.gitlab-ci.yml

@@ -7,7 +7,9 @@ stages:
 
 compile:
   stage: build
-  image: lampepfl/moocs-dotty:2019-09-17-2
+  image: lampepfl/moocs-dotty:2019-10-16
+  except:
+    - tags
   tags:
     - cs210
   script:
@@ -19,10 +21,12 @@ compile:
 
 grade:
   stage: grade
+  except:
+    - tags
   tags:
     - cs210
   image:
-    name: registry.gitlab.com/fnux/cs210-grading-images/progfun1-funsets:20190919-626d0012efc94653bff8736b2570386000f65ea2
+    name: registry.gitlab.com/fnux/cs210-grading-images/progfun2-codecs:20191027-dfbea8aed96096ed3af1cf1958549b97328d4c25
     entrypoint: [""]
   allow_failure: true
   before_script:

README.md

@@ -1,6 +1,6 @@
-# CS-210: Purely Functional Sets
+# CS-210: Codecs
 
 Please follow the [instructions from the main course
-respository](https://gitlab.epfl.ch/lamp/cs-210-functional-programming-2019/blob/master/week2/00-homework2.md).
+respository](https://gitlab.epfl.ch/lamp/cs-210-functional-programming-2019/blob/master/week11/00-homework8.md).
 
 Grading and submission details can be found [here](https://gitlab.epfl.ch/lamp/cs-210-functional-programming-2019/blob/master/week1/02-grading-and-submission.md).

build.sbt

@@ -1,12 +1,16 @@
-course := "progfun1"
-assignment := "funsets"
+course := "progfun2"
+assignment := "codecs"
 name := course.value + "-" + assignment.value
-testSuite := "funsets.FunSetSuite"
+testSuite := "codecs.CodecsSuite"
 
-scalaVersion := "0.19.0-bin-20190918-dd68eb8-NIGHTLY"
-
-scalacOptions ++= Seq("-language:implicitConversions", "-deprecation")
-
-libraryDependencies += "com.novocode" % "junit-interface" % "0.11" % Test
+scalaVersion := "0.19.0-RC1"
+scalacOptions ++= Seq("-deprecation")
+libraryDependencies ++= Seq(
+  ("org.scalacheck" %% "scalacheck" % "1.14.2" % Test).withDottyCompat(scalaVersion.value),
+  ("org.typelevel" %% "jawn-parser" % "0.14.2").withDottyCompat(scalaVersion.value),
+  "com.novocode" % "junit-interface" % "0.11" % Test
+)
 
 testOptions in Test += Tests.Argument(TestFrameworks.JUnit, "-a", "-v", "-s")
+
+initialCommands in console := """import codecs.{_, given}"""

project/MOOCSettings.scala

@@ -18,6 +18,8 @@ object MOOCSettings extends AutoPlugin {
   override def trigger = allRequirements
 
   override val projectSettings: Seq[Def.Setting[_]] = Seq(
-    parallelExecution in Test := false
+    parallelExecution in Test := false,
+    // Report test result after each test instead of waiting for every test to finish
+    logBuffered in Test := false
   )
 }

project/buildSettings.sbt

@@ -1,4 +1,3 @@
-libraryDependencies += "com.novocode" % "junit-interface" % "0.11" % Test
 // Used for base64 encoding
 libraryDependencies += "commons-codec" % "commons-codec" % "1.10"
 

