io.finch.internal.PairJoin.scala Maven / Gradle / Ivy

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package io.finch.internal

import shapeless.ops.adjoin.Adjoin
import shapeless.{::, DepFn2, HNil}

/** We need a version of `Adjoin` that provides slightly different behavior in the case of singleton results (we simply return the value, not a singleton
  * `HList`).
  * @groupname LowPriorityPair Low priority `PairAdjoin`
  * @groupprio LowPriorityPair 0
  */
trait PairAdjoin[A, B] extends DepFn2[A, B]

private[finch] trait LowPriorityPairAdjoin {
  type Aux[A, B, Out0] = PairAdjoin[A, B] { type Out = Out0 }

  /** @group LowPriorityPair
    */
  implicit def pairAdjoin[A, B, Out0](implicit
      adjoin: Adjoin.Aux[A :: B :: HNil, Out0]
  ): Aux[A, B, Out0] =
    new PairAdjoin[A, B] {
      type Out = Out0

      def apply(a: A, b: B): Out0 = adjoin(a :: b :: HNil)
    }
}

object PairAdjoin extends LowPriorityPairAdjoin {
  implicit def singletonPairAdjoin[A, B, C](implicit
      adjoin: Adjoin.Aux[A :: B :: HNil, C :: HNil]
  ): Aux[A, B, C] = new PairAdjoin[A, B] {
    type Out = C

    def apply(a: A, b: B): C = adjoin(a :: b :: HNil).head
  }
}