Immutability is not a language feature in Scala as it's not enforced like it is in Haskell, so you don't have any guarantees about what a piece of code doesn't

Immutability is not a language feature in Scala as it's not enforced like it is in Haskell, so you don't have any guarantees about what a piece of code doesn't do. Granted, you can make a better guess than in Java where mutability is concerned. Scala's syntax is flexible enough to let you build DSLs. It makes use of operator overloading, optional braces after method calls, infix methods, etc. All features that Java doesn't have. Being able to build DSLs is a good thing, but the whole point of DSLs is to hide some of the semantics of the underlying language and let you think in terms of the domain the DSL was built for. So there is clearly a power/complexity/understandability trade-off.