(defn multiply-xf
[]
(fn [xf]
(let [product (volatile! 1)]
(fn
([] (xf))
([result]
(xf result @product)
(xf result))
([result input]
(let [new-product (* input @product)]
(vreset! product new-product)
(if (zero? new-product)
(do
(println "reduced")
(reduced ...)) <----- ???
result)))))))
This is a simple transducer which multiples numbers. I am wondering what would be the reduced value to allow early termination?
I've tried (transient []) but that means the transducer only works with vectors.
I'm assuming you want this transducer to produce a running product sequence and terminate early if the product reaches zero. Although in the example the reducing function xf is never called in the 2-arity step function, and it's called twice in the completion arity.
(defn multiply-xf
[]
(fn [rf]
(let [product (volatile! 1)]
(fn
([] (rf))
([result] (rf result))
([result input]
(let [new-product (vswap! product * input)]
(if (zero? new-product)
(reduced result)
(rf result new-product))))))))
Notice for early termination, we don't care what result is. That's the responsibility of the reducing function rf a.k.a xf in your example. I also consolidated vreset!/@product with vswap!.
(sequence (multiply-xf) [2 2 2 2 2])
=> (2 4 8 16 32)
It will terminate if the running product reaches zero:
(sequence (multiply-xf) [2 2 0 2 2])
=> (2 4)
We can use transduce to sum the output. Here the reducing function is +, but your transducer doesn't need to know anything about that:
(transduce (multiply-xf) + [2 2 2 2])
=> 30
I've tried
(transient [])but that means the transducer only works with vectors.
This transducer also doesn't need to concern itself the type of sequence/collection it's given.
(eduction (multiply-xf) (range 1 10))
=> (1 2 6 24 120 720 5040 40320 362880)
(sequence (multiply-xf) '(2.0 2.0 0.5 2 1/2 2 0.5))
=> (2.0 4.0 2.0 4.0 2.0 4.0 2.0)
(into #{} (multiply-xf) [2.0 2.0 0.5 2 1/2 2 0.5])
=> #{2.0 4.0}
This can be done without transducers as well:
(take-while (complement zero?) (reductions * [2 2 0 2 2]))
=> (2 4)