add Foldable#distinctBy (#1354)

scalaz · May 21, 2017 · bfeb8c8 · bfeb8c8
1 parent 8f38b26
commit bfeb8c8
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diff --git a/core/src/main/scala/scalaz/Foldable.scala b/core/src/main/scala/scalaz/Foldable.scala
@@ -325,6 +325,9 @@ trait Foldable[F[_]]  { self =>
         else seen
     }.reverse
 
+  def distinctBy[A, B: Equal](fa: F[A])(f: A => B): IList[A] =
+    distinctE(fa)(Equal.equalBy(f))
+
   def collapse[X[_], A](x: F[A])(implicit A: ApplicativePlus[X]): X[A] =
     foldRight(x, A.empty[A])((a, b) => A.plus(A.point(a), b))
 

diff --git a/core/src/main/scala/scalaz/syntax/FoldableSyntax.scala b/core/src/main/scala/scalaz/syntax/FoldableSyntax.scala
@@ -60,6 +60,7 @@ final class FoldableOps[F[_],A] private[syntax](val self: F[A])(implicit val F:
   final def minimumBy[B: Order](f: A => B): Option[A] = F.minimumBy(self)(f)
   final def distinct(implicit A: Order[A]): IList[A] = F.distinct(self)
   final def distinctE(implicit A: Equal[A]): IList[A] = F.distinctE(self)
+  final def distinctBy[B: Equal](f: A => B): IList[A] = F.distinctBy(self)(f)
   final def longDigits(implicit d: A <:< Digit): Long = F.longDigits(self)
   final def empty: Boolean = F.empty(self)
   final def element(a: A)(implicit A: Equal[A]): Boolean = F.element(self, a)

diff --git a/tests/src/test/scala/scalaz/FoldableTest.scala b/tests/src/test/scala/scalaz/FoldableTest.scala
@@ -72,6 +72,13 @@ object FoldableTest extends SpecLite {
       if (xs.length > 0) xs.distinctE(Equal.equal((_,_) => true)).length must_== 1
   }
 
+  "distinctBy" ! {
+    case class Foo(a: Int, b: String)
+    val xs = IList(Foo(1, "x"), Foo(2, "x"), Foo(1, "y"))
+    xs.distinctBy(_.a) must_== IList(Foo(1, "x"), Foo(2, "x"))
+    xs.distinctBy(_.b) must_== IList(Foo(1, "x"), Foo(1, "y"))
+  }
+
   "sumr1Opt" ! forAll {
     (xs: List[String]) => xs match {
       case Nil => xs.sumr1Opt must_== None