"concat" two xforms into one?

Question

(let [coll [{:a 1 :b 2}
            {:a 3 :b 4}
            {:a 7 :b 5}
            {:a 1 :b 4}]
      xf1 (comp (map :a) (filter #{1 3}))
      xf2 (comp (map :b) (filter #{2 4}))]
  (concat
    (into [] xf1 coll)
    (into [] xf2 coll)))

is there a way to combine the two xforms above (xf1 and xf2) into a single xform somehow so the collection coll will be traversed only once?

Answer 1

not really a combination of transducers, but:

(let [coll [{:a 1 :b 2}
            {:a 3 :b 4}
            {:a 7 :b 5}
            {:a 1 :b 4}]
      xf1 #(-> % :a #{1 3})
      xf2 #(-> % :b #{2 4})] 
  (into []
    (comp 
      (mapcat (juxt xf1 xf2))
      (remove nil?))
    coll))
=> [1 2 3 4 1 4]

Comments

nice try, but I'm looking for a way to combine transducers...

Answer 2

  (let [coll [{:a 1 :b 2}
              {:a 3 :b 4}
              {:a 7 :b 5}
              {:a 1 :b 4}]
        xf1 (comp (map :a) (filter #{1 3}))
        xf2 (comp (map :b) (filter #{2 4}))]
    (eduction cat [(eduction xf1 coll) (eduction xf2 coll)]))

eduction can be replaced with sequence

Answer 3

(defn facet [m]
  (fn [f]
    (let [m (into {} (for [[k v] m]
                       [k (v f)]))]
      (fn
        ([accum]
         (reduce
          (fn [accum1 [k accum2]]
            (assoc accum1 k ((get m k) accum2)))
          accum
          accum))
        ([accum value]
         (reduce
          (fn [accum1 [k accum2]]
            (assoc accum1 k ((get m k) accum2 value)))
          accum
          accum))))))

(let [coll [{:a 1 :b 2}
            {:a 3 :b 4}
            {:a 7 :b 5}
            {:a 1 :b 4}]
      xf1 (comp (map :a) (filter #{1 3}))
      xf2 (comp (map :b) (filter #{2 4}))]
  ((comp (partial apply concat)
         (juxt :a :b))
   (transduce
    (facet {:a xf1
            :b xf1})
    conj
    {:a []
     :b []}
    coll)))

Comments

a good place to look for interesting transducers is https://github.com/cgrand/xforms and a good place for inspiration for different kinds of processing you can do with folds is https://github.com/aphyr/tesser