Add Foldable extension collectFirstSomeM

typelevel · Aug 11, 2018 · 743f0f9 · 743f0f9
1 parent 9a59eb6
commit 743f0f9
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diff --git a/core/src/main/scala/cats/syntax/foldable.scala b/core/src/main/scala/cats/syntax/foldable.scala
@@ -1,6 +1,8 @@
 package cats
 package syntax
 
+import cats.data.OptionT
+
 trait FoldableSyntax extends Foldable.ToFoldableOps with UnorderedFoldable.ToUnorderedFoldableOps {
   implicit final def catsSyntaxNestedFoldable[F[_]: Foldable, G[_], A](fga: F[G[A]]): NestedFoldableOps[F, G, A] =
     new NestedFoldableOps[F, G, A](fga)
@@ -92,4 +94,29 @@ final class FoldableOps[F[_], A](val fa: F[A]) extends AnyVal {
         b.toString.dropRight(delim.length)
     } + suffix
   }
+
+  /**
+    * Monadic version of `collectFirstSome`.
+    * {{{
+    * scala> import cats.implicits._
+    * scala> def parseInt(s: String): Either[String, Int] = Either.catchOnly[NumberFormatException](s.toInt).leftMap(_.getMessage)
+    * scala> val keys1 = List("1", "2", "4", "5")
+    * scala> val map1 = Map(4 -> "Four", 5 -> "Five")
+    * scala> keys1.collectFirstSomeM(parseInt(_) map map1.get)
+    * res1: scala.util.Either[String,Option[String]] = Right(Some(Four))
+    * scala> val map2 = Map(6 -> "Six", 7 -> "Seven")
+    * scala> keys1.collectFirstSomeM(parseInt(_) map map2.get)
+    * res2: scala.util.Either[String,Option[String]] = Right(None)
+    * scala> val keys2 = List("1", "x", "4", "5")
+    * scala> keys2.collectFirstSomeM(parseInt(_) map map1.get)
+    * res3: scala.util.Either[String,Option[String]] = Left(For input string: "x")
+    * scala> val keys3 = List("1", "2", "4", "x")
+    * scala> keys3.collectFirstSomeM(parseInt(_) map map1.get)
+    * res4: scala.util.Either[String,Option[String]] = Right(Some(Four))
+    * }}}
+    */
+  def collectFirstSomeM[G[_], B](f: A => G[Option[B]])(implicit F: Foldable[F], G: Monad[G]): G[Option[B]] =
+    F.foldRight(fa, Eval.now(OptionT.none[G, B]))((a, lb) =>
+      Eval.now(OptionT(f(a)).orElse(lb.value))
+    ).value.value
 }
diff --git a/tests/src/test/scala/cats/tests/FoldableSuite.scala b/tests/src/test/scala/cats/tests/FoldableSuite.scala
@@ -140,12 +140,20 @@ abstract class FoldableSuite[F[_]: Foldable](name: String)(
       fa.toList should === (iterator(fa).toList)
     }
   }
-  
+
   test(s"Foldable[$name] mkString_") {
     forAll { (fa: F[Int]) =>
       fa.mkString_("L[", ";", "]") should === (fa.toList.mkString("L[", ";", "]"))
     }
   }
+
+  test(s"Foldable[$name].collectFirstSomeM") {
+    forAll { (fa: F[Int], n: Int) =>
+      fa.collectFirstSomeM(x => (x > n).guard[Option].as(x).asRight[String]) should === (fa.toList.collectFirst {
+        case x if x > n => x
+      }.asRight[String])
+    }
+  }
 }
 
 class FoldableSuiteAdditional extends CatsSuite {