Enable asRoot on FilePath.Empty

core/src/main/scala/eyrie/instances/DescendantInstances.scala

@@ -31,23 +31,6 @@ trait DescendantInstances {
 
 private[eyrie]
 trait DescendantByInputInstances {
-  implicit def eyrieTrivialByInputInstance[A](
-    implicit A: TrivialDescendant[A]
-  ): Descendant.ByInput.Aux[A, A] =
-    eyrieAmbiguousTrivialByInputInstance
-
-  implicit def eyrieAmbiguousTrivialByInputInstance[A](
-    implicit A: TrivialDescendant[A]
-  ): Descendant.ByInput.Aux[A, A] =
-    new Descendant.ByInput[A] {
-      override
-      type Root = A
-
-      override
-      def root: A => A =
-        A.root
-    }
-
   implicit def eyrieDescendantByInputInstance[A, B](
     implicit A: Descendant[A, B]
   ): Descendant.ByInput.Aux[A, B] =
@@ -59,6 +42,16 @@ trait DescendantByInputInstances {
       def root: A => B =
         A.root
     }
+
+  implicit def eyrieTrivialDescendantByInputInstance[A](
+    implicit A: TrivialDescendant[A]
+  ): Descendant.ByInput.Aux[A, A] =
+    eyrieDescendantByInputInstance[A, A]
+
+  implicit def eyrieAmbiguousTrivialDescendantByInputInstance[A](
+    implicit A: TrivialDescendant[A]
+  ): Descendant.ByInput.Aux[A, A] =
+    eyrieDescendantByInputInstance[A, A]
 }
 
 private[eyrie]
@@ -90,15 +83,60 @@ trait PotentialDescendantInstances {
 
 private[eyrie]
 trait PotentialDescendantByInputInstances {
-  implicit def eyriePotentialDescendantByInputInstance[A, D](
-    implicit A: PotentialDescendant[A, D]
-  ): PotentialDescendant.ByInput.Aux[A, D] =
+  implicit def eyriePotentialDescendantByInputInstance[A, B](
+    implicit A: PotentialDescendant[A, B]
+  ): PotentialDescendant.ByInput.Aux[A, B] =
     new PotentialDescendant.ByInput[A] {
       override
-      type Root = D
+      type Root = B
 
       override
-      def rootOption: A => Option[D] =
+      def rootOption: A => Option[B] =
         A.rootOption
     }
+
+  implicit def eyrieLeftTrivialPotentialDescendantByInputInstance[A, B](
+    implicit A: PotentialDescendant[A, B], ev: Subdivision[A, B, _]
+  ): PotentialDescendant.ByInput.Aux[A, B] =
+    eyriePotentialDescendantByInputInstance[A, B]
+
+  implicit def eyrieLeftAmbiguousTrivialPotentialDescendantByInputInstance[A, B](
+    implicit A: PotentialDescendant[A, B], ev: Subdivision[A, B, _]
+  ): PotentialDescendant.ByInput.Aux[A, B] =
+    eyriePotentialDescendantByInputInstance[A, B]
+
+  implicit def eyrieRightTrivialPotentialDescendantByInputInstance[A, B](
+    implicit A: PotentialDescendant[A, B], ev: Subdivision[A, _, B]
+  ): PotentialDescendant.ByInput.Aux[A, B] =
+    eyriePotentialDescendantByInputInstance[A, B]
+
+  implicit def eyrieRightAmbiguousTrivialPotentialDescendantByInputInstance[A, B](
+    implicit A: PotentialDescendant[A, B], ev: Subdivision[A, _, B]
+  ): PotentialDescendant.ByInput.Aux[A, B] =
+    eyriePotentialDescendantByInputInstance[A, B]
+}
+
+private[eyrie]
+trait PotentialDescendantTrivialByInputInstances {
+  implicit def eyrieLeftPotentialDescendantTrivialByInputInstance[A, B](
+    implicit A: PotentialDescendant[A, B], ev: Subdivision[A, B, _]
+  ): PotentialDescendant.TrivialByInput.Aux[A, B] =
+    new EyriePotentialDescendantInstance[A, B](A)
+
+  implicit def eyrieRightPotentialDescendantTrivialByInputInstance[A, B](
+    implicit A: PotentialDescendant[A, B], ev: Subdivision[A, _, B]
+  ): PotentialDescendant.TrivialByInput.Aux[A, B] =
+    new EyriePotentialDescendantInstance[A, B](A)
+
+  private
+  class EyriePotentialDescendantInstance[A, B](
+    val A: PotentialDescendant[A, B]
+  ) extends PotentialDescendant.TrivialByInput[A] {
+    override
+    type Root = B
+
+    override
+    def asRoot: A => Option[B] =
+      A.rootOption
+  }
 }

core/src/main/scala/eyrie/ops/PotentialDescendant.scala

@@ -1,6 +1,6 @@
 package eyrie.ops
 
-import eyrie.instances.{PotentialDescendantByInputInstances, PotentialDescendantInstances}
+import eyrie.instances.{PotentialDescendantByInputInstances, PotentialDescendantInstances, PotentialDescendantTrivialByInputInstances}
 import simulacrum.typeclass
 
