.gitlab-ci.yml

@@ -7,7 +7,7 @@ stages:
 
 compile:
   stage: build
-  image: lampepfl/moocs-dotty:2019-10-16
+  image: registry.gitlab.com/fnux/cs210-grading-images/progfun2-interpreter-build:latest
   except:
     - tags
   tags:
@@ -26,7 +26,7 @@ grade:
   tags:
     - cs210
   image:
-    name: registry.gitlab.com/fnux/cs210-grading-images/progfun2-codecs:20191027-dfbea8aed96096ed3af1cf1958549b97328d4c25
+    name: registry.gitlab.com/fnux/cs210-grading-images/progfun2-interpreter:20191205-066e2155681d802554c93c569bbf9871921a3ca1
     entrypoint: [""]
   allow_failure: true
   before_script:

README.md

@@ -1,6 +1,6 @@
-# CS-210: Codecs
+# CS-210: Interpreter
 
 Please follow the [instructions from the main course
-respository](https://gitlab.epfl.ch/lamp/cs-210-functional-programming-2019/blob/master/week11/00-homework8.md).
+respository](https://gitlab.epfl.ch/lamp/cs-210-functional-programming-2019/blob/master/week12/00-homework9.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

@@ -1,16 +1,12 @@
 course := "progfun2"
-assignment := "codecs"
+assignment := "newInterpreter"
 name := course.value + "-" + assignment.value
-testSuite := "codecs.CodecsSuite"
+testSuite := "newInterpreter.RecursiveLanguageSuite"
 
 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}"""

src/main/scala/codecs/codecs.scala

@@ -1,283 +0,0 @@
-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

@@ -1,74 +0,0 @@
-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/newInterpreter/Logger.scala

@@ -0,0 +1,16 @@
+package newInterpreter
+
+object Logger
+  private var logging = false
+  private var indentation = 0
+  def on(): Unit =
+    logging = true
+    indentation = 0
+  def off(): Unit =
+    logging = false
+  def indent(): Unit =
+    indentation = indentation + 1
+  def unindent(): Unit =
+    indentation = indentation - 1
+  def log(s: => String): Unit =
+    if logging then println("|  " * indentation + s)

