/
deephlister.scala
80 lines (66 loc) · 2.5 KB
/
deephlister.scala
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/*
* Copyright (c) 2014 Miles Sabin
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package shapeless.examples
import shapeless._, test._
/*
* Demo of using Lazy to resolve (hidden) divergence issues when traversing
* recursive generic structures.
*
* See http://stackoverflow.com/questions/25923974
*/
trait DeepHLister[R <: HList] extends DepFn1[R] { type Out <: HList }
trait LowPriorityDeepHLister {
type Aux[R <: HList, Out0 <: HList] = DeepHLister[R] { type Out = Out0 }
implicit def headNotCaseClassDeepHLister[H, T <: HList](implicit
dht: Lazy[DeepHLister[T]]
): Aux[H :: T, H :: dht.value.Out] = new DeepHLister[H :: T] {
type Out = H :: dht.value.Out
def apply(r: H :: T) = r.head :: dht.value(r.tail)
}
}
object DeepHLister extends LowPriorityDeepHLister {
implicit object hnilDeepHLister extends DeepHLister[HNil] {
type Out = HNil
def apply(r: HNil) = HNil
}
implicit def headCaseClassDeepHLister[H, R <: HList, T <: HList](implicit
gen: Generic.Aux[H, R],
dhh: Lazy[DeepHLister[R]],
dht: Lazy[DeepHLister[T]]
): Aux[H :: T, dhh.value.Out :: dht.value.Out] = new DeepHLister[H :: T] {
type Out = dhh.value.Out :: dht.value.Out
def apply(r: H :: T) = dhh.value(gen.to(r.head)) :: dht.value(r.tail)
}
def apply[R <: HList](implicit dh: DeepHLister[R]): Aux[R, dh.Out] = dh
}
object DeepHListerDemo extends App {
case class A(x: Int, y: String)
case class B(x: A, y: A)
case class C(b: B, a: A)
case class D(a: A, b: B)
type ARepr = Int :: String :: HNil
type BRepr = ARepr :: ARepr :: HNil
type CRepr = BRepr :: ARepr :: HNil
type DRepr = ARepr :: BRepr :: HNil
val adhl = DeepHLister[A :: HNil]
typed[DeepHLister.Aux[A :: HNil, ARepr :: HNil]](adhl)
val bdhl = DeepHLister[B :: HNil]
typed[DeepHLister.Aux[B :: HNil, BRepr :: HNil]](bdhl)
val cdhl = DeepHLister[C :: HNil]
typed[DeepHLister.Aux[C :: HNil, CRepr :: HNil]](cdhl)
val ddhl = DeepHLister[D :: HNil]
typed[DeepHLister.Aux[D :: HNil, DRepr :: HNil]](ddhl)
}