refs #10: Added AsArray type class

shared/src/main/scala/io/estatico/newtype/arrays/AsArray.scala

@@ -0,0 +1,38 @@
+package io.estatico.newtype.arrays
+
+import scala.reflect.ClassTag
+
+trait AsArray[N] {
+  type Repr
+  def clsTag: ClassTag[Repr]
+
+  final def empty: Array[N] = Array.empty(clsTag).asInstanceOf[Array[N]]
+
+  final def apply(xs: N*): Array[N] =
+    Array(xs.asInstanceOf[Seq[Repr]]: _*)(clsTag).asInstanceOf[Array[N]]
+
+  final def upcast[R](array: Array[R]): Array[N] = array.asInstanceOf[Array[N]]
+
+  final def downcast(array: Array[N]): Array[Repr] = array.asInstanceOf[Array[Repr]]
+}
+
+object AsArray {
+
+  type Aux[N, R] = AsArray[N] { type Repr = R }
+
+  /** Summon the AsArray instance for the given newtype N. */
+  def apply[N](implicit ev: AsArray[N]): Aux[N, ev.Repr] = ev
+
+  /** Construct a new array of newtype N from varargs. */
+  def apply[N](xs: N*)(implicit ev: AsArray[N]): Array[N] = ev(xs: _*)
+
+  /** Construct an empty array of newtype N. */
+  def empty[N](implicit ev: AsArray[N]): Array[N] = ev.empty
+
+  def unsafeDerive[N, R](implicit ct: ClassTag[R]): Aux[N, R] =
+    new AsArray[N] {
+      type Repr = R
+      override def clsTag: ClassTag[Repr] = ct
+    }
+}
+

shared/src/main/scala/io/estatico/newtype/macros/NewTypeMacros.scala

@@ -1,6 +1,7 @@
 package io.estatico.newtype.macros
 
 import io.estatico.newtype.Coercible
+import io.estatico.newtype.arrays.AsArray
 import scala.reflect.ClassTag
 import scala.reflect.macros.blackbox
 
@@ -39,8 +40,8 @@ private[macros] class NewTypeMacros(val c: blackbox.Context)
 
   val CoercibleCls = typeOf[Coercible[Nothing, Nothing]].typeSymbol
   val CoercibleObj = CoercibleCls.companion
+  val AsArrayObj = q"io.estatico.newtype.arrays.AsArray"
   val ClassTagCls = typeOf[ClassTag[Nothing]].typeSymbol
-  val ClassTagObj = ClassTagCls.companion
   val ObjectCls = typeOf[Object].typeSymbol
 
   // We need to know if the newtype is defined in an object so we can report
@@ -123,7 +124,8 @@ private[macros] class NewTypeMacros(val c: blackbox.Context)
       maybeGenerateUnapplyMethod(clsDef, valDef, tparamsNoVar, tparamNames) :::
       maybeGenerateOpsDef(clsDef, valDef, tparamsNoVar, tparamNames) :::
       generateCoercibleInstances(tparamsNoVar, tparamNames, tparamsWild) :::
-      generateDerivingMethods(tparamsNoVar, tparamNames, tparamsWild)
+      generateDerivingMethods(tparamsNoVar, tparamNames, tparamsWild) :::
+      List(generateAsArrayInstance(clsDef, valDef, tparamsNoVar, tparamNames))
 
     val newtypeObjParents = objParents :+ tq"$typesTraitName"
     val newtypeObjDef = q"""
@@ -318,6 +320,36 @@ private[macros] class NewTypeMacros(val c: blackbox.Context)
     )
   }
 
+  def generateAsArrayInstance(
+    clsDef: ClassDef, valDef: ValDef, tparamsNoVar: List[TypeDef], tparamNames: List[TypeName]
+  ): Tree = {
+    val Repr = tq"${valDef.tpt}"
+    val Type = if (tparamNames.isEmpty) tq"${clsDef.name}" else tq"${clsDef.name}[..$tparamNames]"
+    //val clsTagType = if (tparamNames.isEmpty) Repr else tq"$Repr forSome { ..$tparamsNoVar }"
+    summonImplicit(tq"$ClassTagCls[$Repr]") match {
+      case Some(Typed(ct, _)) =>
+        if (tparamNames.isEmpty) {
+          q"""implicit def asArray: $AsArrayObj.Aux[$Type, $Repr] =
+                $AsArrayObj.unsafeDerive[$Type, $Repr]($ct)"""
+        } else {
+          q"""implicit def asArray[..$tparamsNoVar]: $AsArrayObj.Aux[$Type, $Repr] =
+                $AsArrayObj.unsafeDerive[$Type, $Repr]($ct)"""
+        }
+      case _ =>
+        if (tparamsNoVar.isEmpty) {
+          q"""implicit def asArray(
+                implicit ct: $ClassTagCls[$Repr]
+              ): $AsArrayObj.Aux[$Type, $Repr] =
+                $AsArrayObj.unsafeDerive[$Type, $Repr]"""
+        } else {
+          q"""implicit def asArray[..$tparamsNoVar](
+              implicit ct: $ClassTagCls[$Repr]
+            ): $AsArrayObj.Aux[$Type, $Repr] =
+              $AsArrayObj.unsafeDerive[$Type, $Repr]"""
+        }
+    }
+  }
+
   def getConstructor(body: List[Tree]): DefDef = body.collectFirst {
     case dd: DefDef if dd.name == termNames.CONSTRUCTOR => dd
   }.getOrElse(fail("Failed to locate constructor"))
@@ -350,4 +382,13 @@ private[macros] class NewTypeMacros(val c: blackbox.Context)
       fail(s"newtypes do not support inheritance; illegal supertypes: ${unsupported.mkString(", ")}")
     }
   }
+
+    /** Return the implicit value, if exists, for the given type `tpt`. */
+  def summonImplicit(tpt: Tree): Option[Tree] = {
+    val typeResult = c.typecheck(tpt, c.TYPEmode, silent = true)
+    if (typeResult.isEmpty) None else {
+      val implicitResult = c.inferImplicitValue(typeResult.tpe)
+      if (implicitResult.isEmpty) None else Some(implicitResult)
+    }
+  }
 }

shared/src/test/scala/io/estatico/newtype/macros/NewTypeMacrosTest.scala

@@ -1,5 +1,6 @@
 package io.estatico.newtype.macros
 
+import io.estatico.newtype.arrays.AsArray
 import org.scalatest.{FlatSpec, Matchers}
 import io.estatico.newtype.ops._
 import org.scalacheck.Arbitrary
@@ -39,9 +40,7 @@ class NewTypeMacrosTest extends FlatSpec with Matchers {
 
   it should "work in arrays" in {
     val foo = Foo(313)
-    // See https://github.com/estatico/scala-newtype/issues/25
-    // Array(foo).head shouldBe foo
-    Array[Int](313).asInstanceOf[Array[Foo]].head shouldBe foo
+    AsArray(foo).head shouldBe foo
   }
 
   behavior of "@newtype class"
@@ -101,9 +100,7 @@ class NewTypeMacrosTest extends FlatSpec with Matchers {
   it should "work in arrays" in {
     val repr = Set(Option("newtypes"))
     val ot = OptionT(repr)
-    // See https://github.com/estatico/scala-newtype/issues/25
-    // Array(ot).head shouldBe ot
-    Array(repr).asInstanceOf[Array[OptionT[Set, String]]].head shouldBe ot
+    AsArray(ot).head shouldBe ot
   }
 
   behavior of "@newtype with type bounds"