卡在 clojure 中的泛型类类型提示

Question

我试图从 Apache flink 在 clojure 中运行一个小例子，但现在我被卡住了，因为 clojure 中的类型提示和 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，我会从 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 编译器收到一个错误：

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？ 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。有什么想法吗？

lein 配置如下所示：

(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 returns 手动指定返回类型。

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

另外，由于Flink无法处理匿名类，所以需要使用deftype定义tokenizer并实例化一个新对象：

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

(.flatMap dataset (tokenizer.))

这是一个完整的“字数示例”，可以打包到 jar 中并执行。

注意类型提示和类型转换。对于tokenizer，需要输出(int 1)，否则Long 将是Tuple2 的第二种类型。此外，我们使用 String 来声明 tokenizer 的输出类型（类类型是不够的，因为还必须指定反射类型）。最后，我们需要输入提示(int-array [0]) 来解决groupBy 的重载（没有它，该方法对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)
)

Answer 2

作为此评论的后续Stuck with type hints in clojure for generic class

使用最新的 flink 版本（在 1.6.1 上测试），您需要定义一个自定义类，否则您会收到如下错误：

Exception in thread "main" java.lang.IllegalArgumentException: No matching method found: returns for class org.apache.flink.api.java.operators.FlatMapOperator, compiling:(WordCount.clj:69:13)

自定义类：

package org.apache.flink.java;

import org.apache.flink.api.java.tuple.Tuple2;


public class WordCountTuple extends Tuple2<String, Integer> {

}

clojure 代码

(ns org.apache.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.java WordCountTuple)
   (org.apache.flink.util Collector)
   (java.lang String))
  (:require [clojure.string :as str])
  (:gen-class))

(def flink-env (ExecutionEnvironment/getExecutionEnvironment))

(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.)) WordCountTuple))

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

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

工作示例叉在这里https://github.com/guillaume/flink-external