.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-patmat:20191016-626d0012efc94653bff8736b2570386000f65ea2
+    name: registry.gitlab.com/fnux/cs210-grading-images/progfun2-codecs:20191027-dfbea8aed96096ed3af1cf1958549b97328d4c25
     entrypoint: [""]
   allow_failure: true
   before_script:

README.md

@@ -1,7 +1,6 @@
-# CS-210: For-comprehensions and Collections
-
+# CS-210: Codecs
 
 Please follow the [instructions from the main course
-respository](https://gitlab.epfl.ch/lamp/cs-210-functional-programming-2019/blob/master/week5/00-homework5.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 := "forcomp"
+course := "progfun2"
+assignment := "codecs"
 name := course.value + "-" + assignment.value
-testSuite := "forcomp.AnagramsSuite"
+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/forcomp/Anagrams.scala

@@ -1,174 +0,0 @@
-package forcomp
-
-object Anagrams extends AnagramsInterface {
-
-  /** A word is simply a `String`. */
-  type Word = String
-
-  /** A sentence is a `List` of words. */
-  type Sentence = List[Word]
-
-  /** `Occurrences` is a `List` of pairs of characters and positive integers saying
-   *  how often the character appears.
-   *  This list is sorted alphabetically w.r.t. to the character in each pair.
-   *  All characters in the occurrence list are lowercase.
-   *
-   *  Any list of pairs of lowercase characters and their frequency which is not sorted
-   *  is **not** an occurrence list.
-   *
-   *  Note: If the frequency of some character is zero, then that character should not be
-   *  in the list.
-   */
-  type Occurrences = List[(Char, Int)]
-
-  /** The dictionary is simply a sequence of words.
-   *  It is predefined and obtained as a sequence using the utility method `loadDictionary`.
-   */
-  val dictionary: List[Word] = Dictionary.loadDictionary
-
-  /** Converts the word into its character occurrence list.
-   *
-   *  Note: the uppercase and lowercase version of the character are treated as the
-   *  same character, and are represented as a lowercase character in the occurrence list.
-   *
-   *  Note: you must use `groupBy` to implement this method!
-   */
-  def wordOccurrences(w: Word): Occurrences = w.toLowerCase.toSeq.groupBy(e => e).view.mapValues(t => t.length).toList.sorted
-
-  /** Converts a sentence into its character occurrence list. */
-  def sentenceOccurrences(s: Sentence): Occurrences = wordOccurrences(s.mkString)
-
-  /** The `dictionaryByOccurrences` is a `Map` from different occurrences to a sequence of all
-   *  the words that have that occurrence count.
-   *  This map serves as an easy way to obtain all the anagrams of a word given its occurrence list.
-   *
-   *  For example, the word "eat" has the following character occurrence list:
-   *
-   *     `List(('a', 1), ('e', 1), ('t', 1))`
-   *
-   *  Incidentally, so do the words "ate" and "tea".
-   *
-   *  This means that the `dictionaryByOccurrences` map will contain an entry:
-   *
-   *    List(('a', 1), ('e', 1), ('t', 1)) -> Seq("ate", "eat", "tea")
-   *
-   */
-  lazy val dictionaryByOccurrences: Map[Occurrences, List[Word]] = dictionary.groupBy(wordOccurrences)
-
-  /** Returns all the anagrams of a given word. */
-  def wordAnagrams(word: Word): List[Word] = dictionaryByOccurrences(wordOccurrences(word))
-  
-  // val o : List[Occurrences] = occurrences.map( x => (for(i <- 1 until (x._2+1)) yield (x._1,i)).toList)
-  // o.foldRight(List[Occurrences](Nil))((x,y) => y ++ (for(i <- x; j <- y) yield (i :: j)))
-  /** Returns the list of all subsets of the occurrence list.
-   *  This includes the occurrence itself, i.e. `List(('k', 1), ('o', 1))`
-   *  is a subset of `List(('k', 1), ('o', 1))`.
-   *  It also include the empty subset `List()`.
-   *
-   *  Example: the subsets of the occurrence list `List(('a', 2), ('b', 2))` are:
-   *
-   *    List(
-   *      List(),
-   *      List(('a', 1)),
-   *      List(('a', 2)),
-   *      List(('b', 1)),
-   *      List(('a', 1), ('b', 1)),
-   *      List(('a', 2), ('b', 1)),
-   *      List(('b', 2)),
-   *      List(('a', 1), ('b', 2)),
-   *      List(('a', 2), ('b', 2))
-   *    )
-   *
-   *  Note that the order of the occurrence list subsets does not matter -- the subsets
-   *  in the example above could have been displayed in some other order.
-   */
-  def combinations(occurrences: Occurrences): List[Occurrences] = 
