Were you at Clojure/conj in Washington last week? If so, hello again. Wasn't that a great conference? If not, head to Clojure TV, where all the talks are ready for streaming. Assuming some moderate level of Clojure obsession on your part, I couldn't recommend skipping any of them, so the full catch-up might take you a while, but there are two in particular that I strongly recommend.

Avoiding altercations

The first is actually the very last talk of the conference. Brian Goetz, whom you may have encountered previously as the author of Java Concurrency in Practice or heard of …

How strange to think that, a mere week ago, the world had not yet heard my public pronouncement that transducers ought to be stateless. True, the media frenzy has died down a bit, but in its stead comes the quiet awareness that life will never be the same. Anyway, that's the way I like to think about it.

TL;DR

1. Storing state in the transducer makes it mutable, which might be unfortunate on general principles. In any event, it interferes with the metaphor of transducers as recipes for transformation and arguably makes code more difficult to understand.
2. A natural place …

After my recent attempt to provide type annotations for transducers, several people pointed out that I wasn't accounting for state. The signature of a pure function transformation, whether in Clojure

  (t/defalias Transducer (t/TFn [[a :variance :covariant]
                                 [b :variance :contravariant]]
                (t/All [r] [[r a -> r] -> [r b ->r]])))

or Haskell

  type Transducer a b = forall r . (r -> a -> r) -> (r -> b -> r)

nowhere acknowledges that the transducer might need to maintain, for example, the previous value in the series, in order to remove duplicates.

The failure is most obvious to hardcore Haskell programmers, who, as a rule, would …

TL;DR

1. As noted earlier, transducers can be properly annotated in Clojure using core.typed and they probably should be.
2. But... there are a few tricks necessary to make it work.
3. Transducers in Scala require tricks too, but different ones.
4. Oh, but they're so lovely in Haskell.

Update 2015-01-12

Were you led here by Clojure Gazette? Eric Normand is usually more discriminating, but don't worry, this will only waste a little of your time. Per the previous batch of updates, just below, and various subsequent posts on more or less the same topic, it should be clear this wee bagatelle …

TL;DR

It's not too long, but, to summarize the summary, if you read Rich Hickey's 2014 blog post on transducers first, his 2012 post on reducers will be easier to understand.

Brief Definitions

Herewith, all in one place, are Clojuresque definitions of:

• reducible
• reducing function
• transducer
• reducer
• folder
• decomplected

Longer elaborations of these definitions follow in the subsequent section.

reducing function

Anything that can be used as the first argument of the reduce function, e.g. + or conj. Generally, it's a binary function that returns something of the type of its first argument, which is supposed to be a …