Introduce typeclasses making it easier to write generic extension methods (#478)

* Introduce HasSeqOps
* Introduce HasImmutableMapOps
* Introduce HasIterableOps

Author: julienrf

src/main/scala/strawman/collection/decorators/HasImmutableMapOps.scala

@@ -0,0 +1,43 @@
+package strawman.collection
+package decorators
+
+/**
+  * Type class witnessing that a collection type `C` has
+  * a conversion to `immutable.MapOps[K, V, _, _]`.
+  *
+  * @see [[scala.collection.decorators.HasIterableOps]]
+  */
+trait HasImmutableMapOps[C] extends HasMapOps[C] {
+
+  // Convenient intermediate type definitions to satisfy type bounds
+  protected type _CC[X, +Y] <: immutable.MapOps[X, Y, _CC, _]
+  protected type _C <: immutable.MapOps[K, V, _CC, _C]
+
+  /** A conversion from the type `C` to `immutable.MapOps[K, V, _, _]` */
+  def apply(c: C): immutable.MapOps[K, V, _CC, _C]
+
+}
+
+object HasImmutableMapOps {
+
+  // 1. Map collections
+  implicit def mapHasMapOps[CC[X, +Y] <: immutable.MapOps[X, Y, CC, CC[X, Y]], K0, V0]: HasImmutableMapOps[CC[K0, V0]] { type K = K0; type V = V0 } =
+    new HasImmutableMapOps[CC[K0, V0]] {
+      type K = K0
+      type V = V0
+      type _CC[X, +Y] = CC[X, Y]
+      type _C = CC[K, V]
+      def apply(c: CC[K0, V0]): immutable.MapOps[K0, V0, _CC, _C] = c
+    }
+
+  // 2. Sorted Map collections
+  implicit def sortedMapHasMapOps[CC[X, +Y] <: immutable.Map[X, Y] with immutable.SortedMapOps[X, Y, CC, CC[X, Y]], K0, V0]: HasImmutableMapOps[CC[K0, V0]] { type K = K0; type V = V0 } =
+    new HasImmutableMapOps[CC[K0, V0]] {
+      type K = K0
+      type V = V0
+      type _CC[X, +Y] = immutable.Map[X, Y]
+      type _C = _CC[K, V]
+      def apply(c: CC[K0, V0]): immutable.MapOps[K0, V0, _CC, _C] = c
+    }
+
+}

src/main/scala/strawman/collection/decorators/HasIterableOps.scala

@@ -0,0 +1,44 @@
+package strawman.collection
+package decorators
+
+/**
+  * Type class witnessing that a collection type `C`
+  * has elements of type `A` and has a conversion to `IterableOps[A, _, _]`.
+  *
+  * This type enables simple enrichment of `Iterable`s with extension methods.
+  *
+  * @tparam C Collection type (e.g. `List[Int]`)
+  */
+trait HasIterableOps[C] {
+
+  /** The type of elements (e.g. `Int`) */
+  type A
+
+  /** A conversion from the type `C` to `IterableOps[A, _, _]` */
+  def apply(c: C): IterableOps[A, AnyConstr, _]
+
+}
+
+object HasIterableOps extends LowPriorityHasIterableOps {
+
+  implicit def iterableHasIterableOps[CC[X] <: IterableOps[X, AnyConstr, _], A0]: HasIterableOps[CC[A0]] { type A = A0 } =
+    new HasIterableOps[CC[A0]] {
+      type A = A0
+      def apply(c: CC[A0]): IterableOps[A0, AnyConstr, _] = c
+    }
+
+}
+
+trait LowPriorityHasIterableOps {
+
+  // Makes `HasSeqOps` instances visible in `HasIterableOps` companion
+  implicit def hasSeqOpsHasIterableOps[C, A0](implicit
+    hasSeqOps: HasSeqOps[C] { type A = A0 }
+  ): HasIterableOps[C] { type A = A0 } = hasSeqOps
+
+  // Makes `HasMapOps` instances visible in `HasIterableOps` companion
+  implicit def hasMapOpsHasIterableOps[C, K0, V0](implicit
+    hasMapOps: HasMapOps[C] { type K = K0; type V = V0 }
+  ): HasIterableOps[C] { type A = (K0, V0) } = hasMapOps
+
+}

