Complete ch02

Author: Abhijit Sarkar

.github/run.sh

@@ -8,7 +8,7 @@ no_lint=0
 while (( $# > 0 )); do
    case "$1" in
    	--help)
-			printf "run.sh [OPTION]... [PKG]\n"
+			printf "run.sh [OPTION]... [DIR]\n"
 			printf "options:\n"
 			printf "\t--help			Show help\n"
 			printf "\t--no-test		Skip tests\n"
@@ -29,21 +29,24 @@ while (( $# > 0 )); do
    esac
 done
 
+./mill __.compile
+
 if (( no_test == 0 )); then
   if [[ -z "$1" ]]; then
-    sbt test
+    ./mill __.test
+  elif ./mill resolve modules["$1"].__.test &>/dev/null; then
+    ./mill modules["$1"].__.test
   else
-    green='\033[1;32m'
+    red='\033[0;31m'
     no_color='\033[0m'
-	printf "Running tests in packages matching: %b%s*%b\n" "$green" "$1" "$no_color"
-    sbt "Test / testOnly $1*"
+	  printf "%bNo tests found in: %s%b\n" "$red" "$1" "$no_color"
   fi
 fi
 
 if (( no_lint == 0 )); then
 	if [[ -z "${CI}" ]]; then
-	  sbt scalafmtAll
+	  ./mill mill.scalalib.scalafmt.ScalafmtModule/reformatAll modules[_].sources
 	else
-	  sbt scalafmtCheckAll
+		./mill mill.scalalib.scalafmt.ScalafmtModule/checkFormatAll modules[_].sources
 	fi
 fi

.github/workflows/ci.yml

@@ -8,12 +8,12 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Set up JDK
-        uses: actions/setup-java@v3
+        uses: actions/setup-java@v4
         with:
           distribution: temurin
-          java-version: 17
+          java-version: 21
       - name: Test
         run: ./.github/run.sh --no-lint
       - name: Lint

.gitignore

@@ -1,33 +1,20 @@
-# macOS
 .DS_Store
+*.class
+*.log
+*~
 
-# sbt
-dist/*
-target/
-lib_managed/
-src_managed/
-project/boot/
-project/project
-project/plugins/project/
-project/local-plugins.sbt
-.history
-.ensime
-.ensime_cache/
-.sbt-scripted/
-local.sbt
+# idea
+.idea
+.idea_modules
+/.worksheet/
 
-# Bloop
-.bsp
+# mill
+out/
 
-# VS Code
+.bsp/
+.metals/
 .vscode/
-
-# Metals
 .bloop/
-.metals/
-metals.sbt
 
-# IDEA
-.idea
-.idea_modules
-/.worksheet/
+# virtual machine crash logs
+hs_err_pid*

.mill-version

@@ -0,0 +1 @@
+0.12.3

.scalafmt.conf

@@ -1,4 +1,4 @@
-version = "3.7.17"
+version = "3.8.3"
 align.preset = more
 maxColumn = 120
 runner.dialect = scala3

README.md

@@ -4,29 +4,14 @@ My solutions to the exercises from the book [Scala with Cats](https://www.scalaw
 
 Official GitHub repo: https://github.com/scalawithcats/scala-with-cats
 
-## Contents
+The main branch corresponds to the latest edition of the book (currently 2nd). 
+The older code is available in branches.
 
-1. [Introduction](src/main/scala/ch01)
-2. [Monoids and Semigroups](src/main/scala/ch02)
-3. [Functors](src/main/scala/ch03)
-4. [Monads](src/main/scala/ch04)
-5. [Monad Transformers](src/main/scala/ch05)
-6. [Semigroupal and Applicative](src/main/scala/ch06)
-7. [Foldable and Traverse](src/main/scala/ch07)
-8. [Case Study: Testing Asynchronous Code](src/main/scala/ch08)
-9. [Case Study: Map-Reduce](src/main/scala/ch09)
-10. [Case Study: Data Validation](src/main/scala/ch10)
-11. [Case Study: CRDTs](src/main/scala/ch11)
+## Contents
 
 ## Running tests
-
-```
-./.github/run.sh
-```
-
-To run all tests from a package:
 ```
-./.github/run.sh <package prefix>
+./.github/run.sh <directory>
 ```
 