src/main/scala/codecs/codecs.scala

@@ -0,0 +1,283 @@
+package codecs
+
+/**
+  * A data type modeling JSON values.
+  *
+  * For example, the `42` integer JSON value can be modeled as `Json.Num(42)`
+  */
+sealed trait Json {
+  /**
+   * Try to decode this JSON value into a value of type `A` by using
+   * the given decoder.
+   *
+   * Note that you have to explicitly fix `A` type parameter when you call the method:
+   *
+   * {{{
+   *   someJsonValue.decodeAs[User] // OK
+   *   someJsonValue.decodeAs       // Wrong!
+   * }}}
+   */
+  def decodeAs[A](given decoder: Decoder[A]): Option[A] = decoder.decode(this)
+}
+
+object Json {
+  /** The JSON `null` value */
+  case object Null extends Json
+  /** JSON boolean values */
+  case class Bool(value: Boolean) extends Json
+  /** JSON numeric values */
+  case class Num(value: BigDecimal) extends Json
+  /** JSON string values */
+  case class Str(value: String) extends Json
+  /** JSON objects */
+  case class Obj(fields: Map[String, Json]) extends Json
+  /** JSON arrays */
+  case class Arr(items: List[Json]) extends Json
+}
+
+/**
+  * A type class that turns a value of type `A` into its JSON representation.
+  */
+trait Encoder[-A] {
+
+  def encode(value: A): Json
+
+    /**
+    * Transforms this `Encoder[A]` into an `Encoder[B]`, given a transformation function
+    * from `B` to `A`.
+    *
+    * For instance, given a `Encoder[String]`, we can get an `Encoder[UUID]`:
+    *
+    * {{{
+    *   def uuidEncoder(given stringEncoder: Encoder[String]): Encoder[UUID] =
+    *     stringEncoder.transform[UUID](uuid => uuid.toString)
+    * }}}
+    *
+    * This operation is also known as ?contramap?.
+    */
+  def transform[B](f: B => A): Encoder[B] =
+    Encoder.fromFunction[B](value => this.encode(f(value)))  
+}
+
+object Encoder extends GivenEncoders {
+
+  /**
+   * Convenient method for creating an instance of encoder from a function `f`
+   */
+  def fromFunction[A](f: A => Json) = new Encoder[A] {
+    def encode(value: A): Json = f(value)
+  }
+
+}
+
+trait GivenEncoders {
+
+  /** An encoder for the `Unit` value */
+  given Encoder[Unit] = Encoder.fromFunction(_ => Json.Null)
+
+  /** An encoder for `Int` values */
+  given Encoder[Int] = Encoder.fromFunction(n => Json.Num(BigDecimal(n)))
+
+  /** An encoder for `String` values */
+  given Encoder[String] =
+    Encoder.fromFunction(str => Json.Str(str))
+
+  /** An encoder for `Boolean` values */
+  given Encoder[Boolean] =
+    Encoder.fromFunction(v => Json.Bool(v))
+
+  /**
+    * Encodes a list of values of type `A` into a JSON array containing
+    * the list elements encoded with the given `encoder`
+    */
+  given [A](given encoder: Encoder[A]): Encoder[List[A]] =
+    Encoder.fromFunction(as => Json.Arr(as.map(encoder.encode)))
+
+}
+
+/**
+  * A specialization of `Encoder` that returns JSON objects only
+  */
+trait ObjectEncoder[-A] extends Encoder[A] {
+  // Refines the encoding result to `Json.Obj`
+  def encode(value: A): Json.Obj
+
+  /**
+    * Combines `this` encoder with `that` encoder.
+    * Returns an encoder producing a JSON object containing both
+    * fields of `this` encoder and fields of `that` encoder.
+    */
+  def zip[B](that: ObjectEncoder[B]): ObjectEncoder[(A, B)] =
+    ObjectEncoder.fromFunction { (a, b) =>
+      Json.Obj(this.encode(a).fields ++ that.encode(b).fields)
+    }
+}
+
+object ObjectEncoder {
+
+  /**
+    * Convenient method for creating an instance of object encoder from a function `f`
+    */
+  def fromFunction[A](f: A => Json.Obj): ObjectEncoder[A] = new ObjectEncoder[A] {
+    def encode(value: A): Json.Obj = f(value)
+  }
+
+  /**
+    * An encoder for values of type `A` that produces a JSON object with one field
+    * named according to the supplied `name` and containing the encoded value.
+    */
+  def field[A](name: String)(given encoder: Encoder[A]): ObjectEncoder[A] =
+    ObjectEncoder.fromFunction(a => Json.Obj(Map(name -> encoder.encode(a))))
+
+}
+
+/**
+  * The dual of an encoder. Decodes a serialized value into its initial type `A`.
+  */
+trait Decoder[+A] {
+  /**
+    * @param data The data to de-serialize
+    * @return The decoded value wrapped in `Some`, or `None` if decoding failed
+    */
+  def decode(data: Json): Option[A]
+
+  /**
+    * Combines `this` decoder with `that` decoder.
+    * Returns a decoder that invokes both `this` decoder and `that`
+    * decoder and returns a pair of decoded value in case both succeed,
