Continuation.java

import java.util.function.Function;

public interface Continuation<R, A> extends Function<Function<A, R>, R> {

    @Override
    R apply(Function<A, R> f);

    default <B> Continuation<R, B> map(Function<A, B> f) {
        return continuation(g -> apply(g.compose(f)));
    }

    default <B> Continuation<R, B> flatmap(Function<A, Continuation<R, B>> f) {
        return continuation(g -> apply(a -> f.apply(a).apply(g)));
    }

    static <R, A> Continuation<R, A> of(A a) {
        return f -> f.apply(a);
    }

    static <R, A> Continuation<R, A> continuation(Function<Function<A, R>, R> g) {
        return g::apply;
    }

    static <R, A, B> Continuation<R, A> callcc(Function<Function<A, Continuation<R, B>>, Continuation<R, A>> f) {
        return continuation(k -> f.apply(a -> continuation(__ -> k.apply(a))).apply(k));
    }

}