 trait PotentialDescendant[A, B] {
@@ -24,4 +24,17 @@ object PotentialDescendant extends PotentialDescendantInstances {
       type Root = B
     }
   }
+
+  @typeclass
+  trait TrivialByInput[A] {
+    type Root
+
+    def asRoot: A => Option[Root]
+  }
+
+  object TrivialByInput extends PotentialDescendantTrivialByInputInstances {
+    type Aux[A, B] = TrivialByInput[A] {
+      type Root = B
+    }
+  }
 }

core/src/main/scala/eyrie/syntax/PotentialDescendantSyntax.scala

@@ -10,4 +10,7 @@ trait PotentialDescendantSyntax {
 final class PotentialDescendantOps[A](private val a: A) extends AnyVal {
   def rootOption(implicit A: PotentialDescendant.ByInput[A]): Option[A.Root] =
     A.rootOption(a)
+
+  def asRoot(implicit A: PotentialDescendant.TrivialByInput[A]): Option[A.Root] =
+    A.asRoot(a)
 }

file/src/main/scala/eyrie/file/instances/FilePathConvertible.scala

@@ -22,7 +22,7 @@ class FilePathConvertible[A[_]](implicit A: ClassTag[A[_]]) extends Convertible[
 }
 
 private[file]
-trait ConvertibleInstances extends LowPriorityConvertibleInstances {
+trait ConvertibleInstances extends ConvertibleLowPriorityInstances {
   implicit def eyrieFileEmptyConvertibleInstance[C]: Convertible.Aux[
     Emptiness, True, FilePath.Empty[C], FilePath[C]] =
     _emptyConvertibleInstance.of[C, Emptiness, True, FilePath]
@@ -51,7 +51,7 @@ trait ConvertibleInstances extends LowPriorityConvertibleInstances {
 }
 
 private[file]
-trait LowPriorityConvertibleInstances extends LowPriorityConvertibleInstances1 {
+trait ConvertibleLowPriorityInstances extends ConvertibleLowPriorityInstances1 {
   implicit def eyrieFileIdentityFilePathEmptinessConvertibleInstance[C]: Convertible.Aux[
     Relativity, True, IdentityFilePath[C], FilePath.Empty[C]] =
     _identityFilePathConvertibleInstance.of[C, Relativity, True, FilePath.Empty]
@@ -80,7 +80,7 @@ trait LowPriorityConvertibleInstances extends LowPriorityConvertibleInstances1 {
 }
 
 private[file]
-trait LowPriorityConvertibleInstances1 {
+trait ConvertibleLowPriorityInstances1 {
   private[instances] lazy val _emptyConvertibleInstance = new FilePathConvertible[FilePath.Empty]
   private[instances] lazy val _nonEmptyConvertibleInstance = new FilePathConvertible[FilePath.NonEmpty]
   private[instances] lazy val _relativeConvertibleInstance = new FilePathConvertible[FilePath.Relative]

file/src/main/scala/eyrie/file/instances/FilePathInstances.scala

@@ -5,7 +5,7 @@ import eyrie.ops.Subdivision
 import eyrie.{Emptiness, Relativity}
 
 private[file]
-trait FilePathInstances extends LowPriorityFilePathInstances with EqualityInstances[FilePath] {
+trait FilePathInstances extends FilePathLowPriorityInstances with EqualityInstances[FilePath] {
   private
   lazy val _relativitySubdivisionInstance =
     new FilePathSubdivision[Relativity, FilePath, FilePath.Absolute, FilePath.Relative]
@@ -16,7 +16,7 @@ trait FilePathInstances extends LowPriorityFilePathInstances with EqualityInstan
 }
 
 private[file]
-trait LowPriorityFilePathInstances {
+trait FilePathLowPriorityInstances {
   private
   lazy val _emptinessSubdivisionInstance =
     new FilePathSubdivision[Emptiness, FilePath, FilePath.NonEmpty, FilePath.Empty]

file/src/test/scala/eyrie/file/test/PotentialDescendantSpec.scala

@@ -8,6 +8,7 @@ import eyrie.file.{FilePath, RootDirectory}
 import org.junit.runner.RunWith
 import org.scalatest.FreeSpec
 import org.scalatest.junit.JUnitRunner
+import shapeless.test.illTyped
 
 object PotentialDescendantSpec {
   val nonEmpty: FilePath.NonEmpty[Sys] =
@@ -36,8 +37,15 @@ class PotentialDescendantSpec extends FreeSpec {
     }
   }
   "FilePath.Empty" - {
-    "rootOption should be Option of RootDirectory" in {
-      empty.rootOption: Option[RootDirectory[Sys]]
+    "rootOption should not be available" in {
+      illTyped(
+        """
+          empty.rootOption
+        """
+      )
+    }
+    "asRoot should be Option of RootDirectory" in {
+      empty.asRoot: Option[RootDirectory[Sys]]
     }
   }
 }