Add Compose instance for Map (#2402)

* Add Compose instance for Map

* Move Compose instance for Map to separate trait

Fix binary compatibility issues by moving the new Compose instance for Map
to a separate trait.

* Serialization test for Compose instance for Map

* Add doctest for Compose[Map]

core/src/main/scala/cats/implicits.scala

@@ -7,3 +7,4 @@ object implicits
  with syntax.AllSyntaxBinCompat2
  with instances.AllInstances
  with instances.AllInstancesBinCompat0
+ with instances.AllInstancesBinCompat1

core/src/main/scala/cats/instances/all.scala

@@ -36,3 +36,6 @@ trait AllInstances
 trait AllInstancesBinCompat0
  extends FunctionInstancesBinCompat0
  with Tuple2InstancesBinCompat0
+
+trait AllInstancesBinCompat1
+ extends MapInstancesBinCompat0

core/src/main/scala/cats/instances/map.scala

@@ -4,6 +4,7 @@ package instances
 import cats.kernel.CommutativeMonoid
 
 import scala.annotation.tailrec
+import cats.arrow.Compose
 
 trait MapInstances extends cats.kernel.instances.MapInstances {
 
@@ -79,4 +80,34 @@ trait MapInstances extends cats.kernel.instances.MapInstances {
 
  }
  // scalastyle:on method.length
+
+}
+
+trait MapInstancesBinCompat0 {
+
+ implicit val catsStdComposeForMap: Compose[Map] = new Compose[Map] {
+
+ /**
+ * Compose two maps `g` and `f` by using the values in `f` as keys for `g`.
+ * {{{
+ * scala> import cats.arrow.Compose
+ * scala> import cats.implicits._
+ * scala> val first = Map(1 -> "a", 2 -> "b", 3 -> "c", 4 -> "a")
+ * scala> val second = Map("a" -> true, "b" -> false, "d" -> true)
+ * scala> Compose[Map].compose(second, first)
+ * res0: Map[Int, Boolean] = Map(1 -> true, 2 -> false, 4 -> true)
+ * }}}
+ */
+ def compose[A, B, C](f: Map[B, C], g: Map[A, B]): Map[A, C] = {
+ g.foldLeft(Map.empty[A, C]) {
+ case (acc, (key, value)) =>
+ f.get(value) match {
+ case Some(other) => acc + (key -> other)
+ case _ => acc
+ }
+ }
+ }
+
+ }
+
 }

core/src/main/scala/cats/instances/package.scala

@@ -1,7 +1,7 @@
 package cats
 
 package object instances {
- object all extends AllInstances with AllInstancesBinCompat0
+ object all extends AllInstances with AllInstancesBinCompat0 with AllInstancesBinCompat1
  object bigInt extends BigIntInstances
  object bigDecimal extends BigDecimalInstances
  object bitSet extends BitSetInstances
@@ -21,7 +21,7 @@ package object instances {
  object invariant extends InvariantMonoidalInstances
  object list extends ListInstances
  object long extends LongInstances
- object map extends MapInstances
+ object map extends MapInstances with MapInstancesBinCompat0
  object option extends OptionInstances
  object order extends OrderInstances
  object ordering extends OrderingInstances

testkit/src/main/scala/cats/tests/CatsSuite.scala

@@ -2,7 +2,7 @@ package cats
 package tests
 
 import catalysts.Platform
-import cats.instances.{AllInstances, AllInstancesBinCompat0}
+import cats.instances.{AllInstances, AllInstancesBinCompat0, AllInstancesBinCompat1}
 import cats.syntax.{AllSyntax, AllSyntaxBinCompat0, AllSyntaxBinCompat1, AllSyntaxBinCompat2, EqOps}
 import org.scalactic.anyvals.{PosInt, PosZDouble, PosZInt}
 import org.scalatest.{FunSuite, FunSuiteLike, Matchers}
@@ -33,7 +33,7 @@ trait CatsSuite extends FunSuite
  with GeneratorDrivenPropertyChecks
  with Discipline
  with TestSettings
- with AllInstances with AllInstancesBinCompat0
+ with AllInstances with AllInstancesBinCompat0 with AllInstancesBinCompat1
  with AllSyntax with AllSyntaxBinCompat0 with AllSyntaxBinCompat1
  with AllSyntaxBinCompat2
  with StrictCatsEquality { self: FunSuiteLike =>

tests/src/test/scala/cats/tests/MapSuite.scala

@@ -1,7 +1,8 @@
 package cats
 package tests
 
-import cats.laws.discipline.{FlatMapTests, SemigroupalTests, SerializableTests, UnorderedTraverseTests}
+import cats.arrow.Compose
+import cats.laws.discipline.{FlatMapTests, SemigroupalTests, SerializableTests, UnorderedTraverseTests, ComposeTests}
 
 class MapSuite extends CatsSuite {
  implicit val iso = SemigroupalTests.Isomorphisms.invariant[Map[Int, ?]]
@@ -15,6 +16,9 @@ class MapSuite extends CatsSuite {
  checkAll("Map[Int, Int] with Option", UnorderedTraverseTests[Map[Int, ?]].unorderedTraverse[Int, Int, Int, Option, Option])
  checkAll("UnorderedTraverse[Map[Int, ?]]", SerializableTests.serializable(UnorderedTraverse[Map[Int, ?]]))
 
+ checkAll("Map[Int, Long]", ComposeTests[Map].compose[Int, Long, String, Double])
+ checkAll("Compose[Map]", SerializableTests.serializable(Compose[Map]))
+
 
  test("show isn't empty and is formatted as expected") {
  forAll { (map: Map[Int, String]) =>