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/*
* Scala (https://www.scala-lang.org)
*
* Copyright EPFL and Lightbend, Inc.
*
* Licensed under Apache License 2.0
* (http://www.apache.org/licenses/LICENSE-2.0).
*
* See the NOTICE file distributed with this work for
* additional information regarding copyright ownership.
*/
package scala
package util
import scala.runtime.Statics
import scala.util.control.NonFatal
/**
* The `Try` type represents a computation that may either result in an exception, or return a
* successfully computed value. It's similar to, but semantically different from the [[scala.util.Either]] type.
*
* Instances of `Try[T]`, are either an instance of [[scala.util.Success]][T] or [[scala.util.Failure]][T].
*
* For example, `Try` can be used to perform division on a user-defined input, without the need to do explicit
* exception-handling in all of the places that an exception might occur.
*
* Example:
* {{{
* import scala.io.StdIn
* import scala.util.{Try, Success, Failure}
*
* def divide: Try[Int] = {
* val dividend = Try(StdIn.readLine("Enter an Int that you'd like to divide:\n").toInt)
* val divisor = Try(StdIn.readLine("Enter an Int that you'd like to divide by:\n").toInt)
* val problem = dividend.flatMap(x => divisor.map(y => x/y))
* problem match {
* case Success(v) =>
* println("Result of " + dividend.get + "/"+ divisor.get +" is: " + v)
* Success(v)
* case Failure(e) =>
* println("You must've divided by zero or entered something that's not an Int. Try again!")
* println("Info from the exception: " + e.getMessage)
* divide
* }
* }
*
* }}}
*
* An important property of `Try` shown in the above example is its ability to ''pipeline'', or chain, operations,
* catching exceptions along the way. The `flatMap` and `map` combinators in the above example each essentially
* pass off either their successfully completed value, wrapped in the `Success` type for it to be further operated
* upon by the next combinator in the chain, or the exception wrapped in the `Failure` type usually to be simply
* passed on down the chain. Combinators such as `recover` and `recoverWith` are designed to provide some type of
* default behavior in the case of failure.
*
* ''Note'': only non-fatal exceptions are caught by the combinators on `Try` (see [[scala.util.control.NonFatal]]).
* Serious system errors, on the other hand, will be thrown.
*
* ''Note:'': all Try combinators will catch exceptions and return failure unless otherwise specified in the documentation.
*
* `Try` comes to the Scala standard library after years of use as an integral part of Twitter's stack.
*/
sealed abstract class Try[+T] extends Product with Serializable {
/** Returns `true` if the `Try` is a `Failure`, `false` otherwise.
*/
def isFailure: Boolean
/** Returns `true` if the `Try` is a `Success`, `false` otherwise.
*/
def isSuccess: Boolean
/** Returns the value from this `Success` or the given `default` argument if this is a `Failure`.
*
* ''Note:'': This will throw an exception if it is not a success and default throws an exception.
*/
def getOrElse[U >: T](default: => U): U
/** Returns this `Try` if it's a `Success` or the given `default` argument if this is a `Failure`.
*/
def orElse[U >: T](default: => Try[U]): Try[U]
/** Returns the value from this `Success` or throws the exception if this is a `Failure`.
*/
def get: T
/**
* Applies the given function `f` if this is a `Success`, otherwise returns `Unit` if this is a `Failure`.
*
* ''Note:'' If `f` throws, then this method may throw an exception.
*/
def foreach[U](f: T => U): Unit
/**
* Returns the given function applied to the value from this `Success` or returns this if this is a `Failure`.
*/
def flatMap[U](f: T => Try[U]): Try[U]
/**
* Maps the given function to the value from this `Success` or returns this if this is a `Failure`.
*/
def map[U](f: T => U): Try[U]
/**
* Applies the given partial function to the value from this `Success` or returns this if this is a `Failure`.
*/
def collect[U](pf: PartialFunction[T, U]): Try[U]
/**
* Converts this to a `Failure` if the predicate is not satisfied.
*/
def filter(p: T => Boolean): Try[T]
/** Creates a non-strict filter, which eventually converts this to a `Failure`
* if the predicate is not satisfied.
*
* Note: unlike filter, withFilter does not create a new Try.
* Instead, it restricts the domain of subsequent
* `map`, `flatMap`, `foreach`, and `withFilter` operations.
*
* As Try is a one-element collection, this may be a bit overkill,
* but it's consistent with withFilter on Option and the other collections.
*
* @param p the predicate used to test elements.
* @return an object of class `WithFilter`, which supports
* `map`, `flatMap`, `foreach`, and `withFilter` operations.
* All these operations apply to those elements of this Try
* which satisfy the predicate `p`.
*/
@inline final def withFilter(p: T => Boolean): WithFilter = new WithFilter(p)
/** We need a whole WithFilter class to honor the "doesn't create a new
* collection" contract even though it seems unlikely to matter much in a
* collection with max size 1.
*/
final class WithFilter(p: T => Boolean) {
def map[U](f: T => U): Try[U] = Try.this filter p map f
def flatMap[U](f: T => Try[U]): Try[U] = Try.this filter p flatMap f
def foreach[U](f: T => U): Unit = Try.this filter p foreach f
def withFilter(q: T => Boolean): WithFilter = new WithFilter(x => p(x) && q(x))
}
/**
* Applies the given function `f` if this is a `Failure`, otherwise returns this if this is a `Success`.
* This is like `flatMap` for the exception.
*/
def recoverWith[U >: T](pf: PartialFunction[Throwable, Try[U]]): Try[U]
/**
* Applies the given function `f` if this is a `Failure`, otherwise returns this if this is a `Success`.
* This is like map for the exception.
*/
def recover[U >: T](pf: PartialFunction[Throwable, U]): Try[U]
/**
* Returns `None` if this is a `Failure` or a `Some` containing the value if this is a `Success`.
