What is the purpose of clojure.core.reducers/reduce?

Question

A slightly modified version of reduce was introduced with reducers, clojure.core.reducers/reduce (short r/reduce):

(defn reduce
  ([f coll]
   (reduce f (f) coll))
  ([f init coll]
   (if (instance? java.util.Map coll)
     (clojure.core.protocols/kv-reduce coll f init)
     (clojure.core.protocols/coll-reduce coll f init))))

r/reduce differs from its core sibling only in that it uses (f) as the initial value when none is provided, and it delegates to core reduce-kv for maps.

I don’t understand what use such an odd special-purpose reduce might be and why it was worth including in the reducers library.

Curiously, r/reduce is not mentioned in the two introductory blog posts as far as I can tell (first, second). The official documentation notes

In general most users will not call r/reduce directly and instead should prefer r/fold (...) However, it may be useful to execute an eager reduce with fewer intermediate results.

I’m unsure what that last sentence hints at.

What situations can r/reduce handle that the core reduces cannot? When would I reach for r/reduce with conviction?

Answer 1

Two possible reasons:

1. It has different – better! – semantics than clojure.core/reduce in the initless sequential case. During his 2014 Conj presentation Rich Hickey asked "who knows what the semantics of reduce are when you call it with a collection and no initial value?" – follow this link for the exact spot in the presentation – and then described the said semantics as "a ridiculous, complex rule" and "one of the worst things [he] ever copied from Common Lisp" – cf. Common Lisp's reduce contract. The presentation was about transducers and the context of the remark was a discussion of transduce, which has a superior, simpler contract; r/reduce does as well.

2. Even without considering the above, it's sort of nice to have a version of reduce with a contract very close to that of fold. That enables simple "try one, try the other" benchmarking with the same arguments, as well as simply changing one's mind.