Some new tests

 - Some variants of typeclasses
 - Revised Pouring.scala with some namign improvements

Author: odersky

Diff for: tests/pos/typeclasses.scala

@@ -0,0 +1,198 @@
+class Common:
+
+  // this should go in Predef
+  infix type at [A <: { type This}, B] = A { type This = B }
+
+  trait Ord:
+    type This
+    extension (x: This)
+      def compareTo(y: This): Int
+      def < (y: This): Boolean = compareTo(y) < 0
+      def > (y: This): Boolean = compareTo(y) > 0
+
+  trait SemiGroup:
+    type This
+    extension (x: This) def combine(y: This): This
+
+  trait Monoid extends SemiGroup:
+    def unit: This
+
+  trait Functor:
+    type This[A]
+    extension [A](x: This[A]) def map[B](f: A => B): This[B]
+
+  trait Monad extends Functor:
+    def pure[A](x: A): This[A]
+    extension [A](x: This[A])
+      def flatMap[B](f: A => This[B]): This[B]
+      def map[B](f: A => B) = x.flatMap(f `andThen` pure)
+end Common
+
+
+object Instances extends Common:
+
+/*
+  instance Int: Ord as intOrd with
+    extension (x: Int)
+      def compareTo(y: Int) =
+        if x < y then -1
+        else if x > y then +1
+        else 0
+*/
+  given intOrd: Ord with
+    type This = Int
+    extension (x: Int)
+      def compareTo(y: Int) =
+        if x < y then -1
+        else if x > y then +1
+        else 0
+/*
+  instance List[T: Ord]: Ord as listOrd with
+    extension (xs: List[T]) def compareTo(ys: List[T]): Int = (xs, ys) match
+      case (Nil, Nil) => 0
+      case (Nil, _) => -1
+      case (_, Nil) => +1
+      case (x :: xs1, y :: ys1) =>
+        val fst = x.compareTo(y)
+        if (fst != 0) fst else xs1.compareTo(ys1)
+*/
+
+  // Proposed short syntax:
+  // given listOrd[T: Ord as ord]: Ord at T with
+  given listOrd[T](using ord: Ord { type This = T}): Ord with
+    type This = List[T]
+    extension (xs: List[T]) def compareTo(ys: List[T]): Int = (xs, ys) match
+      case (Nil, Nil) => 0
+      case (Nil, _) => -1
+      case (_, Nil) => +1
+      case (x :: xs1, y :: ys1) =>
+        val fst = x.compareTo(y)
+        if (fst != 0) fst else xs1.compareTo(ys1)
+  end listOrd
+
+/*
+  instance List: Monad as listMonad with
+    extension [A](xs: List[A]) def flatMap[B](f: A => List[B]): List[B] =
+      xs.flatMap(f)
+    def pure[A](x: A): List[A] =
+      List(x)
+*/
+
+  given listMonad: Monad with
+    type This[A] = List[A]
+    extension [A](xs: List[A]) def flatMap[B](f: A => List[B]): List[B] =
+      xs.flatMap(f)
+    def pure[A](x: A): List[A] =
+      List(x)
+
+/*
+  type Reader[Ctx] = X =>> Ctx => X
+  instance Reader[Ctx: _]: Monad as readerMonad with
+    extension [A](r: Ctx => A) def flatMap[B](f: A => Ctx => B): Ctx => B =
+      ctx => f(r(ctx))(ctx)
+    def pure[A](x: A): Ctx => A =
+      ctx => x
+*/
+
+  given readerMonad[Ctx]: Monad with
+    type This[X] = Ctx => X
+    extension [A](r: Ctx => A) def flatMap[B](f: A => Ctx => B): Ctx => B =
+      ctx => f(r(ctx))(ctx)
+    def pure[A](x: A): Ctx => A =
+      ctx => x
+
+  extension (xs: Seq[String])
+    def longestStrings: Seq[String] =
+      val maxLength = xs.map(_.length).max
+      xs.filter(_.length == maxLength)
+
+  extension [T](xs: List[T])
+    def second = xs.tail.head
+    def third = xs.tail.tail.head
+
+  //Proposed short syntax:
+  //extension [M: Monad as m, A](xss: M[M[A]])
+  //  def flatten: M[A] =
+  //    xs.flatMap(identity)
+
+  extension [M, A](using m: Monad)(xss: m.This[m.This[A]])
+    def flatten: m.This[A] =
+      xss.flatMap(identity)
+
+  // Proposed short syntax:
+  //def maximum[T: Ord](xs: List[T]: T =
+  def maximum[T](xs: List[T])(using Ord at T): T =
+    xs.reduceLeft((x, y) => if (x < y) y else x)
+
+  // Proposed short syntax:
+  // def descending[T: Ord as asc]: Ord at T = new Ord:
+  def descending[T](using asc: Ord at T): Ord at T = new Ord:
+    type This = T
+    extension (x: T) def compareTo(y: T) = asc.compareTo(y)(x)
+
+  // Proposed short syntax:
+  // def minimum[T: Ord](xs: List[T]) =
+  def minimum[T](xs: List[T])(using Ord at T) =
+    maximum(xs)(using descending)
+
+  def test(): Unit =
+    val xs = List(1, 2, 3)
+    println(maximum(xs))
+    println(maximum(xs)(using descending))
+    println(maximum(xs)(using descending(using intOrd)))
+    println(minimum(xs))
+
+// Adapted from the Rust by Example book: https://doc.rust-lang.org/rust-by-example/trait.html
+//
+//           lines  words  chars
+// wc Scala: 30     115     853
+// wc Rust : 57     193    1466
+trait Animal:
+  type This
+  // Associated function signature; `This` refers to the implementor type.
+  def apply(name: String): This
+
+  // Method signatures; these will return a string.
+  extension (self: This)
+    def name: String
+    def noise: String
+    def talk(): Unit = println(s"$name, $noise")
+end Animal
+
+class Sheep(val name: String):
+  var isNaked = false
+  def shear() =
+    if isNaked then
+      println(s"$name is already naked...")
+    else
+      println(s"$name gets a haircut!")
+      isNaked = true
+
+/*
+instance Sheep: Animal with
+  def apply(name: String) = Sheep(name)
+  extension (self: This)
+    def name: String = self.name
+    def noise: String = if self.isNaked then "baaaaah?" else "baaaaah!"
+    override def talk(): Unit =
+      println(s"$name pauses briefly... $noise")
+*/
+
+// Implement the `Animal` trait for `Sheep`.
+given Animal with
+  type This = Sheep
+  def apply(name: String) = Sheep(name)
+  extension (self: This)
+    def name: String = self.name
+    def noise: String = if self.isNaked then "baaaaah?" else "baaaaah!"
+    override def talk(): Unit =
+      println(s"$name pauses briefly... $noise")
+
+/*
+
+ - In a type pattern, A <: T, A >: T, A: T, A: _ are all allowed and mean
+   T is a fresh type variable (T can start with a capital letter).
+ - instance definitions
+ - `as m` syntax in context bounds and instance definitions
+
+*/