+    * or `None` if at least one failed.
+    */
+  def zip[B](that: Decoder[B]): Decoder[(A, B)] =
+    Decoder.fromFunction { json =>
+      this.decode(json).zip(that.decode(json))
+    }
+
+  /**
+    * Transforms this `Decoder[A]` into a `Decoder[B]`, given a transformation function
+    * from `A` to `B`.
+    *
+    * This operation is also known as ?map?.
+    */
+  def transform[B](f: A => B): Decoder[B] =
+    Decoder.fromFunction(json => this.decode(json).map(f))
+}
+
+object Decoder extends GivenDecoders {
+
+  /**
+    * Convenient method to build a decoder instance from a function `f`
+    */
+  def fromFunction[A](f: Json => Option[A]): Decoder[A] = new Decoder[A] {
+    def decode(data: Json): Option[A] = f(data)
+  }
+
+  /**
+    * Alternative method for creating decoder instances
+    */
+  def fromPartialFunction[A](pf: PartialFunction[Json, A]): Decoder[A] =
+    fromFunction(pf.lift)
+
+}
+
+trait GivenDecoders {
+
+  /** A decoder for the `Unit` value */
+  given Decoder[Unit] =
+    Decoder.fromPartialFunction { case Json.Null => () }
+
+  /** A decoder for `Int` values. Hint: use the `isValidInt` method of `BigDecimal`. */
+  // TODO Define a given `Decoder[Int]` instance
+  given Decoder[Int] = 
+    Decoder.fromFunction{ case Json.Num(v) => if v.isValidInt then Some(v.intValue) else None
+                          case _ => None}
+
+  /** A decoder for `String` values */
+  // TODO Define a given `Decoder[String]` instance
+  given Decoder[String] = 
+    Decoder.fromPartialFunction{ case Json.Str(str) => str}
+
+  /** A decoder for `Boolean` values */
+  // TODO Define a given `Decoder[Boolean]` instance
+  given Decoder[Boolean] = 
+    Decoder.fromPartialFunction{ case Json.Bool(v) => v}
+  /**
+    * A decoder for JSON arrays. It decodes each item of the array
+    * using the given `decoder`. The resulting decoder succeeds only
+    * if all the JSON array items are successfully decoded.
+    */
+  given [A](given decoder: Decoder[A]): Decoder[List[A]] =
+    Decoder.fromFunction {
+      case Json.Arr(items: List[Json]) => Some(items.map(v => decoder.decode(v).get))
+      case _ => None
+    }
+
+  /**
+    * A decoder for JSON objects. It decodes the value of a field of
+    * the supplied `name` using the given `decoder`.
+    */
+  def field[A](name: String)(given decoder: Decoder[A]): Decoder[A] =
+    Decoder.fromFunction{
+      case Json.Obj(field: Map[String, Json]) => decoder.decode(field.get(name).get)
+      case _ => None
+    }
+
+}
+
+case class Person(name: String, age: Int)
+
+object Person extends PersonCodecs
+
+trait PersonCodecs {
+
+  /** The encoder for `Person` */
+  given Encoder[Person] =
+    ObjectEncoder.field[String]("name")
+      .zip(ObjectEncoder.field[Int]("age"))
+      .transform[Person](user => (user.name, user.age))
+
+  /** The corresponding decoder for `Person` */
+  given Decoder[Person] ={
+    Decoder.field[String]("name").zip(Decoder.field[Int]("age")).transform[Person](user => Person(user._1, user._2))
+    }
+
+}
+
+case class Contacts(people: List[Person])
+
+object Contacts extends ContactsCodecs
+
+trait ContactsCodecs {
+
+  // TODO Define the encoder and the decoder for `Contacts`
+  // The JSON representation of a value of type `Contacts` should be
+  // a JSON object with a single field named ?people? containing an
+  // array of values of type `Person` (reuse the `Person` codecs)
+  given Encoder[Contacts] = 
+    ObjectEncoder.field[List[Person]]("people").transform[Contacts](c => c.people)
+
+  given Decoder[Contacts] = 
+    Decoder.field[List[Person]]("people").transform[Contacts](p => Contacts(p))
+
+}
+
+// In case you want to try your code, here is a simple `Main`
+// that can be used as a starting point. Otherwise, you can use
+// the REPL (use the `console` sbt task).
+object Main {
+
+  def main(args: Array[String]): Unit = {
+    println(renderJson(42))
+    println(renderJson("foo"))
+
+    val maybeJsonString = parseJson(""" "foo" """)
+    val maybeJsonObj    = parseJson(""" { "name": "Alice", "age": 42 } """)
+    val maybeJsonObj2   = parseJson(""" { "name": "Alice", "age": "42" } """)
+    // Uncomment the following lines as you progress in the assignment
+    println(maybeJsonString.flatMap(_.decodeAs[Int]))
+    println(maybeJsonString.flatMap(_.decodeAs[String]))
+    println(maybeJsonObj.flatMap(_.decodeAs[Person]))
+    println(maybeJsonObj2.flatMap(_.decodeAs[Person]))
+    println(renderJson(Person("Bob", 66)))
+  }
+
+}

