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Clojure 1.5 adds new threading macros, including:
some->some->>The changelog has this contrived example to illustrate how some-> works:
user=> (defn die [x] (assert false))
#'user/die
user=> (-> 1 inc range next next next die)
AssertionError Assert failed: false user/die (NO_SOURCE_FILE:65)
user=> (some-> 1 inc range next next next die)
nil
Chatting with other programmers, we found it difficult to think of a good, practical example for some->. When have you used some-> to solve a real-world problem?
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Clojure has a programmatic macro system which allows the compiler to be extended by user code. Macros can be used to define syntactic constructs which would require primitives or built-in support in other languages. Many core constructs of Clojure are not, in fact, primitives, but are normal macros.
->> is the "thread-last" macro. It evaluates one form and passes it as the last argument into the next form. Your code is the equivalent of: (reduce str (interpose ", " (map :subject scenes)))
some-> can be used to "auto-guard" a threaded series of processing steps where some part in the chain (especially in the middle) might return nil which would cause a logic failure further down the chain.
Particular examples could include threading clojure functions together with java interop where you would need to guard against null pointer exceptions.
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