A Reflection on Failure

How does exception handling in Java and Scala compare? What are the appealing aspects of handling exceptions as values using scala.util.Try (new in Scala 2.10)? Is scala.util.Try a monad… and does it matter?

Transcript

1. A Reflection on Failure Daniel W. H. James @dwhjames
2. Statements vs. Expressions
3. Exception handlers are functions
4. try { … } catch myHandler
5. PartialFunction[Throwable, T]
6. try … catch (myHandler orElse generalHandler)
7. try … catch (myHandler andThen someLogging)
8. Checked exceptions
9. public Object myMethod(…) throws Exception { …
10. Failure as a type
11. trait Map[A, +B] { def get(key: A): Option[B] }
12. public Object myMethod(…) throws Exception def myMethod(…): Either[Exception, AnyRef]
13. import scala.util.Try
14. sealed abstract class Try[+T] final case class Success[+T](value: T) extends Try[T] final case class Failure[+T](ex: Throwable) extends Try[T]
15. Try[T] ≅ Either[Throwable, T]
16. Option vs. Try
17. Option Try def apply(x: T): Option[T] def apply(x: 㱺 T): Try[T] Option { mightBeNull(…) } Try { mightThrowEx(…) }
18. Option Try def get: T def getOrElse[U >: T](default: 㱺 U): U
19. Option Try def map[U](f: T 㱺 U): Try[U] def flatMap[U](f: T 㱺 Try[U]): Try[U] def foreach[U](f: T 㱺 U): Unit def filter(f: T 㱺 Boolean): Try[T]
20. Try def orElse[U >: T](default: 㱺 Try[U]): Try[U] Option
21. try { val res = try { callRemoteService(…) } catch { case _: Exception => try { callBackupService(…) } catch { case _: Exception => useLocalService(…) } } displayResult(res) } catch { case ex: ServiceException => … case ex: DisplayException => … }
22. callRemoteService(…) orElse callBackupService(…) orElse useLocalService(…) flatMap displayResult(_) recover { case ex: ServiceException => … case ex: DisplayException => … }
23. Is Try a monad? … not everyone thinks so.
24. def map[U](f: T 㱺 U): Try[U] = this match { case Success(x) => Success(f(x)) case _ => this } the functor laws
25. def map[U](f: T 㱺 U): Try[U] = this match { case Success(x) => Success(f(x)) case _ => this } the functor laws myTry map identity ≡ myTry 1
26. def map[U](f: T 㱺 U): Try[U] = this match { case Success(x) => Success(f(x)) case _ => this } the functor laws myTry map identity ≡ myTry 1 myTry map f map g ≡ myTry map (f andThen g) 2
27. def flatMap[U](f: T 㱺 Try[U]): Try[U] = this match { case Success(x) => f(x) case _ => this } the monad laws
28. def flatMap[U](f: T 㱺 Try[U]): Try[U] = this match { case Success(x) => f(x) case _ => this } the monad laws Success(x) flatMap f ≡ f(x) 1
29. def flatMap[U](f: T 㱺 Try[U]): Try[U] = this match { case Success(x) => f(x) case _ => this } the monad laws Success(x) flatMap f ≡ f(x) 1 myTry flatMap (Success(_)) ≡ myTry 2
30. def flatMap[U](f: T 㱺 Try[U]): Try[U] = this match { case Success(x) => f(x) case _ => this } the monad laws Success(x) flatMap f ≡ f(x) 1 myTry flatMap (Success(_)) ≡ myTry 2 myTry flatMap f flatMap g ≡ myTry flatMap { x => f(x) flatMap g } 3
31. def map[U](f: T 㱺 U): Try[U] = this match { case Success(x) => case _ => this } Success(f(x))
32. def map[U](f: T 㱺 U): Try[U] = this match { case Success(x) => case _ => this } Success(f(x))
33. def map[U](f: T 㱺 U): Try[U] = this match { case Success(x) => try Success(f(x)) catch { e: Exception => Failure(e) } case _ => this }
34. def map[U](f: T 㱺 ): Try[U] U
35. def map[U](f: T 㱺 ): Try[U] U
36. def map[U](f: T 㱺 ex[U]): Try[U]
37. def map[U](f: T 㱺 ex[U]): Try[U] <— flatMap?
38. def map[U](f: T 㱺 ex[U]): Try[U] <— flatMap? functor the laws?
39. def map[U](f: T 㱺 ex[U]): Try[U] <— flatMap? the laws? monad
40. the ‘monad’ laws def map[U](f: T 㱺 ex[U]): Try[U] = this match { case Success(x) => try Success(f(x)) catch { e: Exception => Failure(e) } case _ => this }
41. the ‘monad’ laws Success(x) map f ≡ f(x) 1 def map[U](f: T 㱺 ex[U]): Try[U] = this match { case Success(x) => try Success(f(x)) catch { e: Exception => Failure(e) } case _ => this }
42. the ‘monad’ laws Success(x) map f ≡ f(x) 1 def map[U](f: T 㱺 ex[U]): Try[U] = this match { case Success(x) => try Success(f(x)) catch { e: Exception => Failure(e) } case _ => this }
43. the ‘monad’ laws Success(x) map f ≡ f(x) 1 myTry map (Success(_)) ≡ myTry 2 def map[U](f: T 㱺 ex[U]): Try[U] = this match { case Success(x) => try Success(f(x)) catch { e: Exception => Failure(e) } case _ => this }
44. the ‘monad’ laws Success(x) map f ≡ f(x) 1 myTry map (Success(_)) ≡ myTry 2 def map[U](f: T 㱺 ex[U]): Try[U] = this match { case Success(x) => try Success(f(x)) catch { e: Exception => Failure(e) } case _ => this }
45. the ‘monad’ laws Success(x) map f ≡ f(x) 1 myTry map (Success(_)) ≡ myTry 2 myTry map f map g ≡ myTry map { x => f(x) map g } 3 def map[U](f: T 㱺 ex[U]): Try[U] = this match { case Success(x) => try Success(f(x)) catch { e: Exception => Failure(e) } case _ => this }
46. the ‘monad’ laws Success(x) map f ≡ f(x) 1 myTry map (Success(_)) ≡ myTry 2 myTry map f map g ≡ myTry map { x => f(x) map g } 3 def map[U](f: T 㱺 ex[U]): Try[U] = this match { case Success(x) => try Success(f(x)) catch { e: Exception => Failure(e) } case _ => this }
47. the ‘monad’ laws Success(x) map f ≡ Try { f(x) } 1 myTry map identity ≡ myTry 2 myTry map f map g ≡ myTry map { x => Try { f(x) } map g } 3 def map[U](f: T 㱺 ex[U]): Try[U] = this match { case Success(x) => try Success(f(x)) catch { e: Exception => Failure(e) } case _ => this }
48. btw, why do the monad laws even matter? www.haskell.org/haskellwiki/Monad_laws
49. A Closing Thought is Option.map wrong?
50. def map[B](f: A => B): Option[B] = if (isEmpty) None else Some(f(this.get))
51. def map[B](f: A => B): Option[B] = if (isEmpty) None else Some(f(this.get)) <— null?