src/main/scala/codecs/json.scala

@@ -0,0 +1,74 @@
+package codecs
+
+import org.typelevel.jawn.{ Parser, SimpleFacade }
+import scala.collection.mutable
+import scala.util.Try
+
+// Utility methods that decode values from `String` JSON blobs, and
+// render values to `String` JSON blobs
+
+/**
+ * Parse a JSON document contained in a `String` value into a `Json` value, returns
+ * `None` in case the supplied `s` value is not a valid JSON document.
+ */
+def parseJson(s: String): Option[Json] = Parser.parseFromString[Json](s).toOption
+
+/**
+  * Parse the JSON value from the supplied `s` parameter, and then try to decode
+  * it as a value of type `A` using the given `decoder`.
+  *
+  * Returns `None` if JSON parsing failed, or if decoding failed.
+  */
+def parseAndDecode[A](s: String)(given decoder: Decoder[A]): Option[A] =
+  for {
+    json <- parseJson(s)
+    a    <- decoder.decode(json)
+  } yield a
+
+/**
+  * Render the supplied `value` into JSON using the given `encoder`.
+  */
+def renderJson[A](value: A)(given encoder: Encoder[A]): String =
+  render(encoder.encode(value))
+
+private def render(json: Json): String = json match {
+  case Json.Null    => "null"
+  case Json.Bool(b) => b.toString
+  case Json.Num(n)  => n.toString
+  case Json.Str(s)  => renderString(s)
+  case Json.Arr(vs) => vs.map(render).mkString("[", ",", "]")
+  case Json.Obj(vs) => vs.map { case (k, v) => s"${renderString(k)}:${render(v)}" }.mkString("{", ",", "}")
+}
+
+private def renderString(s: String): String = {
+  val sb = new StringBuilder
+  sb.append('"')
+  var i = 0
+  val len = s.length
+  while (i < len) {
+    s.charAt(i) match {
+      case '"' => sb.append("\\\"")
+      case '\\' => sb.append("\\\\")
+      case '\b' => sb.append("\\b")
+      case '\f' => sb.append("\\f")
+      case '\n' => sb.append("\\n")
+      case '\r' => sb.append("\\r")
+      case '\t' => sb.append("\\t")
+      case c =>
+        if (c < ' ') sb.append("\\u%04x" format c.toInt)
+        else sb.append(c)
+    }
+    i += 1
+  }
+  sb.append('"').toString
+}
+
+given SimpleFacade[Json] {
+  def jnull() = Json.Null
+  def jtrue() = Json.Bool(true)
+  def jfalse() = Json.Bool(false)
+  def jnum(s: CharSequence, decIndex: Int, expIndex: Int) = Json.Num(BigDecimal(s.toString))
+  def jstring(s: CharSequence) = Json.Str(s.toString)
+  def jarray(vs: List[Json]) = Json.Arr(vs)
+  def jobject(vs: Map[String, Json]) = Json.Obj(vs)
+}

