Complete ch03

README.md

@@ -9,6 +9,9 @@ The older code is available in branches.
 
 ## Contents
 
+2. [Algebraic Data Types](ch02)
+3. [Objects as Codata](ch03)
+
 ## Running tests
 ```
 ./.github/run.sh <directory>

ch03/src/Bool.scala

@@ -0,0 +1,25 @@
+package ch03
+
+trait Bool:
+  def `if`[A](t: A)(f: A): A
+
+val True = new Bool:
+  def `if`[A](t: A)(f: A): A = t
+
+val False = new Bool:
+  def `if`[A](t: A)(f: A): A = f
+
+def and(l: Bool, r: Bool): Bool =
+  new Bool:
+    def `if`[A](t: A)(f: A): A =
+      l.`if`(r)(False).`if`(t)(f)
+
+def or(l: Bool, r: Bool): Bool =
+  new Bool:
+    def `if`[A](t: A)(f: A): A =
+      l.`if`(True)(r).`if`(t)(f)
+
+def not(b: Bool): Bool =
+  new Bool:
+    def `if`[A](t: A)(f: A): A =
+      b.`if`(True)(False).`if`(f)(t)

ch03/src/Codata.scala

@@ -0,0 +1,37 @@
+package ch03
+
+/*
+codata is a product of destructors, where destructors are functions from
+the codata type (and, optionally, some other inputs) to some type.
+
+In Scala we define codata as a trait, and implement it as a final class,
+anonymous subclass, or an object.
+
+We have two strategies for implementing methods using codata: structural corecursion, which we can
+use when the result is codata, and structural recursion, which we can use when an input is codata.
+
+Data is connected to codata via fold: any data can instead be implemented as codata with
+a single destructor that is the fold for that data. This is called Church encoding.
+The reverse is also: we can enumerate all potential pairs of inputs and outputs of
+destructors to represent codata as data.
+
+Data and codata offer different forms of extensibility: data makes it easy to add
+new functions, but adding new elements requires changing existing code, while it is
+easy to add new elements to codata but we change existing code if we add new functions.
+ */
+// Polymorphic function type, new in Scala 3.
+// https://www.youtube.com/watch?v=sauaDZ-1-zM
+type List[A, B] = (B, (A, B) => B) => B
+
+// Polymorphic function of two types, A and B, that takes no arg
+// and returns the type defined above.
+val Empty: [A, B] => () => List[A, B] =
+  // empty: B
+  [A, B] => () => (empty, _) => empty
+
+val Pair: [A, B] => (A, List[A, B]) => List[A, B] =
+  // empty: B, f: (A, B) => B
+  [A, B] => (head: A, tail: List[A, B]) => (empty, f) => f(head, tail(empty, f))
+
+val list: [B] => () => List[Int, B] =
+  [B] => () => Pair(1, Pair(2, Pair(3, Empty())))

ch03/src/List.scala

@@ -0,0 +1,23 @@
+package ch03
+
+package data:
+  enum List[A]:
+    case Pair(head: A, tail: List[A])
+    case Empty()
+
+    def foldRight[B](empty: B)(f: (A, B) => B): B =
+      this match
+        case Pair(head, tail) => f(head, tail.foldRight(empty)(f))
+        case Empty()          => empty
+
+package codata:
+  trait List[A]:
+    def foldRight[B](empty: B)(f: (A, B) => B): B
+
+    final class Pair(head: A, tail: List[A]) extends List[A]:
+      def foldRight[B](empty: B)(f: (A, B) => B): B =
+        f(head, tail.foldRight(empty)(f))
+
+    final class Empty extends List[A]:
+      def foldRight[B](empty: B)(f: (A, B) => B): B =
+        empty

ch03/src/Set.scala

@@ -0,0 +1,40 @@
+package ch03
+
+trait Set[A]:
+
+  def contains(elt: A): Boolean
+
+  def insert(elt: A): Set[A] =
+    InsertOneSet(elt, this)
+
+  def union(that: Set[A]): Set[A] =
+    UnionSet(this, that)
+
+final class ListSet[A](elements: scala.collection.immutable.List[A]) extends Set[A]:
+
+  def contains(elt: A): Boolean =
+    elements.contains(elt)
+
+  override def insert(elt: A): Set[A] =
+    ListSet(elt :: elements)
+
+  override def union(that: Set[A]): Set[A] =
+    elements.foldLeft(that) { (set, elt) => set.insert(elt) }
+
+object ListSet:
+  def empty[A]: Set[A] = ListSet(List.empty)
+
+final class InsertOneSet[A](element: A, source: Set[A]) extends Set[A]:
+
+  def contains(elt: A): Boolean =
+    elt == element || source.contains(elt)
+
+final class UnionSet[A](first: Set[A], second: Set[A]) extends Set[A]:
+
+  def contains(elt: A): Boolean =
+    first.contains(elt) || second.contains(elt)
+
+object Evens extends Set[Int]:
+
+  def contains(elt: Int): Boolean =
+    (elt % 2 == 0)

ch03/src/Stream.scala

@@ -0,0 +1,58 @@
