lshoo · June 17, 2016 08:01 · Jun 17, 2016
diff --git a/scala b/scala
@@ -0,0 +1,60 @@
+sealed trait KVStoreA[A]
+case class Put[A](key: String, value: A) extends KVStoreA[Unit]
+case class Get[A](key: String) extends KVStoreA[A]
+case class Delete(key: String) extends KVStoreA[Unit]
+
+import cats.free.Free
+type KVStore[A] = Free[KVStoreA, A]
+
+import cats.free.Free.liftF
+
+def put[T](key: String, value: T): KVStore[Unit] = liftF[KVStoreA, Unit](Put(key, value))
+
+def get[T](key: String): KVStore[Option[T]] = liftF[KVStoreA, Option[T]](Get(key))
+
+def delete(key: String): KVStore[Unit] = liftF[KVStoreA, Unit](Delete(key))
+
+def update[T](key: String, f: T => T): KVStore[Unit] = for {
+  vMaybe <- get[T](key)
+  _ <-  vMaybe.map(v => put[T](key, f(v))).getOrElse(Free.pure(()))
+} yield ()
+
+// 3. Build a program
+val key1 = "wild-cats"
+val key2 = "tame-cats"
+
+def program: KVStore[Option[Int]] = for {
+  _ <- put(key1, 2)
+  _ <- update[Int](key1, (_ + 3))
+  _ <- put(key2, 5)
+  n <- get[Int](key1)
+  _ <- delete(key2)
+} yield n
+
+import cats.arrow.NaturalTransformation
+import cats.{Id, ~>}
+import scala.collection.mutable
+
+def impureCompiler: KVStoreA ~> Id  =
+  new (KVStoreA ~> Id) {
+
+    // a very simple (and imprecise) key-value store
+    val kvs = mutable.Map.empty[String, Any]
+
+    def apply[A](fa: KVStoreA[A]): Id[A] =
+      fa match {
+        case Put(key, value) =>
+          println(s"put($key, $value)")
+          kvs(key) = value
+          ()
+        case Get(key) =>
+          println(s"get($key)")
+          kvs.get(key).map(_.asInstanceOf[A])
+        case Delete(key) =>
+          println(s"delete($key)")
+          kvs.remove(key)
+          ()
+      }
+  }
+
+  val result: Option[Int] = program.foldMap(impureCompiler)
No results found