shapeless

问题 I would like to extract the key and value of the head of an HList using these two methods: def getFieldName[K, V](value: FieldType[K, V])(implicit witness: Witness.Aux[K]): K = witness.value def getFieldValue[K, V](value: FieldType[K, V]): V = value I tried a few variations of this function, but I couldn't make it work, I think this might be the closest to the right solution: def getFieldNameValue[Key <: Symbol, Value <: AnyRef, Y <: HList, A <: Product](a : A) (implicit gen : LabelledGeneric

问题 I'm having trouble understanding the way the Shapeless record Selector interacts with scala's type inference. I'm trying to create a method that can grab a field from a Shapeless record by key, only if the value of the field has a certain unary type constructor, in this particular case Vector[_] , and then grab an inner value of inferred type V out of the Vector , in this case with Vector.apply() . I feel like I'm close. This works, with a concrete inner type of Int : val record = ( "a" ->>

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 I would like to have some function applied to fields in a case class, that are annotated with MyAnnotation . The idea is to transform type T into its generic representation, extract annotations, zip, fold right (or left) to reconstruct a generic representation and finally get back to type T . I followed the answer provided here and this gist. I'm using scala 2.11.12 and shapeless 2.3.3. Hereafter is my code: import shapeless._ import shapeless.ops.hlist._ case class MyAnnotation(func: String)

问题 Given 2 GADT Algebras which know about each other and 2 interpreters that are mutually recursive, I am having issues having to cast from type A to type h <: HList even though in the context of the pattern match, it should be implied that type A is type h. Is there a way to avoid the asInstanceOf[h] call in the interpreter? abstract class KvpHList[H<:HList] object KvpNil extends KvpHList[HNil] case class KvpCons[H <: A :: T,A, T<:HList](head: KvpValue[A], tail: KvpHList[T])(implicit isHCons:

问题 I'm using Shapeless to accumulate materialized values in Akka as an HList and convert that to a case class. (You don't have to know Akka much for this question, but the default approach accumulates materialized values as recursively nested 2-tuples, which isn't much fun, so Shapeless HLists seemed a more sensible approach -- and works pretty well. But I don't know how to properly re-use that approach. Here, I'll simplify the kinds of values Akka produces.) For example, let's say we've got two

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