Document exercises

ch02/src/MyList.scala

@@ -10,6 +10,10 @@ enum MyList[A]:
   case Empty()
   case Pair(_head: A, _tail: MyList[A])
 
+  /*
+  Exercise: Map
+  Once you’ve completed iterate, try to implement map in terms of unfold.
+   */
   def map[B](f: A => B): MyList[B] =
 //    this match
 //      case Empty()          => Empty()
@@ -50,5 +54,9 @@ object MyList:
   def fill[A](n: Int)(elem: => A): MyList[A] =
     unfold(0)(_ == n, _ => elem, _ + 1)
 
+  /*
+  Exercise: Iterate
+  Implement iterate using the same reasoning as we did for fill.
+   */
   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/Tree.scala

@@ -10,6 +10,21 @@ enum Tree[A]:
   case Leaf(value: A)
   case Node(left: Tree[A], right: Tree[A])
 
+  /*
+  Exercise: Methods for Tree
+
+  Write some methods for Tree. Implement
+
+  - size, which returns the number of values (Leafs) stored in the Tree;
+  - contains, which returns true if the Tree contains a given element of type A, and false otherwise; and
+  - map, which creates a Tree[B] given a function A => B
+
+  Use whichever you prefer of pattern matching or dynamic dispatch to implement the methods.
+
+  Exercise: Using Fold
+  Prove to yourself that you can replace structural recursion with calls to fold,
+  by redefining size, contains, and map for Tree using only fold.
+   */
   /** @return
     *   the number of values (Leafs) stored in the Tree
     */
@@ -38,6 +53,10 @@ enum Tree[A]:
 //      case Tree.Leaf(value) => Tree.Leaf(f(value))
 //      case Tree.Node(left, right) => Tree.Node(left.map(f), right.map(f))
 
+  /*
+  Exercise: Tree Fold
+  Implement a fold for Tree defined earlier.
+   */
   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))

ch03/src/Bool.scala

@@ -14,6 +14,10 @@ def and(l: Bool, r: Bool): Bool =
     def `if`[A](t: A)(f: A): A =
       l.`if`(r)(False).`if`(t)(f)
 
+/*
+Exercise: Or and Not
+Test your understanding of Bool by implementing or and not.
+ */
 def or(l: Bool, r: Bool): Bool =
   new Bool:
     def `if`[A](t: A)(f: A): A =

ch03/src/Set.scala

@@ -1,5 +1,8 @@
 package ch03
 
+/*
+3.6 Exercise: Sets
+ */
 trait Set[A]:
 
   def contains(elt: A): Boolean

ch03/src/Stream.scala

@@ -16,6 +16,10 @@ trait Stream[A]:
       def head: B         = f(self.head)
       def tail: Stream[B] = self.tail.map(f)
 
+  /*
+  Exercise: Stream Combinators
+  Implement filter, zip, and scanLeft on Stream.
+   */
   def filter(p: A => Boolean): Stream[A] =
     lazy val self = if p(head) then this else tail.filter(p)
     new Stream[A]:

ch04/src/Display.scala

@@ -1,5 +1,8 @@
 package ch04
 
+/*
+4.5 Exercise: Display Library
+*/
 trait Display[A]:
   def display(value: A): String
 

ch05/src/Expression.scala

@@ -16,6 +16,19 @@ has every method call in tail position. Due to Scala runtime limitations not all
 in tail position can be converted to tail calls, so we reified calls and returns into
 data structures used by a recursive loop called a trampoline.
  */
+/*
+Exercise: Arithmetic
+Now it’s your turn to practice using reification. Your task is to implement an interpreter
+for arithmetic expressions. An expression is:
+
+- a literal number, which takes a Double and produces an Expression;
+- an addition of two expressions;
+- a substraction of two expressions;
+- a multiplication of two expressions; or
+- a division of two expressions;
+
+Reify this description as a type Expression.
+ */
 enum Expression:
   case Literal(value: Double)
   case Addition(left: Expression, right: Expression)

ch05/src/ExpressionC.scala

@@ -1,6 +1,7 @@
 package ch05
 
 // Continuation-Passing style.
+/*  Exercise: CPS Arithmetic */
 enum ExpressionC:
   case Literal(value: Double)
   case Addition(left: ExpressionC, right: ExpressionC)

ch05/src/ExpressionT.scala

@@ -1,5 +1,6 @@
 package ch05
 
+/* Exercise: Trampolined Arithmetic */
 enum ExpressionT:
   case Literal(value: Double)
   case Addition(left: ExpressionT, right: ExpressionT)