+package ch03
+
+trait Stream[A]:
+  def head: A
+  def tail: Stream[A]
+  override def toString: String = head.toString
+
+  def take(count: Int): scala.collection.immutable.List[A] =
+    count match
+      case 0 => Nil
+      case n => head :: tail.take(n - 1)
+
+  def map[B](f: A => B): Stream[B] =
+    val self = this
+    new Stream[B]:
+      def head: B         = f(self.head)
+      def tail: Stream[B] = self.tail.map(f)
+
+  def filter(p: A => Boolean): Stream[A] =
+    lazy val self = if p(head) then this else tail.filter(p)
+    new Stream[A]:
+      def head: A         = self.head
+      def tail: Stream[A] = self.tail.filter(p)
+
+  def zip[B](that: Stream[B]): Stream[(A, B)] =
+    val self = this
+    new Stream[(A, B)]:
+      def head: (A, B)         = (self.head, that.head)
+      def tail: Stream[(A, B)] = self.tail.zip(that.tail)
+
+  def scanLeft[B](zero: B)(f: (B, A) => B): Stream[B] =
+    val self = this
+    new Stream[B]:
+      def head: B         = zero
+      def tail: Stream[B] = self.tail.scanLeft(f(zero, self.head))(f)
+
+object Stream:
+  def unfold[A, B](seed: A)(f: A => B, next: A => A): Stream[B] =
+    new Stream[B]:
+      def head: B         = f(seed)
+      def tail: Stream[B] = unfold(next(seed))(f, next)
+
+  val ones: Stream[Int] =
+    new Stream[Int]:
+      def head: Int = 1
+
+      def tail: Stream[Int] = ones
+
+  val alternating: Stream[Int] = Stream.unfold(true)(if _ then 1 else -1, !_)
+
+  val naturals: Stream[Int] = ones.scanLeft(1)(_ + _)
+
+  val naturals2: Stream[Int] = unfold(1)(x => x, _ + 1)
+
+  val naturals3: Stream[Int] =
+    new Stream:
+      def head              = 1
+      def tail: Stream[Int] = naturals.map(_ + 1)

ch03/test/src/BoolSpec.scala

@@ -0,0 +1,23 @@
+package ch03
+
+import org.scalatest.funspec.AnyFunSpec
+import org.scalatest.matchers.should.Matchers.shouldBe
+
+class BoolSpec extends AnyFunSpec:
+  describe("Bool"):
+    it("and"):
+      and(True, True).`if`("yes")("no") shouldBe "yes"
+      and(True, False).`if`("yes")("no") shouldBe "no"
+      and(False, True).`if`("yes")("no") shouldBe "no"
+      and(False, False).`if`("yes")("no") shouldBe "no"
+
+    it("or"):
+      or(True, True).`if`("yes")("no") shouldBe "yes"
+      or(True, False).`if`("yes")("no") shouldBe "yes"
+      or(False, True).`if`("yes")("no") shouldBe "yes"
+      or(False, False).`if`("yes")("no") shouldBe "no"
+
+    it("not"):
+      not(True).`if`("yes")("no") shouldBe "no"
+      not(False).`if`("yes")("no") shouldBe "yes"
+

ch03/test/src/CodataSpec.scala

@@ -0,0 +1,14 @@
+package ch03
+
+import org.scalatest.funspec.AnyFunSpec
+import org.scalatest.matchers.should.Matchers.shouldBe
+
+class CodataSpec extends AnyFunSpec:
+  describe("Codata"):
+    it("list"):
+      val sum = list()(0, (a, b) => a + b)
+      sum shouldBe 6
+
+      val product = list()(1, (a, b) => a * b)
+      product shouldBe 6
+

ch03/test/src/SetSpec.scala

@@ -0,0 +1,19 @@
+package ch03
+
+import org.scalatest.funspec.AnyFunSpec
+import org.scalatest.matchers.should.Matchers.shouldBe
+
+class SetSpec extends AnyFunSpec:
+  describe("Set"):
+    it("evens"):
+      val evensAndOne = Evens.insert(1)
+      val evensAndOthers =
+        Evens.union(ListSet.empty.insert(1).insert(3))
+
+      evensAndOne.contains(1) shouldBe true
+      evensAndOthers.contains(1)  shouldBe true
+      evensAndOne.contains(2)  shouldBe true
+      evensAndOthers.contains(2)  shouldBe true
+      evensAndOne.contains(3)  shouldBe false
+      evensAndOthers.contains(3)  shouldBe true
+

ch03/test/src/StreamSpec.scala

@@ -0,0 +1,23 @@
+package ch03
+
+import org.scalatest.funspec.AnyFunSpec
+import org.scalatest.matchers.should.Matchers.shouldBe
+
+class StreamSpec extends AnyFunSpec:
+  describe("Stream"):
+    it("ones"):
+      Stream.ones.take(5) shouldBe List.fill(5)(1)
+
+    it("alternating"):
+      Stream.alternating.take(5) shouldBe List.iterate(1, 5)(_ * -1)
+
+    it("filter"):
+      Stream.alternating.filter(_ > 0).take(5) shouldBe List.fill(5)(1)
+
+    it("naturals"):
+      Stream.naturals.take(5) shouldBe (1 to 5)
+      Stream.naturals2.take(5) shouldBe (1 to 5)
+      Stream.naturals3.take(5) shouldBe (1 to 5)
+
+
+