src/main/scala/newInterpreter/RecursiveLanguage.scala

@@ -0,0 +1,265 @@
+package newInterpreter
+
+object RecursiveLanguage {
+  /** Expression tree, also called Abstract Syntax Tree (AST) */
+  enum Expr
+    case Constant(value: Int)
+    case Name(name: String)
+    case BinOp(op: BinOps, arg1: Expr, arg2: Expr)
+    case IfNonzero(cond: Expr, caseTrue: Expr, caseFalse: Expr)
+    case Call(function: Expr, arg: Expr)
+    case Fun(param: String, body: Expr)
+
+    /** The empty list, also known as nil. */
+    case Empty
+
+    /** A compound data type composed of a head and a tail. */
+    case Cons(head: Expr, tail: Expr)
+
+    /** A pattern matching expression for Empty and Cons. */
+    case Match(scrutinee: Expr, caseEmpty: Expr, headName: String, tailName: String, caseCons: Expr)
+  import Expr._
+
+  /** Primitive operations that operation on constant values. */
+  enum BinOps
+    case Plus, Minus, Times, DividedBy, Modulo, LessEq
+  import BinOps._
+
+  def evalBinOp(op: BinOps)(ex: Expr, ey: Expr): Expr =
+    (op, ex, ey) match
+      case (Plus,   Constant(x), Constant(y)) => Constant(x + y)
+      case (Minus,  Constant(x), Constant(y)) => Constant(x - y)
+      case (Times,  Constant(x), Constant(y)) => Constant(x * y)
+      case (LessEq, Constant(x), Constant(y)) => Constant(if x <= y then 1 else 0)
+      case (Modulo,    Constant(x), Constant(y)) => if y == 0 then error("Division by zero") else Constant(x % y)
+      case (DividedBy, Constant(x), Constant(y)) => if y == 0 then error("Division by zero") else Constant(x / y)
+      case _ => error(s"Type error in ${show(BinOp(op, ex, ey))}")
+
+  type DefEnv = Map[String, Expr]
+
+  /** Evaluates a progam e given a set of top level definition defs */
+  def eval(e: Expr, defs: DefEnv): Expr =
+    e match
+      case Empty => Empty
+      case Cons(head, tail) => Cons(eval(head, defs), eval(tail, defs))
+      case Match(scrutinee, caseEmpty,hd,tl, caseCons) => 
+        eval(scrutinee, defs) match
+          case Empty => eval(caseEmpty, defs)
+          case Cons(head, tail) => eval(caseCons, defs ++ Map (hd -> head, tl -> tail))
+          case _ => error("Not a list")
+      case Constant(c) => e
+      case Name(n) =>
+        defs.get(n) match
+          case None => error(s"Unknown name $n")
+          case Some(body) => eval(body, defs)
+      case BinOp(op, e1, e2) =>
+        evalBinOp(op)(eval(e1, defs), eval(e2, defs))
+      case IfNonzero(cond, caseTrue, caseFalse) =>
+        if eval(cond, defs) != Constant(0) then eval(caseTrue, defs)
+        else eval(caseFalse, defs)
+      case Fun(n, body) => e
+      case Call(fun, arg) =>
+        Logger.log(show(e))
+        Logger.indent()
+        val eFun = eval(fun, defs)
+        val eArg = eval(arg, defs)
+        eFun match
+          case Fun(n, body) =>
+            Logger.unindent()
+            Logger.log(s"FUN: ${show(eFun)}  ARG: ${show(eArg)}")
+            val bodySub = subst(body, n, eArg)
+            Logger.log(s"${show(bodySub)}")
+            Logger.indent()
+            val res = eval(bodySub, defs)
+            Logger.unindent()
+            Logger.log(s"+--> ${show(res)}")
+            res
+          case _ => error(s"Cannot apply non-function ${show(eFun)} in a call")
+
+
+  /** Substitutes Name(n) by r in e. */
+  def subst(e: Expr, n: String, r: Expr): Expr =
+    e match
+      case Constant(c) => e
+      case Name(s) => if s == n then r else e
+      case BinOp(op, e1, e2) =>
+        BinOp(op, subst(e1, n, r), subst(e2, n, r))
+      case IfNonzero(cond, trueE, falseE) =>
+        IfNonzero(subst(cond, n, r), subst(trueE, n, r), subst(falseE, n, r))
+      case Call(f, arg) =>
+        Call(subst(f, n, r), subst(arg, n, r))
+      case Fun(param, body) =>
+        // If n conflicts with param, there cannot be a reference to n in the
+        // function body, there is nothing so substite.
+        if param == n then e
+        else
