feat(boundaries): add a either boundary

src/main/scala/io/github/tassiLuca/boundaries/either.scala

@@ -0,0 +1,14 @@
+package io.github.tassiLuca.boundaries
+
+import scala.util.boundary
+import scala.util.boundary.{Label, break}
+
+object either:
+
+  inline def apply[L, R](inline body: Label[Left[L, Nothing]] ?=> R): Either[L, R] =
+    boundary(Right(body))
+
+  extension [L, R](e: Either[L, R])
+    inline def ?(using Label[Left[L, Nothing]]): R = e match
+      case Right(value) => value
+      case Left(value) => break(Left(value))

src/main/scala/io/github/tassiLuca/boundaries/examples/BoundaryExamples.scala

@@ -16,3 +16,7 @@ object BoundaryExamples extends App:
   def firstColumn[T](xss: List[List[T]]): Option[List[T]] =
     optional:
       xss.map(_.headOption.?)
+
+  def firstColumn2[T](xss: List[List[T]]): Option[List[T]] =
+    val firstElements = xss.map(_.headOption)
+    if firstElements.forall(_.isDefined) then Some(firstElements.flatten) else None

src/main/scala/io/github/tassiLuca/boundaries/examples/EitherExamples.scala → src/main/scala/io/github/tassiLuca/boundaries/examples/EitherRawExamples.scala

@@ -3,7 +3,7 @@ package io.github.tassiLuca.boundaries.examples
 import scala.util.boundary
 import scala.util.boundary.{Label, break}
 
-trait EitherExamples:
+trait EitherRawExamples:
 
   type User
   type UserId

src/test/scala/io/github/tassiLuca/boundaries/examples/BoundaryExamplesTest.scala

@@ -1,6 +1,6 @@
 package io.github.tassiLuca.boundaries.examples
 
-import io.github.tassiLuca.boundaries.examples.BoundaryExamples.{firstColumn, firstIndex}
+import io.github.tassiLuca.boundaries.examples.BoundaryExamples.{firstColumn, firstColumn2, firstIndex}
 import org.scalatest.flatspec.AnyFlatSpec
 import org.scalatest.matchers.should.Matchers.shouldBe
 
@@ -14,3 +14,8 @@ class BoundaryExamplesTest extends AnyFlatSpec:
     firstColumn(List(List(1, 2, 3), List(3))) shouldBe Some(List(1, 3))
     firstColumn(List(List(), List(1))) shouldBe None
   }
+
+  "BoundaryExamples.firstColumn2" should "return a List with the first element of each List given in input" in {
+    firstColumn2(List(List(1, 2, 3), List(3))) shouldBe Some(List(1, 3))
+    firstColumn2(List(List(), List(1))) shouldBe None
+  }