] trait TypeClass2[A] trait Search[A] object Search extends LPSearch { implicit def case1[A](implicit ev: TypeClass1[A]): Search[A] = null } trait LPSearch { implicit def case2[A](implicit ev: TypeClass2[A]): Search[A] = null } object Test { implicit val ev1: TypeClass1[Int] = null implicit val