Class Try.Try

sealed abstract class Try[+T] extends Product with Serializable

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.

Classlikes

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final class WithFilter(p: T => Boolean)

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.

Abstract methods

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def collect[U](pf: PartialFunction[T, U]): Try[U]

Applies the given partial function to the value from this Success or returns this if this is a Failure.

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def failed: Try[Throwable]

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.

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def filter(p: T => Boolean): Try[T]

Converts this to a Failure if the predicate is not satisfied.

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def flatMap[U](f: T => Try[U]): Try[U]

Returns the given function applied to the value from this Success or returns this if this is a Failure.

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def flatten[U](implicit ev: T <:< Try[U]): Try[U]

Transforms a nested Try, ie, a Try of type Try[Try[T]], into an un-nested Try, ie, a Try of type Try[T].

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def fold[U](fa: Throwable => U, fb: T => U): U

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.

val result: Try[Int] = Try { string.toInt }
log(result.fold(
 ex => "Operation failed with " + ex,
 v => "Operation produced value: " + v
))

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def foreach[U](f: T => U): Unit

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.

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def get: T

Returns the value from this Success or throws the exception if this is a Failure.

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def getOrElse[U >: T](default: => U): U

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.

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def isFailure: Boolean

Returns true if the Try is a Failure, false otherwise.

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def isSuccess: Boolean

Returns true if the Try is a Success, false otherwise.

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def map[U](f: T => U): Try[U]

Maps the given function to the value from this Success or returns this if this is a Failure.

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def orElse[U >: T](default: => Try[U]): Try[U]

Returns this Try if it's a Success or the given default argument if this is a Failure.

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def recover[U >: T](pf: PartialFunction[Throwable, 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.

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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 flatMap for the exception.

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def toEither: Either[Throwable, T]

Returns Left with Throwable if this is a Failure, otherwise returns Right with Success value.

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def toOption: Option[T]

Returns None if this is a Failure or a Some containing the value if this is a Success.

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def transform[U](s: T => Try[U], f: Throwable => Try[U]): Try[U]

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.

Concrete methods

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final def withFilter(p: T => Boolean): WithFilter

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.

Inherited methods

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def canEqual(that: Any): Boolean

A method that should be called from every well-designed equals method that is open to be overridden in a subclass. See Programming in Scala, Chapter 28 for discussion and design.

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def productArity: Int

The size of this product.

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def productElement(n: Int): Any

The nth element of this product, 0-based. In other words, for a product A(x1, ..., xk), returns x(n+1) where 0 <= n < k.

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def productElementName(n: Int): String

The name of the nth element of this product, 0-based. In the default implementation, an empty string.

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def productElementNames: Iterator[String]

An iterator over the names of all the elements of this product.

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def productIterator: Iterator[Any]

An iterator over all the elements of this product.

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def productPrefix: String

A string used in the toString methods of derived classes. Implementations may override this method to prepend a string prefix to the result of toString methods.