That said, in my opinion Scala's blunders are many. You can find out about many of them from Paul Philips, ex committer of Scala, but if he sounds too bitter to you (he does to me; at this point he sounds like there is bad blood between him and Odersky), here are some of mine:
- Null. Null will come back to bite you in Scala code, whenever you call Java libraries, and the occasional Scala library that didn't get the memo. Sure, you can wrap every suspect value with Option(...), but why should you do this?
- Type inference is not as powerful as in a language with a cleaner type system such as Haskell. I'm told this is because Scala tries to be both OOP (inheritance) and FP at the same time. Here is a clear case of Scala's attempt at being a "jack of all trades" resulting in it being inferior than the sum of its parts.
- Type signatures in Scala collections and core types are extremely hard to read. You do not need to read them, but if you try, you are in for a world of hurt. This in itself is not a mortal sin, but of course if you're willing to forgive Scala this much complexity, surely you're willing to forgive other languages as well?
- Tooling is bad. It's getting better, but it's still bad. SBT is painful to use. As for IDEs, IntelliJ is now the recommended choice; I've tried both Scala IDE and IntelliJ, and they both suck. Slow, spurious compilation errors, uncomfortable to use.
- And finally, telling bit of personal experience: I do NOT use Haskell professionally, yet whenever I want to try a quick idea and see if it typechecks, I find it easier to open ghci (the Haskell REPL) and try my idea than do the same with the Scala REPL. What does this say about Scala?