package control
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Type Members
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class
Breaks extends AnyRef
A class that can be instantiated for the break control abstraction.
A class that can be instantiated for the break control abstraction. Example usage:
val mybreaks = new Breaks import mybreaks.{break, breakable} breakable { for (...) { if (...) break() } }
Calls to break from one instantiation of
Breakswill never target breakable objects of some other instantiation. -
trait
ControlThrowable extends Throwable with NoStackTrace
A marker trait indicating that the
Throwableit is mixed into is intended for flow control.A marker trait indicating that the
Throwableit is mixed into is intended for flow control.Note that
Throwablesubclasses which extend this trait may extend any otherThrowablesubclass (eg.RuntimeException) and are not required to extendThrowabledirectly.Instances of
Throwablesubclasses marked in this way should not normally be caught. Where catch-all behaviour is requiredControlThrowableshould be propagated, for example:import scala.util.control.ControlThrowable try { // Body might throw arbitrarily } catch { case c: ControlThrowable => throw c // propagate case t: Exception => log(t) // log and suppress }
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trait
NoStackTrace extends Throwable
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 in scala.sys.SystemProperties.
- Since
2.8
- Note
Since JDK 1.7, a similar effect can be achieved with
class Ex extends Throwable(..., writableStackTrace = false)
Value Members
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object
Breaks extends Breaks
An object that can be used for the break control abstraction.
An object that can be used for the break control abstraction. Example usage:
import Breaks.{break, breakable} breakable { for (...) { if (...) break } }
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object
Exception
Classes representing the components of exception handling.
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,eitherorwithTrymethods. Taken together the classes provide a DSL for composing catch and finally behaviors.Examples
Create a
Catchwhich handles specified exceptions.import scala.util.control.Exception._ import java.net._ val s = "http://www.scala-lang.org/" // Some(http://www.scala-lang.org/) val x1: Option[URL] = catching(classOf[MalformedURLException]) opt new URL(s) // Right(http://www.scala-lang.org/) val x2: Either[Throwable,URL] = catching(classOf[MalformedURLException], classOf[NullPointerException]) either new URL(s) // Success(http://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
Catchwhich logs exceptions usinghandlingandby.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 = "http://www.scala-lang.org/" // Prints page content, // <!DOCTYPE html> // <html> withThrowableLogging { printUrl(goodUrl) }
Use
unwrappingto create aCatchthat 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
failAsValueto 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 withorto build aCatchthat 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"))
- object NoStackTrace extends Serializable
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object
NonFatal
Extractor of non-fatal Throwables.
Extractor of non-fatal Throwables. Will not match fatal errors like
VirtualMachineError(for example,OutOfMemoryErrorandStackOverflowError, subclasses ofVirtualMachineError),ThreadDeath,LinkageError,InterruptedException,ControlThrowable.Note that scala.util.control.ControlThrowable, an internal Throwable, is not matched by
NonFatal(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.") }
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object
TailCalls
Methods exported by this object implement tail calls via trampolining.
Methods exported by this object implement tail calls via trampolining. Tail calling methods have to return their result using
doneor call the next method usingtailcall. Both return aTailRecobject. The result of evaluating a tailcalling function can be retrieved from aTailrecvalue using methodresult. Implemented as described in "Stackless Scala with Free Monads" http://blog.higher-order.com/assets/trampolines.pdfHere'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).result def fib(n: Int): TailRec[Int] = if (n < 2) done(n) else for { x <- tailcall(fib(n - 1)) y <- tailcall(fib(n - 2)) } yield (x + y) fib(40).result
This is the documentation for the Scala standard library.
Package structure
The scala package contains core types like
Int,Float,ArrayorOptionwhich are accessible in all Scala compilation units without explicit qualification or imports.Notable packages include:
scala.collectionand its sub-packages contain Scala's collections frameworkscala.collection.immutable- Immutable, sequential data-structures such asVector,List,Range,HashMaporHashSetscala.collection.mutable- Mutable, sequential data-structures such asArrayBuffer,StringBuilder,HashMaporHashSetscala.collection.concurrent- Mutable, concurrent data-structures such asTrieMapscala.collection.parallel.immutable- Immutable, parallel data-structures such asParVector,ParRange,ParHashMaporParHashSetscala.collection.parallel.mutable- Mutable, parallel data-structures such asParArray,ParHashMap,ParTrieMaporParHashSetscala.concurrent- Primitives for concurrent programming such asFuturesandPromisesscala.io- Input and output operationsscala.math- Basic math functions and additional numeric types likeBigIntandBigDecimalscala.sys- Interaction with other processes and the operating systemscala.util.matching- Regular expressionsOther packages exist. See the complete list on the right.
Additional parts of the standard library are shipped as separate libraries. These include:
scala.reflect- Scala's reflection API (scala-reflect.jar)scala.xml- XML parsing, manipulation, and serialization (scala-xml.jar)scala.swing- A convenient wrapper around Java's GUI framework called Swing (scala-swing.jar)scala.util.parsing- Parser combinators (scala-parser-combinators.jar)Automatic imports
Identifiers in the scala package and the
scala.Predefobject are always in scope by default.Some of these identifiers are type aliases provided as shortcuts to commonly used classes. For example,
Listis an alias forscala.collection.immutable.List.Other aliases refer to classes provided by the underlying platform. For example, on the JVM,
Stringis an alias forjava.lang.String.