Expose FunctionK.liftFunction as a part of the Scala 3 API

core/src/main/scala-2/cats/arrow/FunctionKMacros.scala

@@ -24,8 +24,7 @@ package arrow
 
 import scala.reflect.macros.blackbox
 
-private[arrow] class FunctionKMacroMethods {
-  protected type τ[F[_], G[_]]
+private[arrow] class FunctionKMacroMethods extends FunctionKLift {
 
   /**
    * Lifts function `f` of `F[A] => G[A]` into a `FunctionK[F, G]`.
@@ -48,22 +47,6 @@ private[arrow] class FunctionKMacroMethods {
    */
   def lift[F[_], G[_]](f: (F[α] => G[α]) forSome { type α }): FunctionK[F, G] =
     macro FunctionKMacros.lift[F, G]
-
-  /**
-   * Lifts function `f` of `F[A] => G[A]` into a `FunctionK[F, G]`.
-   *
-   * {{{
-   *   def headOption[A](list: List[A]): Option[A] = list.headOption
-   *   val lifted = FunctionK.liftFunction[List, Option](headOption)
-   * }}}
-   *
-   * Note: The weird `τ[F, G]` parameter is there to compensate for
-   * the lack of polymorphic function types in Scala 2.
-   */
-  def liftFunction[F[_], G[_]](f: F[τ[F, G]] => G[τ[F, G]]): FunctionK[F, G] =
-    new FunctionK[F, G] {
-      def apply[A](fa: F[A]): G[A] = f.asInstanceOf[F[A] => G[A]](fa)
-    }
 }
 
 private[arrow] object FunctionKMacros {

core/src/main/scala-3/cats/arrow/FunctionKMacros.scala

@@ -22,8 +22,7 @@
 package cats
 package arrow
 
-private[arrow] class FunctionKMacroMethods {
-
+private[arrow] class FunctionKMacroMethods extends FunctionKLift {
   /**
    * Lifts function `f` of `[X] => F[X] => G[X]` into a `FunctionK[F, G]`.
    *

core/src/main/scala/cats/arrow/FunctionKLift.scala

@@ -0,0 +1,45 @@
+/*
+ * Copyright (c) 2015 Typelevel
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to
+ * use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+ * the Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+ * IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+package cats
+package arrow
+
+private[arrow] trait FunctionKLift {
+  protected type τ[F[_], G[_]]
+
+  /**
+   * Lifts function `f` of `F[A] => G[A]` into a `FunctionK[F, G]`.
+   *
+   * {{{
+   *   def headOption[A](list: List[A]): Option[A] = list.headOption
+   *   val lifted = FunctionK.liftFunction[List, Option](headOption)
+   * }}}
+   *
+   * Note: The weird `τ[F, G]` parameter is there to compensate for
+   * the lack of polymorphic function types in Scala 2.
+   * 
+   * It is present in the Scala 3 API to simplify cross-compilation.
+   */
+  def liftFunction[F[_], G[_]](f: F[τ[F, G]] => G[τ[F, G]]): FunctionK[F, G] =
+    new FunctionK[F, G] {
+      def apply[A](fa: F[A]): G[A] = f.asInstanceOf[F[A] => G[A]](fa)
+    }
+}

tests/shared/src/test/scala-3/cats/tests/FunctionKLiftSuite.scala

@@ -34,4 +34,12 @@ class FunctionKLiftSuite extends CatsSuite {
       assert(fHeadOption(a) === a.headOption)
     }
   }
+
+  test("lift a function directly using Scala 2 compatible syntax") {
+    def headOption[A](list: List[A]): Option[A] = list.headOption
+    val fHeadOption = FunctionK.liftFunction[List, Option](headOption)
+    forAll { (a: List[Int]) =>
+      assert(fHeadOption(a) === a.headOption)
+    }
+  }
 }