Type insanity, and a non-general path to a functional FizzBuzz in Java 8

PairedFizzBuzz.java

package fizzbuzz;

import java.util.Arrays;
import java.util.Collection;
import java.util.Iterator;
import java.util.List;
import java.util.Optional;
import java.util.function.Function;
import java.util.stream.IntStream;
import java.util.stream.Stream;

public class PairedFizzBuzz {
    static class Pair<L,R> {
        final L left;
        final R right;

        Pair(L left, R right) {
            this.left = left;
            this.right = right;
        }
    }

    public static void main(String[] args) {
        Function<Pair<Integer,Optional<String>>,Pair<Integer,Optional<String>>> id = (p) -> 
                new Pair<>(p.left, p.right);

        Function<Pair<Integer,Optional<String>>,Pair<Integer,Optional<String>>> fizz = (p) -> 
                new Pair<>(p.left, Optional.of(p.right.map(s -> s + "Fizz").orElse("Fizz")));

        Function<Pair<Integer,Optional<String>>,Pair<Integer,Optional<String>>> buzz = (p) -> 
                new Pair<>(p.left, Optional.of(p.right.map(s -> s + "Buzz").orElse("Buzz")));

        List<Function<Pair<Integer, Optional<String>>, Pair<Integer, Optional<String>>>> fizzList =
                Arrays.asList(id, id, fizz);
        List<Function<Pair<Integer, Optional<String>>, Pair<Integer, Optional<String>>>> buzzList =
                Arrays.asList(id, id, id, id, buzz);

        Iterator<Function<Pair<Integer,Optional<String>>,Pair<Integer,Optional<String>>>> fizzes =
                Stream.generate(() -> fizzList)
                      .flatMap(Collection::stream)
                      .iterator();
        Iterator<Function<Pair<Integer,Optional<String>>,Pair<Integer,Optional<String>>>> buzzes =
                Stream.generate(() -> buzzList)
                      .flatMap(Collection::stream)
                      .iterator();

        IntStream.range(1, 1000)
                .mapToObj(num -> new Pair<>(num, Optional.<String>empty()))
                .map(p -> fizzes.next().apply(p))
                .map(p -> buzzes.next().apply(p))
                .map(p -> p.right.orElse(p.left.toString()))
                .limit(20)
                .forEach(System.out::println);
    }
}

PairedFizzBuzzV2.java

package fizzbuzz;

import java.util.Collection;
import java.util.Collections;
import java.util.Iterator;
import java.util.Optional;
import java.util.function.Function;
import java.util.stream.Collectors;
import java.util.stream.IntStream;
import java.util.stream.Stream;

public class PairedFizzBuzzV2 {
    class Pair<L,R> {
        public final L left;
        public final R right;

        Pair(L left, R right) {
            this.left = left;
            this.right = right;
        }
    }

    class IntOptStringPair extends Pair<Integer,Optional<String>> {
        IntOptStringPair(Integer left, Optional<String> right) {
            super(left, right);
        }
    }

    Function<IntOptStringPair,IntOptStringPair> id = p -> p;

    Iterator<Function<IntOptStringPair,IntOptStringPair>> generate(int n, String output) {
        return Stream.generate(() -> Stream.concat(
                Stream.of(Collections.nCopies(n - 1, id)).flatMap(Collection::stream),
                Stream.of(p -> new IntOptStringPair(p.left, Optional.of(p.right.map(s -> s + output).orElse(output)))))
                .collect(Collectors.toList()))
                .flatMap(Collection::stream)
                .iterator();
    }

    public void run() {
        Iterator<Function<IntOptStringPair,IntOptStringPair>> fizzes = generate(3, "Fizz");
        Iterator<Function<IntOptStringPair,IntOptStringPair>> buzzes = generate(5, "Buzz");

        IntStream.range(1, 1000)
                .mapToObj(num -> new IntOptStringPair(num, Optional.<String>empty()))
                .map(p -> fizzes.next().apply(p))
                .map(p -> buzzes.next().apply(p))
                .map(p -> p.right.orElse(p.left.toString()))
                .limit(20)
                .forEach(System.out::println);
    }

    public static void main(String[] args) {
        new PairedFizzBuzz().run();
    }
}

v2 (just posted here) is a bit nicer, but still hard to reason about ;-)

@timyates You can nearly qualify as a C++ programmer 😀