From e799fd031bda1c42d9b5f68f93f7d1d790e543b6 Mon Sep 17 00:00:00 2001
From: Sumedh Mungee <smungee@gmail.com>
Date: Thu, 15 Oct 2015 15:11:52 -0700
Subject: [PATCH] Add FreeApplicative.analyze

In trying to port the FreeApplicative example from @jdegoes's ScalaWorld
2015 talk[1] to cats, I found that cats does not have a
FreeApplicative.analyze method, which allows us to compile a
FreeApplicative into a monoid.

This commit adds FreeApplicative.analyze, as well as a new test for
this, as per a discussion on gitter [2].

[1] https://github.com/jdegoes/scalaworld-2015/blob/master/src/main/scala/asmfree.scala#L32
[2] https://gitter.im/non/cats?at=561ed5373a0d354869528194
---
 .../src/main/scala/cats/free/FreeApplicative.scala |  7 +++++++
 .../scala/cats/free/FreeApplicativeTests.scala     | 14 ++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/free/src/main/scala/cats/free/FreeApplicative.scala b/free/src/main/scala/cats/free/FreeApplicative.scala
index 399ae67eaf..5766ec867e 100644
--- a/free/src/main/scala/cats/free/FreeApplicative.scala
+++ b/free/src/main/scala/cats/free/FreeApplicative.scala
@@ -2,6 +2,7 @@ package cats
 package free
 
 import cats.arrow.NaturalTransformation
+import cats.data.Const
 
 /** Applicative Functor for Free */
 sealed abstract class FreeApplicative[F[_], A] extends Product with Serializable { self =>
@@ -46,6 +47,12 @@ sealed abstract class FreeApplicative[F[_], A] extends Product with Serializable
       }
     }
 
+  /** Interpret this algebra into a Monoid */
+  def analyze[M:Monoid](f: F ~> λ[α => M]): M =
+    foldMap[Const[M, ?]](new (F ~> Const[M, ?]) {
+      def apply[X](x: F[X]): Const[M,X] = Const(f(x))
+    }).getConst
+
   /** Compile this FreeApplicative algebra into a Free algebra. */
   final def monad: Free[F, A] =
     foldMap[Free[F, ?]] {
diff --git a/free/src/test/scala/cats/free/FreeApplicativeTests.scala b/free/src/test/scala/cats/free/FreeApplicativeTests.scala
index e8308ccaae..7084d90bc7 100644
--- a/free/src/test/scala/cats/free/FreeApplicativeTests.scala
+++ b/free/src/test/scala/cats/free/FreeApplicativeTests.scala
@@ -4,6 +4,7 @@ package free
 import cats.arrow.NaturalTransformation
 import cats.laws.discipline.{ArbitraryK, ApplicativeTests, SerializableTests}
 import cats.tests.CatsSuite
+import cats.data.Const
 
 import org.scalacheck.{Arbitrary, Gen}
 
@@ -75,4 +76,17 @@ class FreeApplicativeTests extends CatsSuite {
     val fli2 = FreeApplicative.lift[List, Int](List(1, 3, 5, 7))
     (fli1 |@| fli2).map(_ + _)
   }
+
+  test("FreeApplicative#analyze") {
+    type G[A] = List[Int]
+    val countingNT = new NaturalTransformation[List, G] {
+      def apply[A](la: List[A]): G[A] = List(la.length)
+    }
+
+    val fli1 = FreeApplicative.lift[List, Int](List(1, 3, 5, 7))
+    fli1.analyze[G[Int]](countingNT) should === (List(4))
+
+    val fli2 = FreeApplicative.lift[List, Int](List.empty)
+    fli2.analyze[G[Int]](countingNT) should ===(List(0))
+  }
 }