scala.util.control
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Provides thebreak control abstraction.
Provides thebreak control abstraction.
Thebreak method uses aControlThrowable to transfer control up the stack to an enclosingbreakable.
It is typically used to abruptly terminate afor loop, but can be used to return from an arbitrary computation.
Control resumes after thebreakable.
If there is no matchingbreakable, theBreakControl thrown bybreak is handled in the usual way: if not caught, it may terminate the currentThread.
BreakControl carries no stack trace, so the default exception handler does not print useful diagnostic information; there is no compile-time warning if there is no matchingbreakable.
A catch clause usingNonFatal is safe to use withbreak; it will not short-circuit the transfer of control to the enclosingbreakable.
Abreakable matches a call tobreak if the methods were invoked on the same receiver object, which may be the convenience valueBreaks.
Example usage:
val mybreaks = new Breaksimport mybreaks.{break, breakable}breakable { for (x <- xs) { if (done) break() f(x) }}Calls tobreak from one instance ofBreaks will never resume at thebreakable of some other instance.
Any intervening exception handlers should useNonFatal, or useTry for evaluation:
val mybreaks = new Breaksimport mybreaks.{break, breakable}breakable { for (x <- xs) Try { if (quit) break else f(x) }.foreach(println)}Attributes
- Companion
- object
- Source
- Breaks.scala
- Supertypes
- Known subtypes
- objectBreaks
An object that can be used for the break control abstraction.
A parent class for throwable objects intended for flow control.
A parent class for throwable objects intended for flow control.
Instances ofControlThrowable should not normally be caught.
As a convenience,NonFatal does not matchControlThrowable.
import scala.util.control.{Breaks, NonFatal}, Breaks.{break, breakable}breakable { for (v <- values) { try { if (p(v)) break else ??? } catch { case NonFatal(t) => log(t) // can't catch a break } }}Suppression is disabled, because flow control should not suppress an exceptional condition. Stack traces are also disabled, allowing instances ofControlThrowable to be safely reused.
Instances ofControlThrowable should not normally have a cause. Legacy subclasses may set a cause usinginitCause.
Attributes
- Source
- ControlThrowable.scala
- Supertypes
- Known subtypes
- classNonLocalReturnControl[T]classReturnThrowable[T]
Classes representing the components of exception handling.
Classes representing the components of exception handling.
Each class is independently composable.
This class differs fromscala.util.Try in that it focuses on composing exception handlers rather than composing behavior. All behavior should be composed first and fed to aCatch object using one of theopt,either orwithTry methods. Taken together the classes provide a DSL for composing catch and finally behaviors.
Examples
Create aCatch 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 MalformedURLExceptionval x4: URL = failAsValue(classOf[MalformedURLException])(defaultUrl)(new URL("htt/xx"))Create aCatch which logs exceptions usinghandling andby.
def log(t: Throwable): Unit = t.printStackTraceval 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,// <!DOCTYPE html>// <html>withThrowableLogging { printUrl(goodUrl) }Useunwrapping to create aCatch 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)UsefailAsValue 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 = 0val pick = inputDefaulting(candidatePick.toInt)Compose multipleCatchs withor to build aCatch 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 otherDefaultingdef p(s: String): Int = s.length * s.toInt// Int = 0combinedDefaulting(p("tenty-nine"))// Int = -1combinedDefaulting(p(null: String))// Int = -100combinedDefaulting(throw new IllegalStateException)// Int = 22combinedDefaulting(p("11"))Attributes
- Source
- Exception.scala
- Supertypes
- Self type
- Exception.type
A trait for exceptions which, for efficiency reasons, do not fill in the stack trace.
A trait for exceptions which, for efficiency reasons, do not fill in the stack trace. Stack trace suppression can be disabled on a global basis via a system property wrapper inscala.sys.SystemProperties.
Attributes
- Note
Since JDK 1.7, a similar effect can be achieved with
class Ex extends Throwable(..., writableStackTrace = false)- Companion
- object
- Source
- NoStackTrace.scala
- Supertypes
Attributes
- Companion
- trait
- Source
- NoStackTrace.scala
- Supertypes
- Self type
- NoStackTrace.type
Extractor of non-fatal Throwables.
Extractor of non-fatal Throwables. Will not match fatal errors likeVirtualMachineError (for example,OutOfMemoryError andStackOverflowError, subclasses ofVirtualMachineError),ThreadDeath,LinkageError,InterruptedException,ControlThrowable.
Note thatscala.util.control.ControlThrowable, an internal Throwable, is not matched byNonFatal (and would therefore be thrown).
For example, all harmless Throwables can be caught by:
try { // dangerous stuff} catch { case NonFatal(e) => log.error(e, "Something not that bad.") // or case e if NonFatal(e) => log.error(e, "Something not that bad.")}Attributes
- Source
- NonFatal.scala
- Supertypes
- Self type
- NonFatal.type
Methods exported by this object implement tail calls via trampolining.
Methods exported by this object implement tail calls via trampolining.
Tail calling methods must either return their result usingdone or call the next method usingtailcall. Both return an instance ofTailRec. The result of evaluating a tailcalling function can be retrieved from aTailRec value using methodresult.
Implemented as described in "Stackless Scala with Free Monads"https://blog.higher-order.com/assets/trampolines.pdf
Here's a usage example:
import scala.util.control.TailCalls._def isEven(xs: List[Int]): TailRec[Boolean] = if (xs.isEmpty) done(true) else tailcall(isOdd(xs.tail))def isOdd(xs: List[Int]): TailRec[Boolean] = if (xs.isEmpty) done(false) else tailcall(isEven(xs.tail))isEven((1 to 100000).toList).resultdef fib(n: Int): TailRec[Int] = if (n < 2) done(n) else for { x <- tailcall(fib(n - 1)) y <- tailcall(fib(n - 2)) } yield x + yfib(40).resultAttributes
- Source
- TailCalls.scala
- Supertypes
- Self type
- TailCalls.type
Deprecated classlikes
Library implementation of nonlocal return.
Library implementation of nonlocal return.
Usage:
import scala.util.control.NonLocalReturns.*
returning { ... throwReturn(x) ... }
This API has been deprecated. Its functionality is better served by
scala.util.boundaryin place ofreturningscala.util.breakin place ofthrowReturn
The new abstractions work with plainRuntimeExceptions and are more performant, since returns within the scope of the same method can be rewritten by the compiler to jumps.
Attributes
- Deprecated
[Since version 3.3]Use scala.util.boundary instead- Source
- NonLocalReturns.scala
- Supertypes
- Self type
- NonLocalReturns.type