*/
def toOption: Option[T]
/**
* Transforms a nested `Try`, ie, a `Try` of type `Try[Try[T]]`,
* into an un-nested `Try`, ie, a `Try` of type `Try[T]`.
*/
def flatten[U](implicit ev: T <:< Try[U]): Try[U]
/**
* Inverts this `Try`. If this is a `Failure`, returns its exception wrapped in a `Success`.
* If this is a `Success`, returns a `Failure` containing an `UnsupportedOperationException`.
*/
def failed: Try[Throwable]
/** Completes this `Try` by applying the function `f` to this if this is of type `Failure`, or conversely, by applying
* `s` if this is a `Success`.
*/
def transform[U](s: T => Try[U], f: Throwable => Try[U]): Try[U]
/**
* Returns `Left` with `Throwable` if this is a `Failure`, otherwise returns `Right` with `Success` value.
*/
def toEither: Either[Throwable, T]
/**
* Applies `fa` if this is a `Failure` or `fb` if this is a `Success`.
* If `fb` is initially applied and throws an exception,
* then `fa` is applied with this exception.
*
* @example {{{
* val result: Try[Int] = Try { string.toInt }
* log(result.fold(
* ex => "Operation failed with " + ex,
* v => "Operation produced value: " + v
* ))
* }}}
*
* @param fa the function to apply if this is a `Failure`
* @param fb the function to apply if this is a `Success`
* @return the results of applying the function
*/
def fold[U](fa: Throwable => U, fb: T => U): U
}
object Try {
/** Constructs a `Try` using the by-name parameter. This
* method will ensure any non-fatal exception is caught and a
* `Failure` object is returned.
*/
def apply[T](r: => T): Try[T] =
try Success(r) catch {
case NonFatal(e) => Failure(e)
}
}
final case class Failure[+T](exception: Throwable) extends Try[T] {
override def isFailure: Boolean = true
override def isSuccess: Boolean = false
override def get: T = throw exception
override def getOrElse[U >: T](default: => U): U = default
override def orElse[U >: T](default: => Try[U]): Try[U] =
try default catch { case NonFatal(e) => Failure(e) }
override def flatMap[U](f: T => Try[U]): Try[U] = this.asInstanceOf[Try[U]]
override def flatten[U](implicit ev: T <:< Try[U]): Try[U] = this.asInstanceOf[Try[U]]
override def foreach[U](f: T => U): Unit = ()
override def transform[U](s: T => Try[U], f: Throwable => Try[U]): Try[U] =
try f(exception) catch { case NonFatal(e) => Failure(e) }
override def map[U](f: T => U): Try[U] = this.asInstanceOf[Try[U]]
override def collect[U](pf: PartialFunction[T, U]): Try[U] = this.asInstanceOf[Try[U]]
override def filter(p: T => Boolean): Try[T] = this
override def recover[U >: T](pf: PartialFunction[Throwable, U]): Try[U] = {
val marker = Statics.pfMarker
try {
val v = pf.applyOrElse(exception, (x: Throwable) => marker)
if (marker ne v.asInstanceOf[AnyRef]) Success(v.asInstanceOf[U]) else this
} catch { case NonFatal(e) => Failure(e) }
}
override def recoverWith[U >: T](pf: PartialFunction[Throwable, Try[U]]): Try[U] = {
val marker = Statics.pfMarker
try {
val v = pf.applyOrElse(exception, (x: Throwable) => marker)
if (marker ne v.asInstanceOf[AnyRef]) v.asInstanceOf[Try[U]] else this
} catch { case NonFatal(e) => Failure(e) }
}
override def failed: Try[Throwable] = Success(exception)
override def toOption: Option[T] = None
override def toEither: Either[Throwable, T] = Left(exception)
override def fold[U](fa: Throwable => U, fb: T => U): U = fa(exception)
}
final case class Success[+T](value: T) extends Try[T] {
override def isFailure: Boolean = false
override def isSuccess: Boolean = true
override def get = value
override def getOrElse[U >: T](default: => U): U = get
override def orElse[U >: T](default: => Try[U]): Try[U] = this
override def flatMap[U](f: T => Try[U]): Try[U] =
try f(value) catch { case NonFatal(e) => Failure(e) }
override def flatten[U](implicit ev: T <:< Try[U]): Try[U] = value
override def foreach[U](f: T => U): Unit = f(value)
override def transform[U](s: T => Try[U], f: Throwable => Try[U]): Try[U] = this flatMap s
override def map[U](f: T => U): Try[U] = Try[U](f(value))
override def collect[U](pf: PartialFunction[T, U]): Try[U] = {
val marker = Statics.pfMarker
try {
val v = pf.applyOrElse(value, ((x: T) => marker).asInstanceOf[Function[T, U]])
if (marker ne v.asInstanceOf[AnyRef]) Success(v)
else Failure(new NoSuchElementException("Predicate does not hold for " + value))
} catch { case NonFatal(e) => Failure(e) }
}
override def filter(p: T => Boolean): Try[T] =
try {
if (p(value)) this else Failure(new NoSuchElementException("Predicate does not hold for " + value))
} catch { case NonFatal(e) => Failure(e) }
override def recover[U >: T](pf: PartialFunction[Throwable, U]): Try[U] = this
override def recoverWith[U >: T](pf: PartialFunction[Throwable, Try[U]]): Try[U] = this
override def failed: Try[Throwable] = Failure(new UnsupportedOperationException("Success.failed"))
override def toOption: Option[T] = Some(value)
override def toEither: Either[Throwable, T] = Right(value)
override def fold[U](fa: Throwable => U, fb: T => U): U =
try { fb(value) } catch { case NonFatal(e) => fa(e) }
}
