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September 22, 2014 17:15
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Transducers in java
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interface Fun<A,B> { | |
B apply(A a); | |
} |
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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)); } | |
}; | |
} | |
} |
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interface Reducable<A> { | |
<R> R reduce(Reducer<A,R> reducer); | |
} |
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interface Reducer<A,R> { | |
R init(); | |
R step(R acc, A item); | |
} |
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public class Sum implements Reducer<Long,Long> { | |
public Long init() { return 0L; } | |
public Long step(Long acc, Long item) { return acc + item; } | |
} |
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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); | |
} | |
} |
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interface Transducer<A,B> { | |
<R> Reducer<B,R> transduce(Reducer<A,R> reducer); | |
} |
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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; | |
} | |
} |
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