Java 8: Chain Consumer<Y> to Function<X,Y>

Question

Exploring the new features of Java 8, I stumbled the wish to create a Consumer<X> by chaining a Consumer<Y> to Function<X,Y>.

Does this make sense? And if so, how would a good (general) solution look like?

---

What I've tried (rather a special case by example):

Given

@FunctionalInterface
public interface PartialFunction<X, Y> {
    Y apply(X x) throws Exception;
}

and

import java.util.Optional;
import java.util.function.Consumer;
import java.util.function.Function;

public class PartialFunctions {

    public static <X, Y> Function<X, Optional<Y>> withOptionalResults(final PartialFunction<X, Y> funcThatThrows) {
        return z -> {
            try {
                return Optional.of(funcThatThrows.apply(z));
            } catch (final Exception e) {
                return Optional.empty();
            }
        };
    }

    public static <X, Y> Consumer<X> acceptOnSuccess(final PartialFunction<X, Y> g, final Consumer<Y> c) {
        return x -> withOptionalResults(x).apply(t).ifPresent(c);
    }
}

I end up with a possible usage like:

files.forEach(PartialFunctions.<File, BufferedImage>acceptOnSuccess(
        ImageIO::read, images::add));

However, the need for the explicit generic specification is not optimal. Hopefully there is something better?

Answer 1

interface IgnoreThrowing<F,V> extends Function<F,V> {
    public default V apply(F from) {
        try {
            return ignore(from);
        } catch(Exception e) {
            return null;
        }
    }
    public V ignore(F from) throws Exception;
}

class Throwables {
    public static <F,V> Function<F,V> ignore(IgnoreThrowing<F,V> f) {
        return f;
    }
}

static {
    files.map(Throwables.ignore(ImageIO::read)).collect(...)
}

It will get better if you add a Collector that ignores nulls as input.

edit: i wrote this without syntax checking or compiling, so not totally sure about the placement of the default, and whether the compiler can successfully infer the chained function type parameters.

Answer 2

You could extend the Function interface like so:

public interface ComposableFunction<T, R> extends Function<T, R> {

    default Consumer<T> andThen(Consumer<R> after) {
        Objects.requireNonNull(after);
        return (T t) -> {after.accept(apply(t));};
    }

}

And then use it regularly like so:

ComposableFunction<Throwable, String> getMessage = Throwable::getMessage;
Consumer<String> log = System.out::println;
Consumer<Throwable> logMessage = getMessage.andThen(log);