Direct-style effect for error handling

A direct-style effect for raising and handling errors

core/shared/src/main/scala/krop/raise/Raise.scala

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+/*
+ * Copyright 2023 Creative Scala
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package krop.raise
+
+import scala.util.boundary
+import scala.util.boundary.Label
+import scala.util.boundary.break
+
+/** Effect that can raise an error of type `E`. */
+trait Raise[E] {
+
+  /** Raise an error of type E. The error parameter is call-by-need as the
+    * handler may decide to not evaluate it and instead replace it with another
+    * value. For example, it could be replaced with None if the context is
+    * expecting an Option.
+    */
+  def raise(error: => E): Nothing
+}
+
+object Raise {
+
+  /** Construct a Raise[E] that produces a value in a context expecting a value
+    * of type A.
+    */
+  def apply[E, A](f: E => A)(using Label[A]): Raise[E] =
+    new Raise[E] {
+      def raise(error: => E): Nothing =
+        break(f(error))
+    }
+
+  /** Construct a Raise[E] that produces a value in a context expecting a value
+    * of type A, where the Raise instance ignores the value of type E and simply
+    * produces a constant value of type A.
+    */
+  def const[E, A](a: A)(using Label[A]): Raise[E] =
+    new Raise[E] {
+      def raise(error: => E): Nothing =
+        break(a)
+    }
+
+  def raiseToEither[E, A](using label: Label[Either[E, A]]): Raise[E] =
+    Raise(e => Left(e))
+
+  def raiseToOption[E, A](using label: Label[Option[A]]): Raise[E] =
+    const(None)
+
+  inline def toEither[E, A](inline body: Raise[E] ?=> A): Either[E, A] =
+    boundary {
+      given Raise[E] = raiseToEither
+      Right(body)
+    }
+
+  inline def toOption[E, A](inline body: Raise[E] ?=> A): Option[A] =
+    boundary {
+      given Raise[E] = raiseToOption
+      Some(body)
+    }
+}