+          val fvs = freeVars(r)
+          // If the free variables in r contain param the naive substitution would
+          // change the meaning of param to reference to the function parameter.
+          if fvs.contains(param) then
+            // Perform alpha conversion in body to eliminate the name collision.
+            val param1 = differentName(param, fvs)
+            val body1 = alphaConvert(body, param, param1)
+            Fun(param1, subst(body1, n, r))
+          else
+            // Otherwise, substitute in the function body anyway.
+            Fun(param, subst(body, n, r))
+      case Empty => Empty
+      case Cons(head, tail) => Cons(subst(head, n, r), subst(tail, n, r))
+      case Match(scrutinee, caseEmpty, headName, tailName, caseCons) =>
+        if headName == n || tailName == n then
+          // If n conflicts with headName or tailName, there cannot be a reference
+          // to n in caseCons. Simply substite n by r in scrutinee and caseEmpty.
+          Match(subst(scrutinee, n, r), subst(caseEmpty, n, r), headName, tailName, caseCons)
+        else
+          // If the free variables in r contain headName or tailName, the naive
+          // substitution would change their meaning to reference to pattern binds
+          
+          val fvs = freeVars(r)
+          if fvs.contains(headName) || fvs.contains(tailName) then
+            // Perform alpha conversion in caseCons to eliminate the name collision.
+            val headName1 = differentName(headName, fvs)
+            val caseCons1 = alphaConvert(caseCons, headName, headName1)
+
+            val tailName1 = differentName(tailName,fvs)
+            val caseCons2 = alphaConvert(caseCons1, tailName, tailName1)
+            Match(scrutinee,caseEmpty, headName1, tailName1, subst(caseCons2,n,r))
+            
+          else
+          // Otherwise, substitute in scrutinee, caseEmpty & caseCons anyway.
+            Match(subst(scrutinee, n, r), subst(caseEmpty, n, r), headName, tailName, subst(caseCons, n, r))
+            
+
+  def differentName(n: String, s: Set[String]): String =
+    if s.contains(n) then differentName(n + "'", s)
+    else n
+
+  /** Computes the set of free variable in e. */
+  def freeVars(e: Expr): Set[String] =
+    e match
+      case Constant(c) => Set()
+      case Name(s) => Set(s)
+      case BinOp(op, e1, e2) => freeVars(e1) ++ freeVars(e2)
+      case IfNonzero(cond, trueE, falseE) => freeVars(cond) ++ freeVars(trueE) ++ freeVars(falseE)
+      case Call(f, arg) => freeVars(f) ++ freeVars(arg)
+      case Fun(param, body) => freeVars(body) - param
+      case Empty => Set()
+      case Cons(head, tail) => freeVars(head) ++ freeVars(tail)
+      case Match(scrutinee, caseEmpty,headName,tailName, caseCons) => scrutinee match
+        case Empty => freeVars(caseEmpty)
+        case Cons(head, tail) => freeVars(scrutinee) ++ freeVars(caseEmpty) ++ freeVars(caseCons) 
+      
+
+  /** Substitutes Name(n) by Name(m) in e. */
+  def alphaConvert(e: Expr, n: String, m: String): Expr =
+    e match
+      case Constant(c) => e
+      case Name(s) => if s == n then Name(m) else e
+      case BinOp(op, e1, e2) =>
+        BinOp(op, alphaConvert(e1, n, m), alphaConvert(e2, n, m))
+      case IfNonzero(cond, trueE, falseE) =>
+        IfNonzero(alphaConvert(cond, n, m), alphaConvert(trueE, n, m), alphaConvert(falseE, n, m))
+      case Call(f, arg) =>
+        Call(alphaConvert(f, n, m), alphaConvert(arg, n, m))
+      case Fun(param, body) =>
+        // If n conflicts with param, there cannot be references to n in body,
+        // as these would reference param instead.
+        if param == n then e
+        else Fun(param, alphaConvert(body, n, m))
+      case Empty => e
+      case Cons(head, tail) => Cons(alphaConvert(head, n,m), alphaConvert(tail, n,m))
+      case Match(scrutinee, caseEmpty,headName,tailName, caseCons) => 
