How can I pretty print a PersistentHashMap in Clojure to a string?

(let [s (java.io.StringWriter.)]
  (binding [*out* s]
    (clojure.pprint/pprint {:a 10 :b 20}))
  (.toString s))

Edit: Equivalent succinct version:

(with-out-str (clojure.pprint/pprint {:a 10 :b 20}))