Free.Bind.java

class Bind<A, B> implements Free<B> {

    private final Operation<A> operation;
    private final F<A, Free<B>> fInner;

    private Bind(Operation<A> operation, F<A, Free<B>> fInner) {
        this.operation = operation;
        this.fInner = fInner;
    }

    public <C> Free<C> flatMap(F<B, Free<C>> f) {
        return new Bind<>(operation, a -> fInner.f(a).flatMap(f));
    }

    public B run(Interpreter interpreter) {
        return fInner.f(interpreter.f(operation)).run(interpreter);
    }
}

Free.java

interface Free<A> {

    <B> Free<B> flatMap(F<A, Free<B>> f);

    default <B> Free<B> map(F<A, B> f) {
        return flatMap(a -> new Return<>(f.f(a)));
    }

    A run(Interpreter interpreter);
}

Free.Return.java

class Return<B> implements Free<B> {

    private final B b;

    private Return(B b) {
        this.b = b;
    }

    public <C> Free<C> flatMap(F<B, Free<C>> f) {
        return f.f(b);
    }

    public B run(Interpreter interpreter) {
        return b;
    }
}

Interpreter.java

package org.gtri.framework;

public interface Interpreter {

    <A> A f(Operation<A> operation);
}

Operation.java

package org.gtri.framework;

import fj.F;

public interface Operation<A> {
    default Free<A> free() {
        return new Free.Bind<>(this, Free.Return::new);
    }
}