src/main/scala/strawman/collection/decorators/HasMapOps.scala

@@ -0,0 +1,46 @@
+package strawman.collection
+package decorators
+
+/**
+  * Type class witnessing that a collection type `C`
+  * has keys of type `K`, values of type `V` and has a conversion to `MapOps[K, V, _, _]`.
+  *
+  * This type enables simple enrichment of `Map`s with extension methods.
+  *
+  * @tparam C Collection type (e.g. `Map[Int, String]`)
+  */
+trait HasMapOps[C] extends HasIterableOps[C] {
+
+  /** The type of keys */
+  type K
+
+  /** The type of values */
+  type V
+
+  type A = (K, V)
+
+  /** A conversion from the type `C` to `MapOps[K, V, _, _]` */
+  def apply(c: C): MapOps[K, V, ({ type l[X, +Y] = IterableOps[_, AnyConstr, _] })#l, _]
+
+}
+
+object HasMapOps extends LowPriorityHasMapOps {
+
+  // 1. Map collections
+  implicit def mapHasMapOps[CC[X, +Y] <: MapOps[X, Y, ({ type l[X, +Y] = IterableOps[_, AnyConstr, _] })#l, _], K0, V0]: HasMapOps[CC[K0, V0]] { type K = K0; type V = V0 } =
+    new HasMapOps[CC[K0, V0]] {
+      type K = K0
+      type V = V0
+      def apply(c: CC[K0, V0]): MapOps[K0, V0, ({ type l[X, +Y] = IterableOps[_, AnyConstr, _] })#l, _] = c
+    }
+
+}
+
+trait LowPriorityHasMapOps {
+
+  // Makes `HasImmutableMapOps` instances visible in `HasMapOps` companion
+  implicit def hasImmutableMapOpsHasMapOps[C, K0, V0](implicit
+    hasImmutableMapOps: HasImmutableMapOps[C] { type K = K0; type V = V0 }
+  ): HasMapOps[C] { type K = K0; type V = V0 } = hasImmutableMapOps
+
+}

src/main/scala/strawman/collection/decorators/HasSeqOps.scala

@@ -0,0 +1,60 @@
+package strawman.collection
+package decorators
+
+import scala.{Array, Char, Int}
+import scala.Predef.String
+import strawman.collection.immutable.{ImmutableArray, Range}
+
+/** Type class witnessing that a collection type `C` has
+  * elements of type `A` and has a conversion to `SeqOps[A, _, _]`.
+  *
+  * This type enables simple enrichment of `Seq`s with extension methods which
+  * can make full use of the mechanics of the Scala collections framework in
+  * their implementation.
+  *
+  * @see [[scala.collection.decorators.HasIterableOps]]
+  */
+trait HasSeqOps[C] extends HasIterableOps[C] {
+  /** A conversion from the type `C` to `SeqOps[A, _, _]`. */
+  def apply(c: C): SeqOps[A, AnyConstr, _]
+}
+
+object HasSeqOps {
+
+  // we want to provide implicit instances that unify all possible types `X` with a `SeqOps[A, CC, C]`
+  // 1. Seq collections
+  implicit def seqHasSeqOps[CC[X] <: SeqOps[X, AnyConstr, _], A0]: HasSeqOps[CC[A0]] {type A = A0 } =
+    new HasSeqOps[CC[A0]] {
+      type A = A0
+      def apply(c: CC[A0]): SeqOps[A0, AnyConstr, _] = c
+    }
+
+  // 2. String
+  implicit def stringHasSeqOps: HasSeqOps[String] { type A = Char } =
+    new HasSeqOps[String] {
+      type A = Char
+      def apply(c: String): SeqOps[Char, AnyConstr, _] = stringToStringOps(c)
+    }
+
+  // 3. StringView
+  implicit def stringViewHasSeqOps: HasSeqOps[StringView] { type A = Char } =
+    new HasSeqOps[StringView] {
+      type A = Char
+      def apply(c: StringView): SeqOps[Char, AnyConstr, _] = c
+    }
+
+  // 4. Array
+  implicit def arrayHasSeqOps[A0]: HasSeqOps[Array[A0]] { type A = A0 } =
+    new HasSeqOps[Array[A0]] {
+      type A = A0
+      def apply(c: Array[A0]): SeqOps[A0, AnyConstr, _] = ImmutableArray.unsafeWrapArray(c)
+    }
+
+  // 5. Range collections
+  implicit def rangeHasSeqOps[C <: Range]: HasSeqOps[C] { type A = Int } =
+    new HasSeqOps[C] {
+      type A = Int
+      def apply(c: C): SeqOps[Int, AnyConstr, _] = c
+    }
+
+}

