Examples of the new experimental "safer exceptions".

Author: Dean Wampler

src/main/scala/progscala3/rounding/saferexceptions/SaferExceptions.scala

@@ -0,0 +1,48 @@
+// src/main/scala/progscala3/rounding/saferexceptions/SaferExceptions.scala
+// This example is not in the book. It discusses new work, experimental in
+// Scala 3.1, to incorporate exception throwing into the type system, but without
+// some of the drawbacks of Java-style checked exceptions.
+package progscala3.rounding.saferexceptions
+import java.io.{IOException, File}
+import scala.annotation.experimental
+// Enable safer exceptions
+import language.experimental.saferExceptions
+// If you enable safer exceptions, use the following annotation as a transition
+// feature to allow exceptions to be thrown even when the handler logic isn't in
+// place for all cases. Remove this import when handlers have been added!
+// import unsafeExceptions.canThrowAny
+
+/**
+ * Usage: scala rounding.saferexceptions.SaferExceptions file1 [file2 ...]
+ * Note that an object is used, not a `@def main`, because the compiler tells us
+ * we need to annotate the SaferExceptions type with `@experimental` and this
+ * appears to be the only way to do it. (Also, any methods outside the type that
+ * use this feature would also need this annotation.)
+ *
+ * Here is the output:
+ * ```
+ * sbt:programming-scala-3rd-ed-code-examples> runMain progscala3.rounding.saferexceptions.SaferExceptions build.sbt badfile
+ * [info] compiling 1 Scala source to .../target/scala-3.1.1-RC2/classes ...
+ * [info] running (fork) progscala3.rounding.saferexceptions.SaferExceptions build.sbt badfile
+ * [info] file build.sbt (.../build.sbt) has 4434 bytes.
+ * [info] file badfile: IOException caught: badfile doesn't exist!
+ * [success] Total time: 1 s, completed Jan 16, 2022, 12:20:48 PM
+ * ```
+ */
+@experimental  // Must annotate all the types or methods that use the feature...
+object SaferExceptions:
+  def main(fileNames: Array[String]): Unit =
+    fileNames.foreach { fileName =>
+      try
+        val file = openExistingFile(fileName)
+        val path = file.getCanonicalPath()
+        val size = file.length()
+        println(s"file $fileName ($path) has $size bytes.")
+      catch
+        case ioe: IOException => println(s"file $fileName: IOException caught: ${ioe.getMessage}")
+    }
+
+  def openExistingFile(fileName: String): File throws IOException =
+    val file = new File(fileName)
+    if file.exists() == false then throw new IOException(s"$fileName doesn't exist!")
+    file

src/main/scala/progscala3/rounding/saferexceptions/SaferExceptionsNested.scala

@@ -0,0 +1,54 @@
+// src/main/scala/progscala3/rounding/saferexceptions/SaferExceptionsNested.scala
+// This example is not in the book. It discusses new work, experimental in
+// Scala 3.1, to incorporate exception throwing into the type system, but without
+// the drawbacks of Java-style checked exceptions.
+package progscala3.rounding.saferexceptions
+import java.io.{IOException, File}
+import scala.annotation.experimental
+// Enable safer exceptions
+import language.experimental.saferExceptions
+// If you enable safer exceptions, use the following annotation as a transition
+// feature to allow exceptions to be thrown even when the handler logic isn't in
+// place for all cases. Remove this import when handlers have been added!
+// import unsafeExceptions.canThrowAny
+
+/**
+ * Usage: scala rounding.saferexceptions.SaferExceptionsNested file1 [file2 ...]
+ * Note that an object is used, not a `@def main`, because the compiler tells us
+ * we need to annotate the SaferExceptions type with `@experimental` and this
+ * appears to be the only way to do it. (Also, any methods outside the type that
+ * use this feature would also need this annotation.)
+ *
+ * Here is the output:
+ * ```
+ * sbt:programming-scala-3rd-ed-code-examples> runMain progscala3.rounding.saferexceptions.SaferExceptionsNested build.sbt badfile
+ * [info] compiling 2 Scala sources to .../target/scala-3.1.1-RC2/classes ...
+ * [info] running (fork) progscala3.rounding.saferexceptions.SaferExceptionsNested build.sbt badfile
+ * [info] file build.sbt (.../build.sbt) has 4434 bytes.
+ * [info] file badfile: IOException caught: badfile doesn't exist!
+ * [success] Total time: 1 s, completed Jan 16, 2022, 12:32:51 PM
+ * ```
+ */
+@experimental  // Must annotate all the types or methods that use the feature...
+object SaferExceptionsNested:
+  def main(fileNames: Array[String]): Unit =
+    fileNames.foreach { fileName =>
+      try
+        val file = wrapper(fileName)
+        val path = file.getCanonicalPath()
+        val size = file.length()
+        println(s"file $fileName ($path) has $size bytes.")
+      catch
+        case ioe: IOException => println(s"file $fileName: IOException caught: ${ioe.getMessage}")
+    }
+
+  def wrapper(fileName: String): File throws IOException = openExistingFile(fileName)
+
+  /**
+   * `File throws IOException` is equivalent to this definition of `openExisting`:
+   * `def openExistingFile(fileName: String)(using CanThrow[IOException]) = ...
+   */
+  def openExistingFile(fileName: String): File throws IOException =
+    val file = new File(fileName)
+    if file.exists() == false then throw new IOException(s"$fileName doesn't exist!")
+    file