 ## License

build.mill

@@ -0,0 +1,44 @@
+package build
+
+import mill._, scalalib._, scalafmt._
+import $packages._
+import $file.{versions => v}
+
+def isBookChapter(p: os.Path) = os.isDir(p) && "ch\\d{2}".r.matches(p.baseName)
+
+def moduleNames = interp.watchValue(
+    os.walk(millSourcePath, !isBookChapter(_), maxDepth = 1)
+      .map(_.last)
+)
+
+object modules extends Cross[CatsModule](moduleNames)
+
+trait CatsModule extends ScalaModule with Cross.Module[String] with ScalafmtModule {
+
+  val scalaVersion = v.scalaVersion
+
+  // Ends with 'modules' that need to be removed
+  def millSourcePath = super.millSourcePath / os.up / crossValue
+
+  def scalacOptions: T[Seq[String]] = Seq(
+    "-encoding", "UTF-8",
+    "-feature",
+    "-Werror",
+    "-explain",
+    "-deprecation",
+    "-unchecked",
+    "-Wunused:all",
+    "-rewrite",
+    "-indent",
+    "-source", "future",
+  )
+
+  object test extends ScalaTests with TestModule.ScalaTest {
+   val commonDeps = Seq(
+     ivy"org.scalatest::scalatest:${v.scalatestVersion}",
+     ivy"org.scalatestplus::scalacheck-1-18:${v.scalacheckVersion}"
+   )
+   
+   def ivyDeps = Task{commonDeps}
+  }
+}

build.sbt

@@ -0,0 +1,54 @@
+package ch02
+
+/*
+Structural corecursion works by considering all the possible outputs, which are the constructors of
+the algebraic data type, and then working out the conditions under which we’d call each constructor.
+
+We could abstract structural corecursion as an unfold.
+ */
+enum MyList[A]:
+  case Empty()
+  case Pair(_head: A, _tail: MyList[A])
+
+  def map[B](f: A => B): MyList[B] =
+//    this match
+//      case Empty()          => Empty()
+//      case Pair(head, tail) => Pair(f(head), tail.map(f))
+    MyList.unfold(this)(_.isEmpty, a => f(a.head), _.tail)
+
+  def isEmpty: Boolean =
+    this match
+      case Empty() => true
+      case _       => false
+
+  def head: A =
+    this match
+      case Pair(head, _) => head
+      case _             => scala.sys.error("empty list")
+
+  def tail: MyList[A] =
+    this match
+      case Pair(_, tail) => tail
+      case _             => scala.sys.error("empty list")
+
+  def fold[B](zero: B)(f: (A, B) => B): B =
+    this match
+      case Empty()          => zero
+      case Pair(head, tail) => f(head, tail.fold(zero)(f))
+
+  def toSeq: Seq[A] = fold(Seq.empty)(_ +: _)
+
+object MyList:
+  /*
+  Types inferred for one method parameter cannot be used for other method parameters in the same parameter list.
+  However, types inferred for one method parameter list can be used in subsequent lists.
+   */
+  def unfold[A, B](seed: A)(stop: A => Boolean, f: A => B, next: A => A): MyList[B] =
+    if stop(seed) then MyList.Empty()
+    else MyList.Pair(f(seed), unfold(next(seed))(stop, f, next))
+
+  def fill[A](n: Int)(elem: => A): MyList[A] =
+    unfold(0)(_ == n, _ => elem, _ + 1)
+
+  def iterate[A](start: A, len: Int)(f: A => A): MyList[A] =
+    unfold((0, start))(_._1 == len, _._2, (i, a) => (i + 1, f(a)))

ch02/src/MyList.scala

@@ -0,0 +1,44 @@
+package ch02
+
+/*
+Structural recursion works by considering all the possible inputs (which we usually represent as patterns),
+and then working out what we do with each input case.
+
+We could abstract structural recursion as an fold.
+ */
+enum Tree[A]:
+  case Leaf(value: A)
+  case Node(left: Tree[A], right: Tree[A])
+
+  /** @return
+    *   the number of values (Leafs) stored in the Tree
+    */
+  def size: Int = this.fold(_ => 1, _ + _)
+//    this match
+//      case Tree.Leaf(value) => 1
+//      case Tree.Node(left, right) => left.size + right.size
+
+  /** @param elem
+    *   element to look for
+    * @return
+    *   true if the Tree contains a given element of type A, and false otherwise
+    */
+  def contains(elem: A): Boolean = this.fold(_ == elem, _ || _)
+//    this match
+//      case Tree.Leaf(value) => value == elem
+//      case Tree.Node(left, right) => left.contains(elem) || right.contains(elem)
+
+  /** @param f
+    *   value transformation function
+    * @return
+    *   a Tree[B] given a function A => B
+    */
+  def map[B](f: A => B): Tree[B] = this.fold(v => Tree.Leaf(f(v)), (l, r) => Tree.Node(l, r))
+//    this match
+//      case Tree.Leaf(value) => Tree.Leaf(f(value))
+//      case Tree.Node(left, right) => Tree.Node(left.map(f), right.map(f))
+
+  def fold[B](f: (A => B), g: (B, B) => B): B =
+    this match
+      case Tree.Node(left, right) => g(left.fold(f, g), right.fold(f, g))
+      case Tree.Leaf(value)       => f(value)