Diff for: tests/run/Pouring.check

@@ -1,2 +1 @@
-Vector(Empty(0), Empty(1), Fill(0), Fill(1), Pour(0,1), Pour(1,0))
-Fill(1) Pour(1,0) Empty(0) Pour(1,0) Fill(1) Pour(1,0) --> Vector(4, 6)
+Illegal command line: more arguments expected

Diff for: tests/run/Pouring.scala

@@ -1,37 +1,35 @@
-class Pouring(capacity: Vector[Int]):
-  type Glass = Int
-  type Content = Vector[Int]
+type Glass = Int
+type Levels = Vector[Int]
 
-  enum Move:
-    def apply(content: Content): Content = this match
-      case Empty(g) => content.updated(g, 0)
-      case Fill(g) => content.updated(g, capacity(g))
-      case Pour(from, to) =>
-        val amount = content(from) min (capacity(to) - content(to))
-        extension (s: Content) def adjust(g: Glass, delta: Int) = s.updated(g, s(g) + delta)
-        content.adjust(from, -amount).adjust(to, amount)
+class Pouring(capacity: Levels):
 
+  enum Move:
     case Empty(glass: Glass)
     case Fill(glass: Glass)
     case Pour(from: Glass, to: Glass)
+
+    def apply(levels: Levels): Levels = this match
+      case Empty(glass) =>
+        levels.updated(glass, 0)
+      case Fill(glass) =>
+        levels.updated(glass, capacity(glass))
+      case Pour(from, to) =>
+        val amount = levels(from) min (capacity(to) - levels(to))
+        levels.updated(from, levels(from) - amount)
+              .updated(to,   levels(to)   + amount)
   end Move
 
+  val glasses = 0 until capacity.length
   val moves =
-    val glasses = 0 until capacity.length
-
-       (for g <- glasses yield Move.Empty(g))
+    (for g <- glasses yield Move.Empty(g))
     ++ (for g <- glasses yield Move.Fill(g))
     ++ (for g1 <- glasses; g2 <- glasses if g1 != g2 yield Move.Pour(g1, g2))
 
-  class Path(history: List[Move], val endContent: Content):
+  class Path(history: List[Move], val endContent: Levels):
     def extend(move: Move) = Path(move :: history, move(endContent))
     override def toString = s"${history.reverse.mkString(" ")} --> $endContent"
-  end Path
-
-  val initialContent: Content = capacity.map(x => 0)
-  val initialPath = Path(Nil, initialContent)
 
-  def from(paths: Set[Path], explored: Set[Content]): LazyList[Set[Path]] =
+  def from(paths: Set[Path], explored: Set[Levels]): LazyList[Set[Path]] =
     if paths.isEmpty then LazyList.empty
     else
       val extensions =
@@ -44,14 +42,16 @@ class Pouring(capacity: Vector[Int]):
       paths #:: from(extensions, explored ++ extensions.map(_.endContent))
 
   def solutions(target: Int): LazyList[Path] =
+    val initialContent: Levels = capacity.map(_ => 0)
+    val initialPath = Path(Nil, initialContent)
     for
       paths <- from(Set(initialPath), Set(initialContent))
       path <- paths
       if path.endContent.contains(target)
     yield path
 end Pouring
 
-@main def Test =
-  val problem = Pouring(Vector(4, 7))
-  println(problem.moves)
-  println(problem.solutions(6).head)
+@main def Test(target: Int, capacities: Int*) =
+  val problem = Pouring(capacities.toVector)
+  println(s"Moves: ${problem.moves}")
+  println(s"Solution: ${problem.solutions(target).headOption}")