fold vs transients

[Christophe Grand]

Hi,

The fact that when calling (fold combinef reducef coll):

* (combinef) is directly used as the init for the group reduce,

* and the return value of the group reduce is directly passed to combinef.

preclude using fold instead of a reduce which uses transients (eg #'frequencies).

To fix this issue there should be a way to specify a "cleanup" function to call on the return of reduce.

A potential design choice would be to use the 1-arg arity of combine (that would thus acts as the unit function mapping to the monoid domain)

(r/fold

  (fn 

    ([] (transient {}))

    ([tm] (persistent! tm))

    ([a b] (merge-with + a b)))

  (fn [counts x]

    (assoc! counts x (inc (get counts x 0))))

  coll)

This would be a breaking change for current combinef functions whose 1-arg arity is not the identity.

Is there any interest in this line of work? Or what's the suggested work around?

Thanks,

Christophe

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[Christophe Grand]

Proposed change here https://github.com/cgrand/clojure/commit/65e1acef03362a76f7043ebf3fe2fa277c581912