src/main/scala/funsets/FunSets.scala

@@ -1,89 +0,0 @@
-package funsets
-
-/**
- * 2. Purely Functional Sets.
- */
-trait FunSets extends FunSetsInterface {
-  /**
-   * We represent a set by its characteristic function, i.e.
-   * its `contains` predicate.
-   */
-  override type FunSet = Int => Boolean
-
-  /**
-   * Indicates whether a set contains a given element.
-   */
-  def contains(s: FunSet, elem: Int): Boolean = s(elem)
-
-  /**
-   * Returns the set of the one given element.
-   */
-  def singletonSet(elem: Int): FunSet = (e: Int) => e == elem
-
-  /**
-   * Returns the union of the two given sets,
-   * the sets of all elements that are in either `s` or `t`.
-   */
-  def union(s: FunSet, t: FunSet): FunSet = (e: Int) => s(e) || t(e)
-  
-  /**
-   * Returns the intersection of the two given sets,
-   * the set of all elements that are both in `s` and `t`.
-   */
-  def intersect(s: FunSet, t: FunSet): FunSet = (e: Int) => s(e) && t(e)
-
-  /**
-   * Returns the difference of the two given sets,
-   * the set of all elements of `s` that are not in `t`.
-   */
-  def diff(s: FunSet, t: FunSet): FunSet = (e: Int) => s(e) && !t(e)
-
-  /**
-   * Returns the subset of `s` for which `p` holds.
-   */
-  def filter(s: FunSet, p: Int => Boolean): FunSet = (e: Int) => s(e) && p(e)
-
-  /**
-   * The bounds for `forall` and `exists` are +/- 1000.
-   */
-  val bound = 1000
-
-  /**
-   * Returns whether all bounded integers within `s` satisfy `p`.
-   */
-  def forall(s: FunSet, p: Int => Boolean): Boolean =
-    def iter(a: Int): Boolean =
-      if a == bound + 1 then
-        true
-      else if !p(a) && s(a) then
-        false
-      else
-        iter(a+1)
-    iter(-bound)
-
-  /**
-   * Returns whether there exists a bounded integer within `s`
-   * that satisfies `p`.
-   */
-  def exists(s: FunSet, p: Int => Boolean): Boolean = !forall(s, (e: Int) => !p(e))
-
-  /**
-   * Returns a set transformed by applying `f` to each element of `s`.
-   */
-  def map(s: FunSet, f: Int => Int): FunSet = (e :Int) => exists(s, (x: Int)=> e == f(x))
-
-  /**
-   * Displays the contents of a set
-   */
-  def toString(s: FunSet): String =
-    val xs = for i <- (-bound to bound) if contains(s, i) yield i
-    xs.mkString("{", ",", "}")
-
-  /**
-   * Prints the contents of a set on the console.
-   */
-  def printSet(s: FunSet): Unit =
-    println(toString(s))
-}
-
-object FunSets extends FunSets

src/main/scala/funsets/FunSetsInterface.scala

@@ -1,20 +0,0 @@
-package funsets
-
-/**
- * The interface used by the grading infrastructure. You should not edit any
- * code here, or your submission may fail with a NoSuchMethodError.
- */
-trait FunSetsInterface {
-  type FunSet = Int => Boolean
-
-  def contains(s: FunSet, elem: Int): Boolean
-  def singletonSet(elem: Int): FunSet
-  def union(s: FunSet, t: FunSet): FunSet
-  def intersect(s: FunSet, t: Int => Boolean): FunSet
-  def diff(s: FunSet, t: FunSet): FunSet
-  def filter(s: FunSet, p: Int => Boolean): FunSet
-  def forall(s: FunSet, p: Int => Boolean): Boolean
-  def exists(s: FunSet, p: Int => Boolean): Boolean
-  def map(s: FunSet, f: Int => Int): FunSet
-  def toString(s: FunSet): String
-}

src/main/scala/funsets/Main.scala

@@ -1,6 +0,0 @@
-package funsets
-
-object Main extends App {
-  import FunSets._
-  println(contains(singletonSet(1), 1))
-}