-    occurrences.map( x => (for(i <- 1 until (x._2+1)) yield (x._1,i)).toList)
-    .foldRight(List[Occurrences](Nil))((x,y) => 
-      y ::: (for(i <- x; j <- y) yield (i :: j)))
-
-  /** Subtracts occurrence list `y` from occurrence list `x`.
-   *
-   *  The precondition is that the occurrence list `y` is a subset of
-   *  the occurrence list `x` -- any character appearing in `y` must
-   *  appear in `x`, and its frequency in `y` must be smaller or equal
-   *  than its frequency in `x`.
-   *
-   *  Note: the resulting value is an occurrence - meaning it is sorted
-   *  and has no zero-entries.
-   */
-  def subtract(x: Occurrences, y: Occurrences): Occurrences = 
-    y.foldLeft(Map(x: _*))((map, pair) => {
-      map.updated(pair._1, map.apply(pair._1) - pair._2)
-    }).filter(e => e._2 !=0 ).toList.sorted
-
-  /** Returns a list of all anagram sentences of the given sentence.
-   *
-   *  An anagram of a sentence is formed by taking the occurrences of all the characters of
-   *  all the words in the sentence, and producing all possible combinations of words with those characters,
-   *  such that the words have to be from the dictionary.
-   *
-   *  The number of words in the sentence and its anagrams does not have to correspond.
-   *  For example, the sentence `List("I", "love", "you")` is an anagram of the sentence `List("You", "olive")`.
-   *
-   *  Also, two sentences with the same words but in a different order are considered two different anagrams.
-   *  For example, sentences `List("You", "olive")` and `List("olive", "you")` are different anagrams of
-   *  `List("I", "love", "you")`.
-   *
-   *  Here is a full example of a sentence `List("Yes", "man")` and its anagrams for our dictionary:
-   *
-   *    List(
-   *      List(en, as, my),
-   *      List(en, my, as),
-   *      List(man, yes),
-   *      List(men, say),
-   *      List(as, en, my),
-   *      List(as, my, en),
-   *      List(sane, my),
-   *      List(Sean, my),
-   *      List(my, en, as),
-   *      List(my, as, en),
-   *      List(my, sane),
-   *      List(my, Sean),
-   *      List(say, men),
-   *      List(yes, man)
-   *    )
-   *
-   *  The different sentences do not have to be output in the order shown above - any order is fine as long as
-   *  all the anagrams are there. Every returned word has to exist in the dictionary.
-   *
-   *  Note: in case that the words of the sentence are in the dictionary, then the sentence is the anagram of itself,
-   *  so it has to be returned in this list.
-   *
-   *  Note: There is only one anagram of an empty sentence.
-   */
-  def sentenceAnagrams(sentence: Sentence): List[Sentence] = {
-    def iter(occurrences: Occurrences): List[Sentence] = {
-      if (occurrences.isEmpty) List(Nil)
-      else for {
-        combination <- combinations( occurrences )
-        word <- dictionaryByOccurrences getOrElse (combination, Nil)
-        sentence <- iter( subtract(occurrences,wordOccurrences(word)) )
-        if !combination.isEmpty
-      } yield word :: sentence
-    }
-
-    iter( sentenceOccurrences(sentence) )
-}
-}
-
-object Dictionary {
-  def loadDictionary: List[String] =
-    val wordstream = Option {
-      getClass.getResourceAsStream(List("forcomp", "linuxwords.txt").mkString("/", "/", ""))
-    } getOrElse(sys.error("Could not load word list, dictionary file not found"))
-    try
-      val s = scala.io.Source.fromInputStream(wordstream)
-      s.getLines.toList
-    catch
-      case e: Exception =>
-        println("Could not load word list: " + e)
-        throw e
-    finally
-      wordstream.close()
-}

src/main/scala/forcomp/AnagramsInterface.scala

@@ -1,15 +0,0 @@
-package forcomp
-
-/**
- * The interface used by the grading infrastructure. Do not change signatures
- * or your submission will fail with a NoSuchMethodError.
- */
-trait AnagramsInterface {
-  def wordOccurrences(w: String): List[(Char, Int)]
-  def sentenceOccurrences(s: List[String]): List[(Char, Int)]
-  def dictionaryByOccurrences: Map[List[(Char, Int)], List[String]]
-  def wordAnagrams(word: String): List[String]
-  def combinations(occurrences: List[(Char, Int)]): List[List[(Char, Int)]]
-  def subtract(x: List[(Char, Int)], y: List[(Char, Int)]): List[(Char, Int)]
-  def sentenceAnagrams(sentence: List[String]): List[List[String]]
-}

