Shapeless: Native HList Constraint Using Coproduct

Question

(NOTE: Split from Shapeless: attempt to restrict HList elements to their type )

Question 2 - Own restrictions using Coproduct

What I really wanted to do was write a new constraint using Coproduct.

trait CPConstraint[L <: HList, CP <: Coproduct] extends Serializable object CPConstraint { import shapeless.ops.coproduct.Selector._ def apply[L <: HList, CP <: Coproduct](implicit cpc: CPConstraint[L, CP]): CPConstraint[L, CP] = cpc type <*<[CP <: Coproduct] = { // TODO: just invented a symbol ... what would be an appropriate one? type λ[L <: HList] = CPConstraint[L, CP] } implicit def hnilCP[HN <: HNil, CP <: Coproduct]: CPConstraint[HN, CP] = new CPConstraint[HN, CP] {} implicit def hlistCP[H, T <: HList, CP <: Coproduct](implicit ev: coproduct.Selector[CP, H], cpct: CPConstraint[T, CP]): CPConstraint[H :: T, CP] = new CPConstraint[H :: T, CP] {} } object testCPConstraint { import shapeless.ops.coproduct.Selector._ import CPConstraint._ type CPType = Long :+: String :+: CNil implicit val selLong = implicitly[Selector[CPType, Long]] implicit val selString = implicitly[Selector[CPType, String]] def acceptCP[L <: HList : <*<[CPType]#λ](l: L) = true val hlLong: ::[Long, HNil] = 1L :: HNil val hlString: ::[String, HNil] = "blabla" :: HNil val hlMixed: ::[String, ::[Long, HNil]] = "blabla" :: 1L :: HNil val hlMixedRev: ::[Long, ::[String, HNil]] = 1L :: "blabla" :: HNil val hlInvalid: ::[Double, HNil] = 1.0d :: HNil implicit val scpcEmpty: CPConstraint[HNil, CPType] = implicitly[CPConstraint[HNil, CPType]] implicit val scpcEmptyLong1: CPConstraint[::[Long,HNil], CPType] = new CPConstraint[::[Long,HNil], CPType] {} 

// implicit val scpcEmptyLong2: CPConstraint [hlLong.type, CPType] = new CPConstraint [hlLong.type, CPType] {} // the above line will correspond to the missing implicit - WHY ???

  implicit val cpcLong = implicitly[CPConstraint[hlLong.type, CPType]] val validEmpty = acceptCP(HNil: HNil) val validLong = acceptCP(1l :: HNil) val validMixed = acceptCP("blabla" :: 1l :: HNil) val invalid = acceptCP(1.0d :: HNil) // should fail due to missing evidence } 

Answer 1

During the experiments ... I got his job:

 import shapeless.ops.coproduct import shapeless.{:+:, ::, CNil, Coproduct, HList, HNil} /** * constraint that checks {{{shapeless.HList}}} elements to be of a type contained in a {{{shapeless.Coproduct}}} */ trait CPConstraint[L <: HList, CP <: Coproduct] extends Serializable object CPConstraint { def apply[L <: HList, CP <: Coproduct](implicit cpc: CPConstraint[L, CP]): CPConstraint[L, CP] = cpc type <*<[CP <: Coproduct] = { type λ[L <: HList] = CPConstraint[L, CP] } implicit def hnilCP[HN <: HNil, CP <: Coproduct]: CPConstraint[HN, CP] = new CPConstraint[HN, CP] {} implicit def hlistCP[H, T <: HList, CP <: Coproduct](implicit ev: coproduct.Selector[CP, H], cpct: CPConstraint[T, CP]): CPConstraint[H :: T, CP] = new CPConstraint[H :: T, CP] {} } object testCPConstraint { import CPConstraint._ type CPType = Long :+: String :+: CNil def acceptCP[L <: HList : <*<[CPType]#λ](l: L) = true val hlLong = 1L :: (HNil: HNil) val hlString = "blabla" :: HNil val hlMixed = "blabla" :: 1L :: HNil val hlMixedRev = 1L :: "blabla" :: HNil val hlInvalid = 1.0d :: HNil val validEmpty = acceptCP(HNil) val validEmpty2 = acceptCP(HList()) val validEmpty3 = acceptCP(HNil: HNil) val validString = acceptCP(hlString) val validLong = acceptCP(hlLong) val validMixed = acceptCP(hlMixed) val validMixedRev = acceptCP(hlMixedRev) // val invalid = acceptCP(hlInvalid) // -> fails due to missing evidence (as expected) implicit val scpcEmpty = implicitly[CPConstraint[HNil, CPType]] // implicit val cpcLong = implicitly[CPConstraint[hlLong.type, CPType]] // I still don't understand why above line doesn't work, but the following does ??? implicit val cpcLong2 = implicitly[CPConstraint[::[Long,HNil], CPType]] } 

Answer 2

This answer is shamelessly copied from gitter , they all refer to @SystemFW.

Use a LiftAll witness, for example:

 import shapeless._, ops.hlist.LiftAll, ops.coproduct.Inject type Cop = Int :+: Long :+: CNil def foo[H <: HList](implicit ev: LiftAll[Inject[Cop,?], H]) = null foo[Int :: Long :: HNil] foo[HNil] foo[Int :: Int :: HNil] //foo[Int :: Long :: String :: HNil] doesn't compile 

Where Inject[Cop,?] is type I[X] = Inject[Cop,X] (or can you use the view projector plugin for syntax ? ).