Transducers in java

Fun.java

interface Fun<A,B> {
    B apply(A a);
}

Mapping.java

public class Mapping<A,B> implements Transducer<B,A>
{   
   final Fun<A,B> f;
   public Mapping(Fun<A,B> f) { this.f = f; }
   
   public <R> Reducer<A,R> transduce(final Reducer<B,R> reducer) {
       return new Reducer<A,R>() {
           public R init() { return reducer.init(); }
           public R step(R acc, A item) { return reducer.step(acc, f.apply(item)); }
       };
   }
}

Reducable.java

interface Reducable<A> {
    <R> R reduce(Reducer<A,R> reducer);
}

Reducer.java

interface Reducer<A,R> {
   R init();
   R step(R acc, A item);
}

Sum.java

public class Sum implements Reducer<Long,Long> {
   public Long init() { return 0L; }
   public Long step(Long acc, Long item) { return acc + item; }
}

Test.java

public class Test {
    public static void main(String args[]) {

        Fun<String,Long> decode = new Fun<String,Long>() {
            public Long apply(String s) { return Long.valueOf(s); }
        };
        Reducer<Long,Long> sum = new Sum();
        Reducer<String,Long> process = new Mapping(decode).transduce(sum);
            
        WordReader inputs = new WordReader();
        
        Long result = inputs.reduce(process);
        System.out.println("sum = " + result);
    }
}

Transducer.java

interface Transducer<A,B> {
   <R> Reducer<B,R> transduce(Reducer<A,R> reducer);
}

WordReader.java

import java.util.Scanner;
   
public class WordReader implements Reducable<String> {
 
    public <R> R reduce(Reducer<String,R> reducer) {
        R acc = reducer.init();
        Scanner sc = new Scanner(System.in);
        while (sc.hasNext()) {
            acc = reducer.step(acc, sc.next());
        }
        return acc;
    }
}