带分区的core.async -通过状态传感器不保持状态？

Question

(我有一个先前的问题，https://stackoverflow.com/questions/49097325/clojure-strng-concat-with-group-by-in-sequences-of-maps，并假设我不会有问题进入core.async)。

给定这样的输入数据：

(require '[clojure.core.async :as a])

(def input-data
  [{:itm_na 1 :seq_no 1  :doc_img "this is a very long "}
   {:itm_na 1 :seq_no 2  :doc_img "sentence from a mainframe "}
   {:itm_na 1 :seq_no 3  :doc_img "system that was built before i was "}
   {:itm_na 1 :seq_no 4  :doc_img "born."}
   {:itm_na 2 :seq_no 1  :doc_img "this is a another very long "}
   {:itm_na 2 :seq_no 2  :doc_img "sentence from the same mainframe "}
   {:itm_na 3 :seq_no 1  :doc_img "Ok here we are again. "}
   {:itm_na 3 :seq_no 2  :doc_img "The mainframe only had 40 char per field so"}
   {:itm_na 3 :seq_no 3  :doc_img "they broke it into multiple rows "}
   {:itm_na 3 :seq_no 4  :doc_img "which seems to be common"}
   {:itm_na 3 :seq_no 5  :doc_img " for the time. "}
   {:itm_na 3 :seq_no 6  :doc_img "thanks for your help."}])

partition-by (如预期的)将数据集中到seq中(以便稍后崩溃)：

(count (partition-by :itm_na input-data ))
;;=> 3

然而，当我试图用core.async管道来做这件事时，出于某种原因，它似乎不这么做……在异步管道中，如何获得partition-by的状态转换器部分实际保持状态？

(let
    [source-chan (a/to-chan input-data)
     target-chan (a/chan 100)
     xf (comp (partition-by :itm_na))
     ]
  (a/pipeline 1
              target-chan
              xf
              source-chan)
  (count (<!! (a/into [] target-chan))))

;;=>12

这应该是3？

奇怪的是，当我像下面这样将xf绑定到通道时，我就得到了预期的结果。我不知道为什么a/pipeline的行为会有所不同。

(let [xf (comp (partition-by :itm_na))
      ch (a/chan 1 xf)]
  (a/onto-chan ch input-data)
  (count (<!! (a/into [] ch))))
=>3

医生的..。提到有状态比特：

(clojure.repl/doc partition-by) 
-------------------------
clojure.core/partition-by
([f] [f coll])
  Applies f to each value in coll, splitting it each time f returns a
   new value.  Returns a lazy seq of partitions.  Returns a stateful
   transducer when no collection is provided.

Answer 1

这个特殊的例子是Rich在他的演讲中的简略突出：您不能在有状态的传感器上使用pipeline，这主要是因为pipeline的并行性。