New test: covariant hmaps

Type inference tends to take quite different paths for non-variant
and variant data structures. Since, non-variant hmap has already exposed quite
a few problems, it's good to test it also in the covariant case.

Author: odersky

tests/run/hmap-covariant.scala

@@ -0,0 +1,97 @@
+trait Tuple
+case class TCons[+H, +T <: Tuple](h: H, t: T) extends Tuple
+final case object TNil extends Tuple
+
+// Type level natural numbers -------------------------------------------------
+
+sealed trait Nat
+sealed trait Succ[P <: Nat] extends Nat
+sealed trait Zero extends Nat
+
+// Accessor type class to compute the N'th element of an Tuple L --------------
+
+trait At[L <: Tuple, N <: Nat, Out] {
+  def apply(l: L): Out
+}
+
+object At {
+  implicit def caseZero[H, T <: Tuple]: At[H TCons T, Zero, H] =
+    new At[H TCons T, Zero, H] {
+      def apply(l: H TCons T): H = {
+        val (h TCons _) = l
+        h
+      }
+    }
+
+  implicit def caseN[H, T <: Tuple, N <: Nat, O]
+    (implicit a: At[T, N, O]): At[H TCons T, Succ[N], O] =
+      new At[H TCons T, Succ[N], O] {
+        def apply(l: H TCons T): O = {
+          val (_ TCons t) = l
+          a(t)
+        }
+      }
+}
+
+// An HMap is an Tuple with HEntry elements. We are reusing Tuple for it's nice syntax
+
+final case class HEntry[K, V](value: V)
+
+// Accessor type class to compute the element of type K in a HMap L -----------
+
+trait PhantomGet[K, M <: Tuple, I <: Nat] // extends PhantomAny
+
+object PhantomGet {
+  implicit def getHead[K, V, T <: Tuple]
+    : PhantomGet[K, HEntry[K, V] TCons T, Zero] = null
+
+  implicit def getTail[K, H, T <: Tuple, I <: Nat]
+    (implicit t: PhantomGet[K, T, I])
+    : PhantomGet[K, H TCons T, Succ[I]] = null
+}
+
+// Syntax ---------------------------------------------------------------------
+
+object syntax {
+  object hmap {
+    implicit class HmapGet[M <: Tuple](m: M) {
+      def get[K, V, I <: Nat](k: K)
+        (implicit
+          g: PhantomGet[k.type, M, I],
+          a: At[M, I, HEntry[k.type, V]]
+        ): V = a(m).value
+    }
+
+    def --[K, V](key: K, value: V) = HEntry[key.type, V](value)
+  }
+}
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    import syntax.hmap._
+
+    val map1 =
+      TCons(HEntry[K = "name"]("foo"),
+      TCons(HEntry[K = "genre"](true),
+      TCons(HEntry[K = "moneyz"](123),
+      TCons(HEntry[K = "cat"]("bar"),
+      (TNil: TNil.type)))))
+
+    assert(map1.get("name") == "foo")
+    assert(map1.get("genre") == true)
+    assert(map1.get("moneyz") == 123)
+    assert(map1.get("cat") == "bar")
+
+    val map2 =
+      TCons(--("name"  , "foo"),
+      TCons(--("genre" , true),
+      TCons(--("moneyz", 123),
+      TCons(--("cat"   , "bar"),
+      TNil))))
+
+    assert(map2.get("name") == "foo")
+    assert(map2.get("genre") == true)
+    assert(map2.get("moneyz") == 123)
+    assert(map2.get("cat") == "bar")
+  }
+}