src/test/scala/codecs/CodecsSuite.scala

@@ -0,0 +1,115 @@
+package codecs
+
+import org.scalacheck
+import org.scalacheck.{ Gen, Prop }
+import org.scalacheck.Prop.propBoolean
+import org.junit.{ Assert, Test }
+import scala.reflect.ClassTag
+
+class CodecsSuite extends GivenEncoders, GivenDecoders, PersonCodecs, ContactsCodecs, TestEncoders, TestDecoders {
+
+  def checkProperty(prop: Prop): Unit = {
+    val result = scalacheck.Test.check(scalacheck.Test.Parameters.default, prop)
+    def fail(labels: Set[String], fallback: String): Nothing =
+      if labels.isEmpty then throw new AssertionError(fallback)
+      else throw new AssertionError(labels.mkString(". "))
+    result.status match {
+      case scalacheck.Test.Passed | _: scalacheck.Test.Proved => ()
+      case scalacheck.Test.Failed(_, labels)                  => fail(labels, "A property failed.")
+      case scalacheck.Test.PropException(_, e, labels)        => fail(labels, s"An exception was thrown during property evaluation: $e.")
+      case scalacheck.Test.Exhausted                          => fail(Set.empty, "Unable to generate data.")
+    }
+  }
+
+  /**
+    * Check that a value of an arbitrary type `A` can be encoded and then successfully
+    * decoded with the given pair of encoder and decoder.
+    */
+  def encodeAndThenDecodeProp[A](a: A)(given encA: Encoder[A], decA: Decoder[A]): Prop = {
+    val maybeDecoded = decA.decode(encA.encode(a))
+    maybeDecoded.contains(a) :| s"Encoded value '$a' was not successfully decoded. Got '$maybeDecoded'."
+  }
+
+  @Test def `it is possible to encode and decode the 'Unit' value (0pts)`(): Unit = {
+    checkProperty(Prop.forAll((unit: Unit) => encodeAndThenDecodeProp(unit)))
+  }
+
+  @Test def `it is possible to encode and decode 'Int' values (1pt)`(): Unit = {
+    checkProperty(Prop.forAll((x: Int) => encodeAndThenDecodeProp(x)))
+  }
+
+  @Test def `the 'Int' decoder should reject invalid 'Int' values (2pts)`(): Unit = {
+    val decoded = summon[Decoder[Int]].decode(Json.Num(4.2))
+    assert(decoded.isEmpty, "decoding 4.2 as an integer value should fail")
+  }
+
+  @Test def `a 'String' value should be encoded as a JSON string (1pt)`(): Unit = {
+    assert(summon[Encoder[String]].encode("foo") == Json.Str("foo"))
+  }
+
+  @Test def `it is possible to encode and decode 'String' values (1pt)`(): Unit = {
+    checkProperty(Prop.forAll((s: String) => encodeAndThenDecodeProp(s)))
+  }
+
+  @Test def `a 'Boolean' value should be encoded as a JSON boolean (1pt)`(): Unit = {
+    val encoder = summon[Encoder[Boolean]]
+    assert(encoder.encode(true) == Json.Bool(true))
+    assert(encoder.encode(false) == Json.Bool(false))
+  }
+
+  @Test def `it is possible to encode and decode 'Boolean' values (1pt)`(): Unit = {
+    checkProperty(Prop.forAll((b: Boolean) => encodeAndThenDecodeProp(b)))
+  }
+
+  @Test def `a 'List[A]' value should be encoded as a JSON array (0pts)`(): Unit = {
+    val xs = 1 :: 2 :: Nil
+    val encoder = summon[Encoder[List[Int]]]
+    assert(encoder.encode(xs) == Json.Arr(List(Json.Num(1), Json.Num(2))))
+  }
+
+  @Test def `it is possible to encode and decode lists (5pts)`(): Unit = {
+    checkProperty(Prop.forAll((xs: List[Int]) => encodeAndThenDecodeProp(xs)))
+  }
+
+  @Test def `a 'Person' value should be encoded as a JSON object (1pt)`(): Unit = {
+    val person = Person("Alice", 42)
+    val json = Json.Obj(Map("name" -> Json.Str("Alice"), "age" -> Json.Num(42)))
+    val encoder = summon[Encoder[Person]]
+    assert(encoder.encode(person) == json)
+  }
+
+  @Test def `it is possible to encode and decode people (4pts)`(): Unit = {
+    checkProperty(Prop.forAll((s: String, x: Int) => encodeAndThenDecodeProp(Person(s, x))))
+  }
+  
+  @Test def `a 'Contacts' value should be encoded as a JSON object (1pt)`(): Unit = {
+    val contacts = Contacts(List(Person("Alice", 42)))
+    val json = Json.Obj(Map("people" ->
+      Json.Arr(List(Json.Obj(Map("name" -> Json.Str("Alice"), "age" -> Json.Num(42)))))
+    ))
+    val encoder = summon[Encoder[Contacts]]
+    assert(encoder.encode(contacts) == json)
+  }
+
+  @Test def `it is possible to encode and decode contacts (4pts)`(): Unit = {
+    val peopleGenerator = Gen.listOf(Gen.resultOf((s: String, x: Int) => Person(s, x)))
+    checkProperty(Prop.forAll(peopleGenerator)(people => encodeAndThenDecodeProp(Contacts(people))))
+  }
+
+}
+
+trait TestEncoders extends EncoderFallbackInstance
+
+trait EncoderFallbackInstance {
+
+  given [A](given ct: ClassTag[A]): Encoder[A] = throw new AssertionError(s"No given instance of `Encoder[${ct.runtimeClass.getSimpleName}]`")
+
+}
+
+trait TestDecoders extends DecoderFallbackInstance
+
+trait DecoderFallbackInstance {
+
+  given [A](given ct: ClassTag[A]): Decoder[A] = throw new AssertionError(s"No given instance of `Decoder[${ct.runtimeClass.getSimpleName}]")
+
+}