src/main/scala/strawman/collection/decorators/ImmutableMapDecorator.scala

@@ -2,7 +2,7 @@ package strawman
 package collection
 package decorators
 
-class ImmutableMapDecorator[K, V, CC[X, +Y] <: immutable.Map[X, Y]](`this`: CC[K, V]) {
+class ImmutableMapDecorator[C, M <: HasImmutableMapOps[C]](coll: C)(implicit val map: M) {
 
   /**
     * Updates an existing binding or create a new one according to the
@@ -23,15 +23,16 @@ class ImmutableMapDecorator[K, V, CC[X, +Y] <: immutable.Map[X, Y]](`this`: CC[K
     *
     * @return A new updated `Map`
     */
-  def updatedWith[C](key: K)(f: PartialFunction[Option[V], Option[V]])(implicit bf: BuildFrom[CC[K, V], (K, V), C]): C = {
+  def updatedWith[That](key: map.K)(f: PartialFunction[Option[map.V], Option[map.V]])(implicit bf: BuildFrom[C, (map.K, map.V), That]): That = {
     val pf = f.lift
+    val `this` = map(coll)
     val previousValue = `this`.get(key)
     pf(previousValue) match {
-      case None => bf.fromSpecificIterable(`this`)(`this`)
+      case None => bf.fromSpecificIterable(coll)(`this`.toIterable)
       case Some(result) =>
         result match {
-          case None => bf.fromSpecificIterable(`this`)(`this` - key)
-          case Some(v) => bf.fromSpecificIterable(`this`)(`this` + (key -> v))
+          case None => bf.fromSpecificIterable(coll)((`this` - key).toIterable)
+          case Some(v) => bf.fromSpecificIterable(coll)((`this` + (key -> v)).toIterable)
         }
     }
   }

src/main/scala/strawman/collection/decorators/IterableDecorator.scala

@@ -2,7 +2,7 @@ package strawman
 package collection
 package decorators
 
-class IterableDecorator[A](val `this`: Iterable[A]) extends AnyVal {
+class IterableDecorator[C, I <: HasIterableOps[C]](coll: C)(implicit val it: I) {
 
   /**
     * Left to right fold that stops if the combination function `op`
@@ -14,8 +14,8 @@ class IterableDecorator[A](val `this`: Iterable[A]) extends AnyVal {
     *         going left to right with the start value `z` on the left, and stopping when
     *         all the elements have been traversed or earlier if the operator returns `None`
     */
-  def foldSomeLeft[B](z: B)(op: (B, A) => Option[B]): B =
-    `this`.iterator().foldSomeLeft(z)(op)
+  def foldSomeLeft[B](z: B)(op: (B, it.A) => Option[B]): B =
+    it(coll).iterator().foldSomeLeft(z)(op)
 
   /**
     * Right to left fold that can be interrupted before traversing the whole collection.
@@ -28,6 +28,7 @@ class IterableDecorator[A](val `this`: Iterable[A]) extends AnyVal {
     *         then `result` is returned without iterating further; if it returns `Right(f)`, the function
     *         `f` is applied to the previous result to produce the new result and the fold continues.
     */
-  def lazyFoldRight[B](z: B)(op: A => Either[B, B => B]): B = `this`.iterator().lazyFoldRight(z)(op)
+  def lazyFoldRight[B](z: B)(op: it.A => Either[B, B => B]): B =
+    it(coll).iterator().lazyFoldRight(z)(op)
 
 }