坚持使用clojure中的类型提示进行泛型类

Question

我试图从一个在clojure中运行的Apache flink中得到一个小例子,但是现在我被卡住了,因为clojure中的类型暗示和flink中的一些奇怪的怪癖.

这是我的代码:

(ns pipeline.core
 (:import
 (org.apache.flink.api.java ExecutionEnvironment)
 (org.apache.flink.api.common.functions FlatMapFunction)
 (org.apache.flink.api.java.tuple Tuple2)
 (org.apache.flink.util Collector)
 (java.lang String)))

(def flink-env (ExecutionEnvironment/createLocalEnvironment))

(def dataset (.fromElements flink-env (to-array ["please test me"])))

(defn tokenizer [] (reify FlatMapFunction
                 ( flatMap [this value collector] 
                   (println value))))

(.flatMap dataset (tokenizer))

如果我不提供类型提示,我从flink api收到错误:

Caused by: java.lang.IllegalArgumentException: The types of the interface org.apache.flink.api.common.functions.FlatMapFunction could not be inferred. Support for synthetic interfaces, lambdas, and generic types is limited at this point.
at org.apache.flink.api.java.typeutils.TypeExtractor.getParameterType(TypeExtractor.java:662)

如果我提供类型提示:

(defn tokenizer [] (reify FlatMapFunction
                 ( ^void flatMap [this ^String value ^Collector collector] 
                   (println value))))

我从clojure编译器得到一个错误:

Caused by: java.lang.IllegalArgumentException: Can't find matching method: flatMap, leave off hints for auto match.
at clojure.lang.Compiler$NewInstanceMethod.parse(Compiler.java:8065) 

有没有办法在clojure中使用泛型类添加类型提示？它应该是这样的:

(defn tokenizer [] (reify FlatMapFunction
                 ( ^void flatMap [this ^String value ^Collector<Tuple2<String, Integer>> collector] 
                   (println value))))

但这不起作用.有任何想法吗？

lein配置看起来像这样:

(defproject pipeline "0.1.0-SNAPSHOT"
 :description "FIXME: write description"
 :url "http://example.com/FIXME"
 :license {:name "Eclipse Public License"
        :url "http://www.eclipse.org/legal/epl-v10.html"}
 :dependencies [[org.clojure/clojure "1.7.0"]               
             [org.apache.flink/flink-java "0.9.0"]              
             ]
  :aot :all)

Answer 1

Clojure 无法处理反射，因此需要通过 Flink 方法手动指定返回类型returns。

(.returns (.flatMap dataset (tokenizer)) String)

此外，由于 Flink 无法处理匿名类，因此在使用时需要使用 usedeftype来定义和实例化一个新对象：tokenizer

(deftype tokenizer [] FlatMapFunction
                      (flatMap [this value collector] 
                        (println value)))

(.flatMap dataset (tokenizer.))

这是一个完整的“字数统计示例”，可以打包到 jar 中并执行。

注意类型提示和强制转换。对于tokenizer输出(int 1)是必需的，否则Long将是第二种类型的Tuple2. 此外，我们使用 String 来声明输出类型tokenizer（类类型是不够的，因为还必须指定反射类型）。最后，我们需要输入hint(int-array [0])来解决重载问题groupBy（如果没有它，该方法对于Clojure编译器来说是不明确的）。

(ns org.apache.flink.flink-clojure.WordCount
 (:import
 (org.apache.flink.api.common.functions FlatMapFunction)
 (org.apache.flink.api.java DataSet)
 (org.apache.flink.api.java ExecutionEnvironment)
 (org.apache.flink.api.java.tuple Tuple2)
 (org.apache.flink.util Collector)
 (java.lang String))
 (:require [clojure.string :as str])
 (:gen-class))

(def flink-env (ExecutionEnvironment/createLocalEnvironment))

(def text (.fromElements flink-env (to-array ["please test me and me too"])))

(deftype tokenizer [] FlatMapFunction
                      (flatMap [this value collector]
                        (doseq [v (str/split value #"\s")]
                          (.collect collector (Tuple2. v (int 1))))))

(def tokens (.returns (.flatMap text (tokenizer.)) "Tuple2<String,Integer>"))

(def counts (.sum (.groupBy tokens (int-array [0])) 1))

(defn -main []
  (.print counts)
)