TL;DR As noted earlier, transducers can be properly annotated in Clojure using core.typed and they probably should be. But... there are a few tricks necessary to make it work. Transducers in Scala require tricks too, but different ones. Oh, but they're so lovely in Haskell. Transducers I won't explain transducers here. The canonical introduction is Rich Hickey's blog post, with further explanation in his Strangeloop talk. I contributed a brief glossary, which may possibly be helpful. Why bother with typed transducers in Clojure At the end of an earlier post, I noted that, despite some ...