Composition of BiFunctions

Question

I have a stream of BiFunctions which I want to reduce to a single BiFunction.

To be more specific I have a stream of BiFunctions

BiFunction<x,y,y>

Where x and y denote the types of the arguments.

Assuming I have two functions

f (x, y) -> y
g (x, y) -> y

I want to compose them to the function

h(x, y) -> f(x, g(x, y))

Is this possible using Java 8 streams, and if not, what would be the most elegant way asides of just looping over all the available functions and composing them in another way?

Answer 1

Using x=Integer and y=String, you could define f and g as:

BiFunction<Integer, String, String> f = (i, s) -> i + "-f-" + s;
BiFunction<Integer, String, String> g = (i, s) -> i + "-g-" + s;

And the h function could be built using a reduction:

BiFunction<Integer, String, String> h = Stream.of(f, g)
        .reduce((f_, g_) -> (i, s) -> f_.apply(i, g_.apply(i, s)))
        .get(); //we know the stream is not empty so we can call get directly

If you apply h to 1 and "s", it will return 1-f-1-g-s.

Answer 2

As a slight modification to the answer by assylias : Depending on the application pattern, you can also use the Stream#reduce method that accepts an identity as the first argument, which in this case is the "identity BiFunction" (actually, a projection on the second argument).

import java.util.function.BiFunction;
import java.util.stream.Stream;

public class BiFunctionsComposition
{
    public static void main(String[] args)
    {
        BiFunction<String, String, String> f = (x,y) -> "f("+x+","+y+")";
        BiFunction<String, String, String> g = (x,y) -> "g("+x+","+y+")";

        BiFunction<String, String, String> h = Stream.of(f, g)
            .reduce((x,y) -> y, (ff,gg) -> (x,y) -> ff.apply(x, gg.apply(x, y)));

        String result = h.apply("A", "B");
        System.out.println(result); // Prints f(A,g(A,B))
    }
}