Merge pull request #1129 from joroKr21/select-range

Optimize SelectRange typeclass derivation

core/src/main/scala/shapeless/ops/hlists.scala

@@ -916,26 +916,42 @@ object hlist {
   }
 
   /**
-   * Type class supporting supporting access to the elements in range [a,b[ of this `HList`.
-   * Avaialable only if this `HList` contains all elements in range
+   * Type class supporting supporting access to the elements in range [a,b] of this `HList`.
+   * Available only if this `HList` contains all elements in range
    *
-   * @author Andreas Koestler
+   * @author Ivan Aristov
    */
   @implicitNotFound("Implicit not found: shapeless.Ops.SelectRange[${L}, ${A}, ${B}]. You requested the elements in range [${A},${B}[, but HList ${L} does not contain all of them.")
   trait SelectRange[L <: HList, A <: Nat, B <: Nat] extends DepFn1[L] { type Out <: HList }
 
   object SelectRange {
     def apply[L <: HList, A <: Nat, B <: Nat](implicit sel: SelectRange[L, A, B]): Aux[L, A, B, sel.Out] = sel
 
-    type Aux[L <: HList, A <: Nat, B <: Nat, Out0 <: HList] = SelectRange[L, A, B] {type Out = Out0}
+    type Aux[L <: HList, A <: Nat, B <: Nat, Out0 <: HList] = SelectRange[L, A, B] {
+      type Out = Out0
+    }
+
+    @deprecated("Slower version of take and drop", "2.3.5")
+    def SelectRangeAux[L <: HList, A <: Nat, B <: Nat, Ids <: HList, SelOut <: HList](
+      implicit range: nat.Range.Aux[A, B, Ids], sel: SelectMany.Aux[L, Ids, SelOut]
+    ): Aux[L, A, B, SelOut] = new SelectRange[L, A, B] {
+      type Out = SelOut
+      def apply(l: L): Out = sel(l)
+    }
 
-    implicit def SelectRangeAux[L <: HList, A <: Nat, B <: Nat, Ids <: HList, SelOut <: HList]
-    (implicit range: shapeless.ops.nat.Range.Aux[A, B, Ids], sel: SelectMany.Aux[L, Ids, SelOut]): Aux[L, A, B, SelOut] =
-      new SelectRange[L, A, B] {
-        type Out = SelOut
+    implicit def take[L <: HList, T <: Nat](
+      implicit take: Take[L, T]
+    ): Aux[L, _0, T, take.Out] = new SelectRange[L, _0, T] {
+      type Out = take.Out
+      def apply(t: L): take.Out = take(t)
+    }
 
-        def apply(l: L): Out = sel(l)
-      }
+    implicit def drop[H, L <: HList, A <: Nat, B <: Nat](
+      implicit select: SelectRange[L, A, B]
+    ): Aux[H :: L, Succ[A], Succ[B], select.Out] = new SelectRange[H :: L, Succ[A], Succ[B]] {
+      type Out = select.Out
+      def apply(t: H :: L): select.Out = select(t.tail)
+    }
   }