+        if headName == n || tailName == n then Match(scrutinee, caseEmpty,headName,tailName, caseCons)
+        else Match(scrutinee, alphaConvert(caseEmpty,n,m),headName,tailName, alphaConvert(caseCons,n,m))
+
+  case class EvalException(msg: String) extends Exception(msg)
+
+  def error(msg: String) = throw EvalException(msg)
+
+  // Printing and displaying
+
+  /** Pretty print an expression as a String. */
+  def show(e: Expr): String =
+    e match
+      case Constant(c) => c.toString
+      case Name(n) => n
+      case BinOp(op, e1, e2) =>
+        val opString = op match
+          case Plus      => "+"
+          case Minus     => "-"
+          case Times     => "*"
+          case DividedBy => "/"
+          case Modulo    => "%"
+          case LessEq    => "<="
+        s"($opString ${show(e1)} ${show(e2)})"
+      case IfNonzero(cond, caseTrue, caseFalse) =>
+        s"(if ${show(cond)} then ${show(caseTrue)} else ${show(caseFalse)})"
+      case Call(f, arg) => show(f) + "(" + show(arg) + ")"
+      case Fun(n, body) => s"($n => ${show(body)})"
+      case Empty => ""
+      case Cons(head, tail) => show(head) + "::" + show(tail)
+      case Match(scrutinee, caseEmpty, headName, tailName, caseCons) => show(scrutinee) + "Match " + "case Nil => " + show(caseEmpty) + "case " + headName + "::" + tailName + "=> " + show(caseCons)
+
+  /** Pretty print top-level definition as a String. */
+  def showEnv(env: Map[String, Expr]): String =
+    env.map { case (name, body) => s"def $name =\n  ${show(body)}" }.mkString("\n\n") + "\n"
+
+  /** Evaluates an expression with the given top-level definitions with logging enabled. */
+  def tracingEval(e: Expr, defs: DefEnv): Expr =
+    Logger.on()
+    val evaluated = eval(e, defs)
+    println(s" ~~> $evaluated\n")
+    Logger.off()
+    evaluated
+
+  def minus(e1: Expr, e2: Expr)     = BinOp(BinOps.Minus,  e1, e2)
+  def plus(e1: Expr, e2: Expr)      = BinOp(BinOps.Plus,   e1, e2)
+  def leq(e1: Expr, e2: Expr)       = BinOp(BinOps.LessEq, e1, e2)
+  def times(e1: Expr, e2: Expr)     = BinOp(BinOps.Times,  e1, e2)
+  def modulo(e1: Expr, e2: Expr)    = BinOp(BinOps.Modulo, e1, e2)
+  def dividedBy(e1: Expr, e2: Expr) = BinOp(BinOps.DividedBy, e1, e2)
+  // The {Name => N} import syntax renames Name to N in this scope
+  import Expr.{Name => N, Constant => C, _}
+
+  /** Examples of top-level definitions (used in tests) */
+  val definitions: DefEnv = Map[String, Expr](
+    "fact" -> Fun("n",
+       IfNonzero(N("n"),
+         times(N("n"),
+               Call(N("fact"), minus(N("n"), C(1)))),
+         C(1))),
+
+    "square" -> Fun("x",
+      times(N("x"), N("x"))),
+
+    "twice" -> Fun("f", Fun("x",
+      Call(N("f"), Call(N("f"), N("x"))))),
+
+    // TODO Implement map (see recitation session)
+    "map" -> Fun("ls", Fun("f", Match( Name("ls"), Empty, "x", "xs", Cons( Call(Name("f"), Name("x") ), Call( Call ( Name("map"), Name("xs") ), Name("f") ) )))),
+
+    // TODO Implement gcd (see recitation session)
+    "gcd" -> Fun("a", Fun("b", IfNonzero( Name("b"), Call( Call ( Name("gcd"), Name ("b")), modulo ( Name("a") , Name("b"))), Name("a")))),
+
+    // TODO Implement foldLeft (see recitation session)
+    "foldLeft" -> Fun("ls", Fun("acc", Fun("f", Match( Name("ls"), Name("acc"), "x", "xs" , 
+    Call(
+      Call(
+        Call( Name("foldLeft"), Name("xs")) , 
+        Call(Call(Name("f"), Name("acc")), Name("x"))
+      ), Name("f") 
+    )
+    )))),
+
+    // TODO Implement foldRight (analogous to foldLeft, but operate right-to-left)
+    "foldRight" -> Fun("ls", Fun("z", Fun("fold", Match( Name("ls"), Name("z"), "x", "xs" , 
+    Call(Call(Name("fold"), Name("x")), 
+    Call(Call(Call(Name("foldRight"), Name("xs")), Name("z")), Name("fold")))
+    )))),
+  )
+
+  def main(args: Array[String]): Unit =
+    println(showEnv(definitions))
+    tracingEval(Call(N("fact"), C(6)), definitions)
+    tracingEval(Call(Call(N("twice"), N("square")), C(3)), definitions)
+}

