TailRec.java

import java.util.function.Function;
import java.util.function.Supplier;

interface TailRec<A> {
    default <B> TailRec<B> map(Function<A, B> f) {
        return flatmap(x -> new Call<>(() -> new Done<>(f.apply(x))));
    }

    default <B> TailRec<B> flatmap(Function<A, TailRec<B>> f) {
        if (this instanceof Done) {
            Done<A> done = (Done<A>) this;
            return new Call<B>(() -> f.apply(done.val()));
        } else if (this instanceof Call) {
            Call<A> call = (Call<A>) this;
            return new Cont<>(call, f);
        } else if (this instanceof Cont) {
            Cont<Object, A> c = (Cont<Object, A>) this;
            return new Cont<>(c.head(), x -> c.fun().apply(x).flatmap(f));
        } else {
            throw new RuntimeException("illegal state");
        }
    }

    //
//    default A result() {
//        if (this instanceof Done) {
//            Done<A> done = (Done<A>) this;
//            return done.val();
//        } else if (this instanceof Call) {
//            Call<A> call = (Call<A>) this;
//            return call.next().result();
//        } else if (this instanceof Cont) {
//            Cont<Object, A> cont = (Cont<Object, A>) this;
//            TailRec<Object> head = cont.head();
//            Function<Object, TailRec<A>> fun = cont.fun();
//            if (head instanceof Done) {
//                Done<Object> done = (Done<Object>) head;
//                return fun.apply(done).result();
//            } else if (head instanceof Call) {
//                Call<Object> call = (Call<Object>) head;
//                return call.next().flatmap(fun).result();
//            } else if (head instanceof Cont) {
//                Cont<Object, Object> cont1 = (Cont<Object, Object>) head;
//                return cont1.head().flatmap(x -> cont1.fun().apply(x).flatmap(fun)).result();
//            } else {
//                throw new RuntimeException("123");
//            }
//        } else {
//            throw new RuntimeException("123");
//        }
//
//    }
    default A result() {
        TailRec<A> t = this;
        while (!(t instanceof Done)) {
            if (t instanceof Call) {
                Call<A> call = (Call<A>) t;
                t = call.next();
            } else if (t instanceof Cont) {
                Cont<Object, A> cont = (Cont<Object, A>) t;
                TailRec<Object> head = cont.head();
                Function<Object, TailRec<A>> fun = cont.fun();
                if (head instanceof Done) {
                    Done<Object> done = (Done<Object>) head;
                    t= fun.apply(done);
                } else if (head instanceof Call) {
                    Call<Object> call = (Call<Object>) head;
                    t = call.next().flatmap(fun);
//                    return call.next().flatmap(fun).result();
                } else if (head instanceof Cont) {
                    Cont<Object, Object> cont1 = (Cont<Object, Object>) head;
                    t = cont1.head().flatmap(x -> cont1.fun().apply(x).flatmap(fun));
                } else {
                    throw new RuntimeException("123");
                }
            } else {
                throw new RuntimeException("123");
            }
        }


        Done<A> done = (Done<A>) t;
        return done.val();

    }

    class Done<A> implements TailRec<A> {
        A val;

        Done(A val) {
            this.val = val;
        }

        A val() {
            return val;
        }
    }

    class Call<A> implements TailRec<A> {

        Supplier<TailRec<A>> sup;

        Call(Supplier<TailRec<A>> sup) {
            this.sup = sup;
        }

        TailRec<A> next() {
            return sup.get();
        }

    }

    class Cont<A, B> implements TailRec<B> {
        TailRec<A> a;
        Function<A, TailRec<B>> f;

        Cont(TailRec<A> a, Function<A, TailRec<B>> f) {
            this.a = a;
            this.f = f;
        }

        TailRec<A> head() {
            return a;
        }

        Function<A, TailRec<B>> fun() {
            return f;
        }
    }
}