As disclaimer, I've been a Scala zealot for the last 3 years. On Clojure vs Scala, usually comparisons are kind of unfair I think. As somebody interested in FP, I tried Clojure first. And FP being all about immutable data-structures, pure functions operating on immutable data-structures and dealing with side-effects explicitly by means of abstractions such as the I/O Monad, I expected to see that in Clojure, seeing how people describe it as being very FP.

And it was like a big WTF for me seeing how for example everything in Clojure is a Var that can be changed. And eventually I got it, I mean Clojure is a LISP and it's meant to be compiled by a REPL and the unit of compilation is not a file and this is in LISP's tradition and so on and so forth. But it's still a WTF given its usual characterization. Even in Java classes are completely immutable (until you start manipulating bytecode of course) and you've got "final" declarations. You have no "final" in Clojure. And this is actually a pity because it (probably) means that those persistent data-structures can't be implemented in Clojure, as "final" on top of the JVM comes with a pretty cool contract in the Java Memory Model that you need. And then instead of Scala's implicit parameters, that are actually very explicit and type-checked by the compiler, in Clojure you have package-wide vars or refs that people manipulate as a poor man's dependency injection mechanism. And state changes are usually done by mutating atoms. Well that's cool, in Scala people also do that, but in Scala you also have a mature implementation of the I/O monad and monad transformers if you want it, or various FRP libraries. And I'm sure that Clojure has plenty of libraries, but overall I feel that in Scala the ecosystem of design patterns borrowed-from-Haskell is more mature.

Scala provides mutable variants of its data-structures in the standard library. But that's not extra rope to hang yourself, because in Clojure the use cases still exist and people end up using Java's mutable data-structures instead. In Scala the blend of OOP and FP can sound overwhelming, but it's actually the healthiest blend of FP + OOP to ever come up in a programming language. And in Clojure, just because you can't declare a class, that doesn't mean that OOP doesn't exist. It does and the blend is not healthy. This is why "reify" and "proxy" exist and are used quite frequently. And things like clojure.lang.ISeq are a Java OOP interface and not a protocol. Basically Scala is trying to make the best of OOP, whereas Clojure considers it a necessary evil of the host platform that it can't get rid of. Pick your poison.