src/test/scala/codecs/CodecsSuite.scala

@@ -1,115 +0,0 @@
-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/newInterpreter/RecursiveLanguageSuite.scala

@@ -0,0 +1,126 @@
+package newInterpreter
+
+class RecursiveLanguageSuite {
+  import org.junit._
+  import org.junit.Assert.{assertEquals, fail}
+  import RecursiveLanguage._, Expr.{Constant => C, Name => N, _}
+
+  @Test def gcdTests(): Unit = {
+    def test(a: Int, b: Int, c: Int): Unit = {
+      val call   = Call(Call(N("gcd"), C(a)), C(b))
+      val result = eval(call, definitions)
+      assertEquals(C(c), result)
+    }
+    test(60,  90,  30)
+    test(36,  48,  12)
+    test(25,  85,  5 )
+    test(12,  15,  3 )
+    test(60,  75,  15)
+    test(35,  56,  7 )
+    test(96,  128, 32)
+    test(120, 135, 15)
+    test(150, 225, 75)
+  }
+
+  @Test def evalTests: Unit = {
+    assertEquals(Empty, eval(Empty, Map()))
+    assertEquals(Cons(C(1), Empty), eval(Cons(C(1), Empty), Map()))
+    assertEquals(Cons(C(2), Empty), eval(Cons(BinOp(BinOps.Plus, C(1), C(1)), Empty), Map()))
+
+    assertEquals(C(0),  eval(Match(Empty,              C(0), "_", "_",  C(1)),    Map()))
+    assertEquals(C(1),  eval(Match(Cons(Empty, Empty), C(0), "_", "_",  C(1)),    Map()))
+    assertEquals(C(2),  eval(Match(Cons(C(2), Empty),  C(0), "x", "xs", N("x")),  Map()))
+    assertEquals(Empty, eval(Match(Cons(C(2), Empty),  C(0), "x", "xs", N("xs")), Map()))
+
+    try {
+      eval(Match(C(0), C(0), "_", "_", C(1)), Map())
+      fail()
+    } catch {
+      case EvalException(msg) => // OK!
+    }
+  }
+
+  @Test def freeVarsTests: Unit = {
+    assertEquals(Set(),         freeVars(Empty))
+    assertEquals(Set("a", "b"), freeVars(Cons(N("a"), N("b"))))
+    assertEquals(Set(),         freeVars(Match(Empty, C(0),   "a", "b", N("a"))))
+    assertEquals(Set(),         freeVars(Match(Empty, C(0),   "a", "b", N("b"))))
+    assertEquals(Set("c"),      freeVars(Match(Empty, N("c"), "_", "_", C(1))))
+  }
+
+  @Test def alphaConvertTests: Unit = {
+    assertEquals(Cons(N("b"), N("b")), alphaConvert(Cons(N("a"), N("a")), "a", "b"))
+    assertEquals(Match(Empty, C(0),   "a", "b", N("a")), alphaConvert(Match(Empty, C(0),   "a", "b", N("a")), "a", "b"))
+    assertEquals(Match(Empty, C(0),   "a", "b", N("b")), alphaConvert(Match(Empty, C(0),   "a", "b", N("b")), "a", "b"))
+    assertEquals(Match(Empty, N("a"), "a", "b", C(1)),   alphaConvert(Match(Empty, N("c"), "a", "b", C(1)),   "c", "a"))
+    assertEquals(Match(Empty, N("b"), "a", "b", C(1)),   alphaConvert(Match(Empty, N("c"), "a", "b", C(1)),   "c", "b"))
+    assertEquals(Match(Empty, C(0),   "a", "b", N("e")), alphaConvert(Match(Empty, C(0),   "a", "b", N("d")), "d", "e"))
+  }
+
+  val sum = "sum" -> Fun("a", Fun("b", BinOp(BinOps.Plus, N("a"), N("b"))))
+  val div = "div" -> Fun("a", Fun("b", BinOp(BinOps.DividedBy, N("a"), N("b"))))
