add as extension method to optics in Scala 2 and 3 (#1110)

* add as extension method to optics in Scala 2 and 3

* use extension syntax in Scala 3

* use as syntax within GenPrism

core/shared/src/main/scala-2.x/monocle/syntax/MacroSyntax.scala

@@ -0,0 +1,3 @@
+package monocle.syntax
+
+trait MacroSyntax

core/shared/src/main/scala-3.x/monocle/syntax/MacroSyntax.scala

@@ -0,0 +1,47 @@
+package monocle.syntax
+
+import monocle.{Focus, Fold, Iso, Optional, Prism, Setter, Traversal}
+
+import scala.quoted.{Expr, Quotes, Type}
+
+trait MacroSyntax {
+
+  extension [From, To] (optic: Prism[From, To]) {
+    inline def as[CastTo <: To]: Prism[From, CastTo] =
+      optic.andThen(GenPrism[To, CastTo])
+  }
+
+  extension [From, To] (optic: Optional[From, To]) {
+    inline def as[CastTo <: To]: Optional[From, CastTo] =
+      optic.andThen(GenPrism[To, CastTo])
+  }
+
+  extension [From, To] (optic: Traversal[From, To]) {
+    inline def as[CastTo <: To]: Traversal[From, CastTo] =
+      optic.andThen(GenPrism[To, CastTo])
+  }
+
+  extension [From, To] (optic: Setter[From, To]) {
+    inline def as[CastTo <: To]: Setter[From, CastTo] =
+      optic.andThen(GenPrism[To, CastTo])
+  }
+
+  extension [From, To] (optic: Fold[From, To]) {
+    inline def as[CastTo <: To]: Fold[From, CastTo] =
+      optic.andThen(GenPrism[To, CastTo])
+  }
+
+}
+
+private[monocle] object GenPrism {
+  inline def apply[From, To <: From]: Prism[From, To] =
+    ${ GenPrismImpl.apply }
+}
+
+private[monocle] object GenPrismImpl {
+  def apply[From: Type, To: Type](using Quotes): Expr[Prism[From, To]] =
+    '{
+      Prism[From, To]((from: From) => if (from.isInstanceOf[To]) Some(from.asInstanceOf[To]) else None)(
+        (to: To) => to.asInstanceOf[From])
+    }
+}

core/shared/src/main/scala/monocle/syntax/All.scala

@@ -2,4 +2,4 @@ package monocle.syntax
 
 object all extends Syntaxes
 
-trait Syntaxes extends ApplySyntax with AppliedFocusSyntax with FieldsSyntax
+trait Syntaxes extends ApplySyntax with AppliedFocusSyntax with MacroSyntax with FieldsSyntax

core/shared/src/test/scala-3.x/monocle/syntax/AsSyntaxSpec.scala

@@ -0,0 +1,31 @@
+package monocle.syntax
+
+import monocle._
+import monocle.syntax.all._
+import munit.DisciplineSuite
+
+class AsSyntaxSpec extends DisciplineSuite {
+
+  sealed trait StringOrInt
+  case class S(value: String) extends StringOrInt
+  case class I(value: Int) extends StringOrInt
+
+  val iso: Iso[StringOrInt, StringOrInt]             = Iso.id
+  val lens: Lens[StringOrInt, StringOrInt]           = Iso.id
+  val prism: Prism[StringOrInt, StringOrInt]         = Iso.id
+  val optional: Optional[StringOrInt, StringOrInt]   = Iso.id
+  val traversal: Traversal[StringOrInt, StringOrInt] = Iso.id
+  val setter: Setter[StringOrInt, StringOrInt]       = Iso.id
+  val getter: Getter[StringOrInt, StringOrInt]       = Iso.id
+  val fold: Fold[StringOrInt, StringOrInt]           = Iso.id
+
+  test("iso.as"){ assertEquals(iso.as[I].getOption(I(1)), Some(I(1))) }
+  test("lens.as"){ assertEquals(lens.as[I].getOption(I(1)), Some(I(1))) }
+  test("prism.as"){ assertEquals(prism.as[I].getOption(I(1)), Some(I(1))) }
+  test("optional.as"){ assertEquals(optional.as[I].getOption(I(1)), Some(I(1))) }
+  test("traversal.as"){ assertEquals(traversal.as[I].getAll(I(1)), List(I(1))) }
+  test("setter.as"){ assertEquals(setter.as[I].replace(I(5))(I(1)), I(5)) }
+  test("getter.as"){ assert(getter.as[I].getAll(I(1)) == List(I(1))) }
+  test("fold.as"){ assert(fold.as[I].getAll(I(1)) == List(I(1))) }
+
+}

