Merge pull request #3421 from pk044/fproductLeft

Functor - fproductLeft
typelevel · May 21, 2020 · 2bb358d · 2bb358d
2 parents 4465141 + 409015f
commit 2bb358d
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diff --git a/core/src/main/scala/cats/Functor.scala b/core/src/main/scala/cats/Functor.scala
@@ -99,6 +99,20 @@ import simulacrum.{noop, typeclass}
    */
   def fproduct[A, B](fa: F[A])(f: A => B): F[(A, B)] = map(fa)(a => a -> f(a))
 
+  /**
+   *  Pair the result of function application with `A`.
+   *
+   * Example:
+   * {{{
+   * scala> import cats.Functor
+   * scala> import cats.implicits.catsStdInstancesForOption
+   *
+   * scala> Functor[Option].fproductLeft(Option(42))(_.toString)
+   * res0: Option[(String, Int)] = Some((42,42))
+   * }}}
+   */
+  def fproductLeft[A, B](fa: F[A])(f: A => B): F[(B, A)] = map(fa)(a => f(a) -> a)
+
   /**
    * Replaces the `A` value in `F[A]` with the supplied value.
    *

diff --git a/docs/src/main/tut/nomenclature.md b/docs/src/main/tut/nomenclature.md
@@ -26,6 +26,7 @@ _WARNING_: this page is written manually, and not automatically generated, so ma
 | `F[A] => B => F[B]`  | `as`   |
 | `F[A] => (A => B) => F[B]` | `map`   |
 | `F[A] => (A => B) => F[(A,B)]` | `fproduct`   |
+| `F[A] => (A => B) => F[(B,A)]` | `fproductLeft`   |
 | `F[A] => B => F[(B, A)]`  | `tupleLeft`  |
 | `F[A] => B => F[(A, B)]`  | `tupleRight` |
 | `(A => B) => (F[A] => F[B])` | `lift`   |

diff --git a/tests/src/test/scala/cats/tests/SyntaxSuite.scala b/tests/src/test/scala/cats/tests/SyntaxSuite.scala
@@ -274,6 +274,7 @@ object SyntaxSuite {
     val fb0: F[B] = fa.map(f)
     val fu: F[Unit] = fa.void
     val fab: F[(A, B)] = fa.fproduct(f)
+    val fba: F[(B, A)] = fa.fproductLeft(f)
 
     val b = mock[B]
     val fb1: F[B] = fa.as(b)