+
+  @Test def foldLeft: Unit = {
+    val list1 = Cons(C(1), Cons(C(2), Cons(C(3), Empty)))
+    val call1 = Call(Call(Call(N("foldLeft"), list1), C(100)), N("sum"))
+    assertEquals(C(106), eval(call1, definitions + sum))
+
+    val list2 = Cons(C(1), Cons(C(2), Cons(C(3), Cons(C(4), Cons(C(5), Cons(C(6), Empty))))))
+    val call2 = Call(Call(Call(N("foldLeft"), list2), C(100000)), N("div"))
+    assertEquals(C(138), eval(call2, definitions + div))
+  }
+
+  @Test def foldRight: Unit = {
+    val list1 = Cons(C(1), Cons(C(2), Cons(C(3), Empty)))
+    val call1 = Call(Call(Call(N("foldRight"), list1), C(100)), N("sum"))
+    assertEquals(C(106), eval(call1, definitions + sum))
+
+    val list2 = Cons(C(1000000), Cons(C(20000), Cons(C(3000), Cons(C(400), Cons(C(50), Cons(C(6), Empty))))))
+    val call2 = Call(Call(Call(N("foldRight"), list2), C(1)), N("div"))
+    assertEquals(C(3003), eval(call2, definitions + div))
+  }
+
+  @Test def substitutionSimple: Unit = {
+    // Substitution should happend everywhere in the Match. In scrutinee and in caseCons:
+    assertEquals(Cons(C(1), Empty), eval(
+      Call(N("bar"), Cons(C(1), Empty)),
+      Map("bar" -> Fun("x", Match(N("x"), C(0), "y", "z", N("x"))))
+    ))
+
+    // In scrutinee and in caseEmpty:
+    assertEquals(Empty, eval(
+      Call(N("bar"), Empty),
+      Map("bar" -> Fun("x", Match(N("x"), N("x"), "y", "z", C(0))))
+    ))
+
+    // But not inside caseCons when the binding name clashes with the functions name:
+    assertEquals(C(1), eval(
+      Call(N("bar"), Cons(C(1), Empty)),
+      Map("bar" -> Fun("x", Match(N("x"), C(0), "x", "z", N("x"))))
+    ))
+  }
+
+  @Test def substitutionFunctionCapture: Unit = {
+    // Here comes the real fun, plus_one uses "map" as it's first argument name,
+    // incorrect implementation will accidentally capture the recursion in map
+    // turn into that name into a reference to first argument of plus_one.
+    val plus_one = "plus_one" -> Fun("map", BinOp(BinOps.Plus, C(1), N("map")))
+    val list1 = Cons(C(1), Cons(C(2), Cons(C(3), Empty)))
+    val list2 = Cons(C(2), Cons(C(3), Cons(C(4), Empty)))
+    assertEquals(list2, eval(Call(Call(N("map"), list1), N("plus_one")), definitions + plus_one))
+  }
+
+  @Test def substitutionMatchCapture: Unit = {
+    // More fun, this uses "fact" as a first binding in the pattern match:
+    val pairMap = "pairMap" -> Fun("pair", Fun("function",
+      Match(
+        N("pair"),
+        Empty,
+        "fact", "tcaf",
+        Cons(Call(N("function"), N("fact")), Call(N("function"), N("tcaf"))))
+    ))
+    assertEquals(Cons(C(6), C(24)), eval(Call(Call(N("pairMap"), Cons(C(3), C(4))), N("fact")), definitions + pairMap))
+  }
+
+  @Rule def individualTestTimeout = new org.junit.rules.Timeout(200 * 1000)
+}