shapeless

问题 Given the following data model: sealed trait Fruit case class Apple(id: Int, sweetness: Int) extends Fruit case class Pear(id: Int, color: String) extends Fruit I've been looking to implement a segregate basket function which for the given basket of fruits will return separate baskets of apples and pears: def segregateBasket(fruitBasket: Set[Fruit]): (Set[Apple], Set[Pear]) I've attempted a couple of approaches, but none of them seems to be fitting the bill perfectly. Below are my attempts:

问题 Is there a way to derive a type from an existing one in Scala? For example, for case class Person(name: String, age: Int) I'd like to get a Product / Tuple of (Option[String], Option[Int]) , i.e. a type mapped from an existing one. There's a feature in Typescript (mapped types) that allows this relatively easily, which is how I started thinking down this path. But I'm not sure how something like this would be done in Scala. I feel like the solution involves using shapeless in some way but I'm

问题 This question may be asked and answered before, but I would like to understand this with an example and I could not reason out where the Aux pattern might be helpful! So here is the trait: trait Foo[A] { type B def value: B } Why do I have a type that is kind of bound to the return type of the value function? What do I achieve doing this? In particular, where would I use such patterns? 回答1: Imagine a typeclass for getting the last element of any tuple. trait Last[A] { type B def last(a: A): B

问题 I'm using shapeless in Scala, and I'd like to write a function allPairs that will take two HLists and return an HList of all pairs of elements. For example: import shapeless._ val list1 = 1 :: "one" :: HNil val list2 = 2 :: "two" :: HNil // Has value (1, 2) :: (1, "two") :: ("one", 2) :: ("one", "two") :: HNil val list3 = allPairs(list1, list2) Any idea how to do this? Also, I'd like to emphasize I'm looking for a function , not an inlined block of code. 回答1: You can't use a for

问题 I'm using shapeless in Scala, and I'd like to write a function allPairs that will take two HLists and return an HList of all pairs of elements. For example: import shapeless._ val list1 = 1 :: "one" :: HNil val list2 = 2 :: "two" :: HNil // Has value (1, 2) :: (1, "two") :: ("one", 2) :: ("one", "two") :: HNil val list3 = allPairs(list1, list2) Any idea how to do this? Also, I'd like to emphasize I'm looking for a function , not an inlined block of code. 回答1: You can't use a for

问题 If I have a Shapeless HMap[MappingA] (with implicits properly defined for the type MappingA[K, V] ), can I type-safely transform/map it to a HMap[MappingB] . class MappingA[K, V] implicit val intToString = new MappingA[Int, String] implicit val stringToInt = new MappingA[String, Int] class MappingB[K, V] implicit val longToString = new MappingA[Long, String] implicit val stringToLong = new MappingA[String, Long] val hm1 = HMap[MappingA](1 -> "one", "two" -> 2) // How to... val hm2: HMap

问题 Is it possible to generically replace arguments in a case class? More specifically, say I wanted a substitute function that received a "find" case class and a "replace" case class (like the left and right sides of a grammar rule) as well as a target case class, and the function would return a new case class with arguments of the find case class replaced with the replace case class? The function could also simply take a case class (Product?) and a function to be applied to all arguments

问题 I'm trying to extend the Parser type class from this blog post to support nested case classes parsing. Since I'm new to shapeless, I've decided to start with the non-general case, that would only support a case class with 2 fields that are themselves case classes. To illustrate this better, here's the case classes definition first: case class Person(name: String, age: Int) case class Family(husband: Person, wife: Person) In addition to the code from that blog post, I've created one more

问题 I'm trying to extend the Parser type class from this blog post to support nested case classes parsing. Since I'm new to shapeless, I've decided to start with the non-general case, that would only support a case class with 2 fields that are themselves case classes. To illustrate this better, here's the case classes definition first: case class Person(name: String, age: Int) case class Family(husband: Person, wife: Person) In addition to the code from that blog post, I've created one more

问题 In the following, I'm trying to make a polymorphic function to convert a RawFeatureValue into a RefinedFeatureValue . import shapeless._ object test { type RawFeatureValue = Int :+: Double :+: String :+: CNil type RefinedFeatureValue = Int :+: Double :+: CNil private object convert extends Poly1 { implicit def caseInt = at[Int](i => i) implicit def caseDouble = at[Double](d => d) implicit def caseString = at[String](s => s.hashCode) } val a = Coproduct[RawFeatureValue](12) val b: