backported #3084 Add minimumBy/maximumBy/Option to Foldable

core/src/main/scala/cats/syntax/foldable.scala

@@ -303,6 +303,34 @@ final class FoldableOps0[F[_], A](private val fa: F[A]) extends AnyVal {
     import cats.syntax.foldable._
     F.partitionEitherM[G, A, B, C](fa)(f)(A, M)
   }
+
+  /**
+   * Find the minimum `A` item in this structure according to an `Order.by(f)`.
+   *
+   * @return `None` if the structure is empty, otherwise the minimum element
+   * wrapped in a `Some`.
+   *
+   * @see [[Reducible#minimum]] for a version that doesn't need to return an
+   * `Option` for structures that are guaranteed to be non-empty.
+   *
+   * @see [[maximumOptionBy]] for maximum instead of minimum.
+   */
+  def minimumOptionBy[B: Order](f: A => B)(implicit F: Foldable[F]): Option[A] =
+    F.minimumOption(fa)(Order.by(f))
+
+  /**
+   * Find the maximum `A` item in this structure according to an `Order.by(f)`.
+   *
+   * @return `None` if the structure is empty, otherwise the maximum element
+   * wrapped in a `Some`.
+   *
+   * @see [[Reducible#maximum]] for a version that doesn't need to return an
+   * `Option` for structures that are guaranteed to be non-empty.
+   *
+   * @see [[minimumOptionBy]] for minimum instead of maximum.
+   */
+  def maximumOptionBy[B: Order](f: A => B)(implicit F: Foldable[F]): Option[A] =
+    F.maximumOption(fa)(Order.by(f))
 }
 
 final private[syntax] class FoldableOps1[F[_]](private val F: Foldable[F]) extends AnyVal {

core/src/main/scala/cats/syntax/reducible.scala

@@ -31,4 +31,20 @@ final class ReducibleOps0[F[_], A](private val fa: F[A]) extends AnyVal {
    * */
   def reduceMapK[G[_], B](f: A => G[B])(implicit F: Reducible[F], G: SemigroupK[G]): G[B] =
     F.reduceLeftTo(fa)(f)((b, a) => G.combineK(b, f(a)))
+
+  /**
+   * Find the minimum `A` item in this structure according to an `Order.by(f)`.
+   *
+   * @see [[maximumBy]] for maximum instead of minimum.
+   */
+  def minimumBy[B: Order](f: A => B)(implicit F: Reducible[F]): A =
+    F.minimum(fa)(Order.by(f))
+
+  /**
+   * Find the maximum `A` item in this structure according to an `Order.by(f)`.
+   *
+   * @see [[minimumBy]] for minimum instead of maximum.
+   */
+  def maximumBy[B: Order](f: A => B)(implicit F: Reducible[F]): A =
+    F.maximum(fa)(Order.by(f))
 }

tests/src/test/scala/cats/tests/FoldableSuite.scala

@@ -192,6 +192,20 @@ abstract class FoldableSuite[F[_]: Foldable](name: String)(implicit ArbFInt: Arb
     }
   }
 
+  test(s"Foldable[$name].maximumBy/minimumBy") {
+    forAll { (fa: F[Int], f: Int => Int) =>
+      val maxOpt = fa.maximumOptionBy(f).map(f)
+      val minOpt = fa.minimumOptionBy(f).map(f)
+      val nelOpt = fa.toList.toNel
+      maxOpt should ===(nelOpt.map(_.maximumBy(f)).map(f))
+      maxOpt should ===(nelOpt.map(_.toList.maxBy(f)).map(f))
+      minOpt should ===(nelOpt.map(_.minimumBy(f)).map(f))
+      minOpt should ===(nelOpt.map(_.toList.minBy(f)).map(f))
+      maxOpt.forall(i => fa.forall(f(_) <= i)) should ===(true)
+      minOpt.forall(i => fa.forall(f(_) >= i)) should ===(true)
+    }
+  }
+
   test(s"Foldable[$name].reduceLeftOption/reduceRightOption") {
     forAll { (fa: F[Int]) =>
       val list = fa.toList