hlist

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)

来源： https://stackoverflow.com/questions/64102795/how-to-convert-json-to-scala-shapeless-hlist

来源： https://stackoverflow.com/questions/64102795/how-to-convert-json-to-scala-shapeless-hlist

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 Thanks to https://github.com/milessabin/shapeless/wiki/Feature-overview:-shapeless-2.0.0 I understand how to zip shapeless HLists: Import some stuff from Shapeless 2.0.0-M1: import shapeless._ import shapeless.ops.hlist._ import syntax.std.tuple._ import Zipper._ Create two HLists: scala> val h1 = 5 :: "a" :: HNil h1: shapeless.::[Int,shapeless.::[String,shapeless.HNil]] = 5 :: a :: HNil scala> val h2 = 6 :: "b" :: HNil h2: shapeless.::[Int,shapeless.::[String,shapeless.HNil]] = 6 :: b ::

问题 Question 1 - Basic LUBConstraints My first try playing around with existing LUBConstraints fails for missing evidence (see code block below). Any hint why? Isn't an empty list a valid list of longs? no element violates the constraint. import shapeless.ops.coproduct import shapeless.{::, :+:, Coproduct, HNil, HList} object testLUBConstraints { import shapeless.LUBConstraint._ // !!! see comment on question - this satisfies the implicit below!!! // implicit val hnilLUBForLong = new

问题 I would like to create the equivalent of: def toTupleN[A1, ..., AN, L <: HList](l: L): TupleN[A1, ..., AN] Code using toTupleN should compile only when there is exactly one N combination of values in l that the tuple could be created from. Anything else should generate a compile-time error. Available implicit conversions should be taken into account. Note that there are no restrictions on the size of l or the ordering of values in it. Example: val l = 23 :: (1, "wibble") :: (2, "wobble") ::

问题 I have this type which will be generated via shapeless: type hlistt = STUDENT.type :: AUTO_LOANS.type :: HNil Basically I have a bunch of case objects extending a trait so I managed to create a method which gives me instances of all case objects as an HList Then using import shapeless.ops.hlist.Last and init I wrote a method to retrieve one of the nodes in the HList if the value is equal to the string "student": def getLast(hl:hlistt) = { val last0=Last[hlistt] val la=last0(hl) if (la.value =