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/forcomp/AnagramsSuite.scala

@@ -1,91 +0,0 @@
-package forcomp
-
-import org.junit._
-import org.junit.Assert.assertEquals
-
-
-class AnagramsSuite {
-  import Anagrams._
-
-  @Test def `wordOccurrences: abcd (3pts)`: Unit =
-    assertEquals(List(('a', 1), ('b', 1), ('c', 1), ('d', 1)), wordOccurrences("abcd"))
-
-  @Test def `wordOccurrences: Robert (3pts)`: Unit =
-    assertEquals(List(('b', 1), ('e', 1), ('o', 1), ('r', 2), ('t', 1)), wordOccurrences("Robert"))
-
-
-  @Test def `sentenceOccurrences: abcd e (5pts)`: Unit =
-    assertEquals(List(('a', 1), ('b', 1), ('c', 1), ('d', 1), ('e', 1)), sentenceOccurrences(List("abcd", "e")))
-
-
-  @Test def `dictionaryByOccurrences.get: eat (10pts)`: Unit =
-    assertEquals(Some(Set("ate", "eat", "tea")), dictionaryByOccurrences.get(List(('a', 1), ('e', 1), ('t', 1))).map(_.toSet))
-
-
-  @Test def `wordAnagrams married (2pts)`: Unit =
-    assertEquals(Set("married", "admirer"), wordAnagrams("married").toSet)
-
-  @Test def `wordAnagrams player (2pts)`: Unit =
-    assertEquals(Set("parley", "pearly", "player", "replay"), wordAnagrams("player").toSet)
-
-
-
-  @Test def `subtract: lard - r (10pts)`: Unit =
-    val lard = List(('a', 1), ('d', 1), ('l', 1), ('r', 1))
-    val r = List(('r', 1))
-    val lad = List(('a', 1), ('d', 1), ('l', 1))
-    assertEquals(lad, subtract(lard, r))
-
-
-  @Test def `combinations: [] (8pts)`: Unit =
-    assertEquals(List(Nil), combinations(Nil))
-
-  @Test def `combinations: abba (8pts)`: Unit =
-    val abba = List(('a', 2), ('b', 2))
-    val abbacomb = List(
-      List(),
-      List(('a', 1)),
-      List(('a', 2)),
-      List(('b', 1)),
-      List(('a', 1), ('b', 1)),
-      List(('a', 2), ('b', 1)),
-      List(('b', 2)),
-      List(('a', 1), ('b', 2)),
-      List(('a', 2), ('b', 2))
-    )
-    assertEquals(abbacomb.toSet, combinations(abba).toSet)
-
-
-  @Test def `sentence anagrams: [] (10pts)`: Unit =
-    val sentence = List()
-    assertEquals(List(Nil), sentenceAnagrams(sentence))
-
-  @Test def `sentence anagrams: Linux rulez (10pts)`: Unit =
-    val sentence = List("Linux", "rulez")
-    val anas = List(
-      List("Rex", "Lin", "Zulu"),
-      List("nil", "Zulu", "Rex"),
-      List("Rex", "nil", "Zulu"),
-      List("Zulu", "Rex", "Lin"),
-      List("null", "Uzi", "Rex"),
-      List("Rex", "Zulu", "Lin"),
-      List("Uzi", "null", "Rex"),
-      List("Rex", "null", "Uzi"),
-      List("null", "Rex", "Uzi"),
-      List("Lin", "Rex", "Zulu"),
-      List("nil", "Rex", "Zulu"),
-      List("Rex", "Uzi", "null"),
-      List("Rex", "Zulu", "nil"),
-      List("Zulu", "Rex", "nil"),
-      List("Zulu", "Lin", "Rex"),
-      List("Lin", "Zulu", "Rex"),
-      List("Uzi", "Rex", "null"),
-      List("Zulu", "nil", "Rex"),
-      List("rulez", "Linux"),
-      List("Linux", "rulez")
-    )
-    assertEquals(anas.toSet, sentenceAnagrams(sentence).toSet)
-
-
-  @Rule def individualTestTimeout = new org.junit.rules.Timeout(10 * 1000)
-}