object 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.


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.(URL.java:586) withThrowableLogging { printUrl(badUrl) } val goodUrl = "https://www.scala-lang.org/" // Prints page content, // <!DOCTYPE html> // <html> 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(<console>: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"))
class Object
trait Matchable
class Any

Catch behavior composition

Build Catch objects from exception lists and catch logic

def catching[T](exceptions: Class[_]*): Catch[T]

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.

def ignoring(exceptions: Class[_]*): Catch[Unit]

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

def failing[T](exceptions: Class[_]*): Catch[Option[T]]

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

def failAsValue[T](exceptions: Class[_]*)(value: => T): Catch[T]

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

def unwrapping[T](exceptions: Class[_]*): Catch[T]

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

Finally behavior composition

Build Catch objects from finally logic

def ultimately[T](body: => Unit): Catch[T]

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

General purpose catch objects

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

final val noCatch: Catch[Nothing]

The empty Catch object.

final def allCatch[T]: Catch[T]

A Catch object which catches everything.

final def nonFatalCatch[T]: Catch[T]

A Catch object which catches non-fatal exceptions.

DSL behavior composition

Expressive Catch behavior composition

def handling[T](exceptions: Class[_]*): By[Throwable => T, Catch[T]]

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


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

Promiscuous Catch behaviors

Useful if catching ControlThrowable or InterruptedException is required.

def catchingPromiscuously[T](exceptions: Class[_]*): Catch[T]

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.

Logic Containers

Containers for catch and finally behavior.

class Finally extends Described

A container class for finally code.

class Catch[+T](val pf: Catcher[T], val fin: Option[Finally], val rethrow: Throwable => Boolean) extends Described

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 Params

result type of bodies used in try and catch blocks

Value Params

Finally logic which if defined will be invoked after catch logic


Partial function used when applying catch logic to determine result value


Predicate on throwables determining when to rethrow a caught Throwable


Type members


class By[T, R](f: T => R)

Value members

Concrete methods

final def allCatcher[T]: Catcher[T]
def catching[T](c: Catcher[T]): Catch[T]
def mkCatcher[Ex <: Throwable, T](isDef: Ex => Boolean, f: Ex => T)(`evidence$1`: ClassTag[Ex]): PartialFunction[Throwable, T]

!!! Not at all sure of every factor which goes into this, and/or whether we need multiple standard variations.


true if x is ControlThrowable or InterruptedException otherwise false.


Concrete fields

final val nothingCatcher: Catcher[Nothing]



implicit def throwableSubtypeToCatcher[Ex <: Throwable, T](pf: PartialFunction[Ex, T])(`evidence$2`: ClassTag[Ex]): Catcher[T]