Exception

Classes representing the components of exception handling.

Each class is independently composable.

This class differs from scala.util.Try in that it focuses on composing exception handlers rather than composing behavior. All behavior should be composed first and fed to a Catch object using one of the opt, either or withTry methods. Taken together the classes provide a DSL for composing catch and finally behaviors.

Examples

Create a Catch which handles specified exceptions.

import scala.util.control.Exception._
import java.net._

val s = "https://www.scala-lang.org/"

// Some(https://www.scala-lang.org/)
val x1: Option[URL] = catching(classOf[MalformedURLException]) opt new URL(s)

// Right(https://www.scala-lang.org/)
val x2: Either[Throwable,URL] =
  catching(classOf[MalformedURLException], classOf[NullPointerException]) either new URL(s)

// Success(https://www.scala-lang.org/)
val x3: Try[URL] = catching(classOf[MalformedURLException], classOf[NullPointerException]) withTry new URL(s)

val defaultUrl = new URL("http://example.com")
//  URL(http://example.com) because htt/xx throws MalformedURLException
val x4: URL = failAsValue(classOf[MalformedURLException])(defaultUrl)(new URL("htt/xx"))

Create a Catch which logs exceptions using handling and by.

def log(t: Throwable): Unit = t.printStackTrace

val withThrowableLogging: Catch[Unit] = handling(classOf[MalformedURLException]) by (log)

def printUrl(url: String) : Unit = {
  val con = new URL(url) openConnection()
  val source = scala.io.Source.fromInputStream(con.getInputStream())
  source.getLines().foreach(println)
}

val badUrl = "htt/xx"
// Prints stacktrace,
//   java.net.MalformedURLException: no protocol: htt/xx
//     at java.net.URL.<init>(URL.java:586)
withThrowableLogging { printUrl(badUrl) }

val goodUrl = "https://www.scala-lang.org/"
// Prints page content,
//   &lt;!DOCTYPE html&gt;
//   &lt;html&gt;
withThrowableLogging { printUrl(goodUrl) }

Use unwrapping to create a Catch that unwraps exceptions before rethrowing.

class AppException(cause: Throwable) extends RuntimeException(cause)

val unwrappingCatch: Catch[Nothing] = unwrapping(classOf[AppException])

def calcResult: Int = throw new AppException(new NullPointerException)

// Throws NPE not AppException,
//   java.lang.NullPointerException
//     at .calcResult(&lt;console&gt;:17)
val result = unwrappingCatch(calcResult)

Use failAsValue to provide a default when a specified exception is caught.

val inputDefaulting: Catch[Int] = failAsValue(classOf[NumberFormatException])(0)
val candidatePick = "seven" // scala.io.StdIn.readLine()

// Int = 0
val pick = inputDefaulting(candidatePick.toInt)

Compose multiple Catchs with or to build a Catch that provides default values varied by exception.

val formatDefaulting: Catch[Int] = failAsValue(classOf[NumberFormatException])(0)
val nullDefaulting: Catch[Int] = failAsValue(classOf[NullPointerException])(-1)
val otherDefaulting: Catch[Int] = nonFatalCatch withApply(_ => -100)

val combinedDefaulting: Catch[Int] = formatDefaulting or nullDefaulting or otherDefaulting

def p(s: String): Int = s.length * s.toInt

// Int = 0
combinedDefaulting(p("tenty-nine"))

// Int = -1
combinedDefaulting(p(null: String))

// Int = -100
combinedDefaulting(throw new IllegalStateException)

// Int = 22
combinedDefaulting(p("11"))

Source:: Exception.scala

class Object

trait Matchable

class Any

Exception.type

Catch behavior composition

Build Catch objects from exception lists and catch logic

Creates a Catch object which will catch any of the supplied exceptions.

Creates a Catch object which will catch any of the supplied exceptions. Since the returned Catch object has no specific logic defined and will simply rethrow the exceptions it catches, you will typically want to call opt, either or withTry on the return value, or assign custom logic by calling "withApply".

Note that Catch objects automatically rethrow ControlExceptions and others which should only be caught in exceptional circumstances. If you really want to catch exactly what you specify, use catchingPromiscuously instead.

Source:: Exception.scala

Creates a Catch object which maps all the supplied exceptions to the given value.

Source:: Exception.scala

Creates a Catch object which maps all the supplied exceptions to None.

Source:: Exception.scala

Creates a Catch object which catches and ignores any of the supplied exceptions.

Source:: Exception.scala

Creates a Catch object which unwraps any of the supplied exceptions.

Source:: Exception.scala

Finally behavior composition

Build Catch objects from finally logic

Returns a Catch object with no catch logic and the argument as the finally logic.

Source:: Exception.scala

General purpose catch objects

Catch objects with predefined behavior. Use combinator methods to compose additional behavior.

A Catch object which catches everything.

Source:: Exception.scala

The empty Catch object.

Source:: Exception.scala

A Catch object which catches non-fatal exceptions.

Source:: Exception.scala

DSL behavior composition

Expressive Catch behavior composition

Returns a partially constructed Catch object, which you must give an exception handler function as an argument to by.

Example:

handling(classOf[MalformedURLException], classOf[NullPointerException]) by (_.printStackTrace)

Source:

Exception.scala

Promiscuous Catch behaviors

Useful if catching ControlThrowable or InterruptedException is required.

Creates a Catch object which will catch any of the supplied exceptions.

Creates a Catch object which will catch any of the supplied exceptions. Unlike "catching" which filters out those in shouldRethrow, this one will catch whatever you ask of it including ControlThrowable or InterruptedException.

Source:: Exception.scala

Logic Containers

Containers for catch and finally behavior.

A container class for catch/finally logic.

Pass a different value for rethrow if you want to probably unwisely allow catching control exceptions and other throwables which the rest of the world may expect to get through.

Type parameters:

T: result type of bodies used in try and catch blocks

Value parameters:

fin: Finally logic which if defined will be invoked after catch logic
pf: Partial function used when applying catch logic to determine result value
rethrow: Predicate on throwables determining when to rethrow a caught Throwable

Source: