What is the reasoning behind the Transducers eduction function?

[Jason Gilman]

There's a few new functions that have been added with Transducers which mostly have very clear use cases to me. What are the use cases for the new eduction function? (which looks like it was previously called iteration). The example at http://clojure.org/transducers is informative. It looks similar to sequence in that it produces something that is seq-able with the major difference being that the transform is applied each time the sequence is traversed.

You can see the difference here in this code (This uses iteration instead of eduction because I was on 1.7.0-alpha2)

(def to-str-iteration 

  (iteration (map (fn [i]

                    (println "iteration" i)

                    (str i)))

             (range 3)))

(def to-str-seq 

  (sequence (map (fn [i]

                   (println "sequence" i)

                   (str i)))

            (range 3)))

(println "First iteration")

(dorun to-str-iteration)

(println "Second iteration")

(dorun to-str-iteration)

(println "First sequence")

(dorun to-str-seq)

(println "Second sequence")

(dorun to-str-seq)

Printed:

First iteration

iteration 0

iteration 0

iteration 1

iteration 2

Second iteration

iteration 0

iteration 0

iteration 1

iteration 2

First sequence

sequence 0

sequence 1

sequence 2

Second sequence

Should we use eduction when we need side effects or reading the current state during a transform and sequence when the transform is pure? Or is there a subtler difference between the two that I'm missing?

[Laurent PETIT]

Hello,

eduction does not return a seq, or anything already "concrete" like that, rather it returns a new reducible, with the interesting property lying in the retention of the xform argument (a transducer). 

This allows you, for instance, to pass around a collection with all the processing steps but the final rendering of the result (a seq? an aggregate result? a vector ? nil?) => it's up to the user of the educible. 

Since the educible is an unreduced form of the initial collection, then every time it will be "realized" into a concrete value, the hidden (behind the xform) collection will be traversed, and the transducer-returned reducing function called. 

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Laurent Petit