What are the key differences between Java 8's Optional, Scala's Option and Haskell's Maybe?

Question

I've read a few posts on Java 8's upcoming Optional type, and I'm trying to understand why people keep suggesting it's not as powerful as Scala's Option. As far as I can tell it has:

• Higher order functions like map and filter using Java 8 lambdas.
• Monadic flatMap
• Short circuiting through getOrElse type functions.

What am I missing?

Answer 1

Some possibilities come to mind (OTOH, I haven't seen people actually saying that, so they might mean something else):

1. No pattern matching.

2. No equivalent to Scala's fold or Haskell's fromMaybe: you have to do optional.map(...).orElseGet(...) instead.

3. No monadic syntax.

I also wouldn't call any of these "less powerful" myself, since you can express everything you can with the corresponding Scala/Haskell types; these are all conciseness/usability concerns.

Answer 2

Optional and Maybe are effectively in correspondence. Scala has None and Some[A] as subclassing Option[A] which might be more directly comparable to Java since Java could have done the same.

Most other differences either have to do with the ease of handling Maybe/Option in Haskell/Scala which won't translate since Java is less expressive as a language or the consistency of use of Maybe/Option in Haskell/Scala where many of the guarantees and conveniences afforded by the type only kick in once most libraries have agreed to use optional types instead of nulls or exceptions.