feat(scala): m07 io and m08 stream with tests

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scala/solutions/IO.scala

@@ -0,0 +1,62 @@
+package snake
+
+/** An operation an IO program can ask the outside world to perform. */
+enum Op:
+  case Draw(shape: Shape)
+  case RandomSeed
+
+/** What the outside world answers: `Done` for a `Draw`, `Seed(value)` for a `RandomSeed`. */
+enum OpResult:
+  case Done
+  case Seed(value: Long)
+
+/**
+ * m07 io: effects as values. An IO is a plain-data description of a program; nothing runs until interpreted.
+ *   - `Pure(a)`         done, with a value
+ *   - `Effect(op, k)`   perform `op`, then continue with `k(result)`
+ */
+enum IO[+A]:
+  case Pure(a: A)
+  case Effect(op: Op, k: OpResult => IO[A])
+
+  def map[B](f: A => B): IO[B] = flatMap(a => IO.Pure(f(a)))
+
+  // Pure: apply f now. Effect: keep the op, and extend the continuation so f runs after k.
+  // Nothing is performed here, we only build a bigger description.
+  def flatMap[B](f: A => IO[B]): IO[B] = this match
+    case Pure(a)       => f(a)
+    case Effect(op, k) => Effect(op, result => k(result).flatMap(f))
+
+final case class Traced[A](result: A, ops: List[Op])
+
+object IO:
+
+  def pure[A](a: A): IO[A] = Pure(a)
+
+  def effect(op: Op): IO[OpResult] = Effect(op, pure)
+
+  def sequence(ios: List[IO[Unit]]): IO[Unit] =
+    ios.foldRight(pure(()))((io, rest) => io.flatMap(_ => rest))
+
+  def draw(shape: Shape): IO[Unit] = effect(Op.Draw(shape)).map(_ => ())
+
+  // An interpreter that answers a RandomSeed with anything but a Seed is broken; say so loudly.
+  lazy val randomSeed: IO[Long] = effect(Op.RandomSeed).map {
+    case OpResult.Seed(value) => value
+    case OpResult.Done        => throw new IllegalStateException("RandomSeed must be answered with a Seed")
+  }
+
+  def drawScene(scene: Scene): IO[Unit] = sequence(scene.map(draw))
+
+  // Answer each op with a canned result, collecting the ops as we go. Tail recursion: the compiler turns it
+  // into a loop, so long programs cannot overflow the stack, and still nothing is mutated.
+  def trace[A](io: IO[A], seed: Long): Traced[A] =
+    @annotation.tailrec
+    def go(io: IO[A], done: List[Op]): Traced[A] = io match
+      case Pure(a) => Traced(a, done.reverse)
+      case Effect(op, k) =>
+        val answer = op match
+          case Op.RandomSeed => OpResult.Seed(seed)
+          case Op.Draw(_)    => OpResult.Done
+        go(k(answer), op :: done)
+    go(io, Nil)