[09] update for 2022

README.md

@@ -18,6 +18,9 @@ usually marked with `???`. Lectures not listed here are not migrated to Scala
 | 4: Laziness                     | [`laziness`](src/main/scala/laziness/)                            | `testOnly laziness.*`
 | 5: Algebras, Laws and monoids   | [`algebra`](src/main/scala/algebra/)                              | `testOnly algebra.*`
 | 6: Foldables and Functors       | [`foldfunc`](src/main/scala/foldfunc/)                            | `testOnly foldfunc.*`
+| 7: Monads                       | [`monads`](src/main/scala/monads/)                                | `testOnly monads.*`
+| 8: Applicative Functors         | [`applicative`](src/main/scala/applicative/)                      | `testOnly applicative.*`
+| 9: An algebraic View on Monads  | [`readerwriter`](src/main/scala/readerwriter/)                    | `testOnly readerwriter.*`
 
 ## Usage tips:
 To keep your local solutions to the exercises when pulling from the repository,

src/main/scala/readerwriter/Randoms.scala

 package readerwriter
 
-import readerwriter.internal.State, State._
-import util._
+import readerwriter.internal.State, State.*
 
 object Randoms:
   def threeInts: State[RNG, (Int, Int, Int)] = ???
 
-  /*@formatter:off*/
   def randomInt: State[RNG, Int] = for
     rng      <- get[RNG]
     (rng2, i) = rng.nextInt
     _        <- set(rng2)
   yield i
-  /*@formatter:on*/
-
 
   def nonNegativeInt: State[RNG, Int] = for
     i <- randomInt
@@ -23,6 +19,8 @@ trait RNG:
   def nextInt: (RNG, Int)
 
 case class Simple(seed: Long) extends RNG:
+  /** Basically the same formula as in java.util.Random
+   *  See https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/Random.html#next(int) */
   def nextInt: (RNG, Int) =
     val newSeed = (seed * 0x5DEECE66DL + 0xBL) & 0xFFFFFFFFFFFFL
     val nextRNG = Simple(newSeed)

src/main/scala/readerwriter/Readers.scala

@@ -2,8 +2,7 @@ package readerwriter
 
 import java.time.LocalDate
 
-import readerwriter.internal.Reader, Reader._
-import util._
+import readerwriter.internal.Reader, Reader.*
 
 final case class Request(
     user: Option[String],

src/main/scala/readerwriter/Writers.scala

 package readerwriter
 
 
-import readerwriter.internal.Writer, Writer._
-import util._
-
+import readerwriter.internal.Writer, Writer.{*, given}
 
 object Writers:
   /* everything required to use tell and the monad operations
@@ -13,7 +11,6 @@ object Writers:
 
   def collatzSearch(start: Int, limit: Int): Writer[List[String], Int] = ???
 
-
 object CollatzWithoutWriter:
   def collatzDepth(n: Int): (List[String], Int) =
     if n == 1 then

src/main/scala/readerwriter/internal/Reader.scala

@@ -2,7 +2,7 @@ package readerwriter.internal
 
 import applicative.Monad
 
-case class Reader[R, A](run: R => A)
+case class Reader[-R, A](run: R => A)
 
 object Reader:
   def ask[R]: Reader[R, R] = Reader(x => x)

src/main/scala/readerwriter/internal/State.scala

@@ -8,6 +8,9 @@ import applicative.Monad
 case class State[S, A] private (run: S => (S, A))
 
 case object State:
+  def get[S]: State[S, S] = State(s => (s, s))
+  def set[S](s: S): State[S, Unit] = State(_ => (s, ()))
+
   given stateMonad[S]: Monad[[a] =>> State[S, a]] with
     def pure[A](a: A): State[S, A] = State(s => (s, a))
     extension [A](fa: State[S, A])
@@ -15,6 +18,3 @@ case object State:
         val (s1, a) = fa.run(s)
         f(a).run(s1)
       )
-
-  def get[S]: State[S, S] = State(s => (s, s))
-  def set[S](s: S): State[S, Unit] = State(_ => (s, ()))

src/main/scala/readerwriter/internal/Writer.scala

@@ -6,6 +6,7 @@ import applicative.Monad
 final case class Writer[L, A](v: (L, A))
 
 object Writer:
+
   given [A]: Monoid[List[A]] with
     def zero: List[Nothing] = List.empty
     def combine(l1: List[A], l2: List[A]): List[A] = l1 ++ l2

src/main/scala/util/package.scala

-import applicative.Monad
-
-package object util:
-  implicit class MonadOps[F[_] : Monad, A](fa: F[A]):
-    def flatMap[B](f: A => F[B]): F[B] = implicitly[Monad[F]].flatMap(fa)(f)
-    def map[B](f: A => B): F[B] = implicitly[Monad[F]].map(fa)(f)

src/test/scala/readerwriter/ReaderSpec.scala

@@ -27,6 +27,6 @@ class ReaderSpec extends AnyFlatSpec with Matchers with AppendedClues with Pendi
   }
 
   "sayBye" should "contain the correct user name and date" in {
-    Readers.sayBye.run(exampleRequest) shouldBe "Goodbye Mister X, today is 2019-06-26"
+    Readers.sayBye.run(exampleRequest) shouldBe "Goodbye Mister X, now is 2019-06-26"
   }