src/test/scala/funsets/FunSetSuite.scala

@@ -1,73 +0,0 @@
-package funsets
-
-import org.junit._
-
-/**
- * This class is a test suite for the methods in object FunSets.
- *
- * To run this test suite, start "sbt" then run the "test" command.
- */
-class FunSetSuite {
-
-  import FunSets._
-
-  @Test def `contains is implemented`: Unit =
-    assert(contains(x => true, 100))
-
-  /**
-   * When writing tests, one would often like to re-use certain values for multiple
-   * tests. For instance, we would like to create an Int-set and have multiple test
-   * about it.
-   *
-   * Instead of copy-pasting the code for creating the set into every test, we can
-   * store it in the test class using a val:
-   *
-   *   val s1 = singletonSet(1)
-   *
-   * However, what happens if the method "singletonSet" has a bug and crashes? Then
-   * the test methods are not even executed, because creating an instance of the
-   * test class fails!
-   *
-   * Therefore, we put the shared values into a separate trait (traits are like
-   * abstract classes), and create an instance inside each test method.
-   *
-   */
-
-  trait TestSets {
-    val s1 = singletonSet(1)
-    val s2 = singletonSet(2)
-    val s3 = singletonSet(3)
-  }
-
-  /**
-   * This test is currently disabled (by using @Ignore) because the method
-   * "singletonSet" is not yet implemented and the test would fail.
-   *
-   * Once you finish your implementation of "singletonSet", remvoe the
-   * @Ignore annotation.
-   */
-  @Ignore("not ready yet") @Test def `singleton set one contains one`: Unit =
-    /**
-     * We create a new instance of the "TestSets" trait, this gives us access
-     * to the values "s1" to "s3".
-     */
-    new TestSets {
-      /**
-       * The string argument of "assert" is a message that is printed in case
-       * the test fails. This helps identifying which assertion failed.
-       */
-      assert(contains(s1, 1), "Singleton")
-    }
-
-  @Test def `union contains all elements of each set`: Unit =
-    new TestSets {
-      val s = union(s1, s2)
-      assert(contains(s, 1), "Union 1")
-      assert(contains(s, 2), "Union 2")
-      assert(!contains(s, 3), "Union 3")
-    }
-
-
-
-  @Rule def individualTestTimeout = new org.junit.rules.Timeout(10 * 1000)
-}