macro/src/main/scala-2.x/monocle/macros/syntax/MacroSyntax.scala

@@ -0,0 +1,48 @@
+package monocle.macros.syntax
+
+import monocle.{Fold, Optional, Prism, Setter, Traversal}
+
+import scala.reflect.macros.blackbox
+
+trait MacroSyntax {
+  implicit def toMacroPrismOps[S, A](optic: Prism[S, A]): MacroPrismOps[S, A]             = new MacroPrismOps(optic)
+  implicit def toMacroOptionalOps[S, A](optic: Optional[S, A]): MacroOptionalOps[S, A]    = new MacroOptionalOps(optic)
+  implicit def toMacroTraversalOps[S, A](optic: Traversal[S, A]): MacroTraversalOps[S, A] = new MacroTraversalOps(optic)
+  implicit def toMacroSetterOps[S, A](optic: Setter[S, A]): MacroSetterOps[S, A]          = new MacroSetterOps(optic)
+  implicit def toMacroFoldOps[S, A](optic: Fold[S, A]): MacroFoldOps[S, A]                = new MacroFoldOps(optic)
+}
+
+class MacroPrismOps[S, A](private val optic: Prism[S, A]) extends AnyVal {
+  def as[CastTo <: A]: Prism[S, CastTo] = macro MacroAsOpsImpl.as_impl[Prism, S, A, CastTo]
+}
+
+class MacroOptionalOps[S, A](private val optic: Optional[S, A]) extends AnyVal {
+  def as[CastTo <: A]: Optional[S, CastTo] = macro MacroAsOpsImpl.as_impl[Optional, S, A, CastTo]
+}
+
+class MacroTraversalOps[S, A](private val optic: Traversal[S, A]) extends AnyVal {
+  def as[CastTo <: A]: Traversal[S, CastTo] = macro MacroAsOpsImpl.as_impl[Traversal, S, A, CastTo]
+}
+
+class MacroSetterOps[S, A](private val optic: Setter[S, A]) extends AnyVal {
+  def as[CastTo <: A]: Setter[S, CastTo] = macro MacroAsOpsImpl.as_impl[Setter, S, A, CastTo]
+}
+
+class MacroFoldOps[S, A](private val optic: Fold[S, A]) extends AnyVal {
+  def as[CastTo <: A]: Fold[S, CastTo] = macro MacroAsOpsImpl.as_impl[Fold, S, A, CastTo]
+}
+
+class MacroAsOpsImpl(val c: blackbox.Context) {
+  def as_impl[Optic[_, _], From, To: c.WeakTypeTag, CastTo: c.WeakTypeTag]: c.Expr[Optic[From, CastTo]] = {
+    import c.universe._
+
+    val subj = c.prefix.tree match {
+      case Apply(TypeApply(_, _), List(x)) => x
+      case t                               => c.abort(c.enclosingPosition, s"Invalid prefix tree ${show(t)}")
+    }
+
+    c.Expr[Optic[From, CastTo]](
+      q"""$subj.andThen(_root_.monocle.macros.GenPrism[${c.weakTypeOf[To]}, ${c.weakTypeOf[CastTo]}])"""
+    )
+  }
+}

macro/src/main/scala-2.x/monocle/macros/syntax/all.scala

@@ -1,3 +1,3 @@
 package monocle.macros.syntax
 
-object all extends ApplyFocusSyntax
+object all extends ApplyFocusSyntax with MacroSyntax

macro/src/main/scala-3.x/monocle/macros/GenPrism.scala

@@ -1,8 +1,9 @@
 package monocle.macros
 
-import monocle.Focus
+import monocle.{Focus, Prism}
+import monocle.syntax.all._
 
 object GenPrism {
-  transparent inline def apply[Source, Target <: Source] =
-    Focus[Source](_.as[Target])
+  inline def apply[Source, Target <: Source]: Prism[Source, Target] =
+    Focus[Source]().as[Target]
 }

macro/src/test/scala-2.x/monocle/macros/AsSyntaxSpec.scala

@@ -0,0 +1,31 @@
+package monocle.macros
+
+import monocle.macros.syntax.all._
+import monocle._
+import munit.DisciplineSuite
+
+class AsSyntaxSpec extends DisciplineSuite {
+
+  sealed trait StringOrInt
+  case class S(value: String) extends StringOrInt
+  case class I(value: Int)    extends StringOrInt
+
+  val iso: Iso[StringOrInt, StringOrInt]             = Iso.id
+  val lens: Lens[StringOrInt, StringOrInt]           = Iso.id
+  val prism: Prism[StringOrInt, StringOrInt]         = Iso.id
+  val optional: Optional[StringOrInt, StringOrInt]   = Iso.id
+  val traversal: Traversal[StringOrInt, StringOrInt] = Iso.id
+  val setter: Setter[StringOrInt, StringOrInt]       = Iso.id
+  val getter: Getter[StringOrInt, StringOrInt]       = Iso.id
+  val fold: Fold[StringOrInt, StringOrInt]           = Iso.id
+
+  test("iso.as")(assertEquals(iso.as[I].getOption(I(1)), Some(I(1))))
+  test("lens.as")(assertEquals(lens.as[I].getOption(I(1)), Some(I(1))))
+  test("prism.as")(assertEquals(prism.as[I].getOption(I(1)), Some(I(1))))
+  test("optional.as")(assertEquals(optional.as[I].getOption(I(1)), Some(I(1))))
+  test("traversal.as")(assertEquals(traversal.as[I].getAll(I(1)), List(I(1))))
+  test("setter.as")(assertEquals(setter.as[I].replace(I(5))(I(1)), I(5)))
+  test("getter.as")(assert(getter.as[I].getAll(I(1)) == List(I(1))))
+  test("fold.as")(assert(fold.as[I].getAll(I(1)) == List(I(1))))
+
+}