Packages

  • package root

    This is the documentation for the Scala standard library.

    This is the documentation for the Scala standard library.

    Package structure

    The scala package contains core types like Int, Float, Array or Option which are accessible in all Scala compilation units without explicit qualification or imports.

    Notable packages include:

    Other packages exist. See the complete list on the right.

    Additional parts of the standard library are shipped as separate libraries. These include:

    Automatic imports

    Identifiers in the scala package and the scala.Predef object are always in scope by default.

    Some of these identifiers are type aliases provided as shortcuts to commonly used classes. For example, List is an alias for scala.collection.immutable.List.

    Other aliases refer to classes provided by the underlying platform. For example, on the JVM, String is an alias for java.lang.String.

    Definition Classes
    root
  • package scala

    Core Scala types.

    Core Scala types. They are always available without an explicit import.

    Definition Classes
    root
  • package util
    Definition Classes
    scala
  • package control
    Definition Classes
    util
  • 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, 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,
    //   <!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"))
    Definition Classes
    control
  • By
  • Catch
  • Described
  • Finally

class By[T, R] extends AnyRef

Source
Exception.scala
Linear Supertypes
Type Hierarchy
Ordering
  1. Alphabetic
  2. By Inheritance
Inherited
  1. By
  2. AnyRef
  3. Any
Implicitly
  1. by any2stringadd
  2. by StringFormat
  3. by Ensuring
  4. by ArrowAssoc
  1. Hide All
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Visibility
  1. Public
  2. Protected

Instance Constructors

  1. new By(f: (T) => R)

Value Members

  1. def by(x: T): R