MWord.java

M.java

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.function.Function;

interface Result<T> {
    <R> Result<R> then(Function<T, Result<R>> nextFn);
}

record Success<T>(T value) implements Result<T> {
    @Override
    public <R> Result<R> then(Function<T, Result<R>> nextFn) {
        return nextFn.apply(this.value);
    }
}

record Failure<T>(Throwable err) implements Result<T> {
    @Override
    @SuppressWarnings("unchecked")
    public <R> Result<R> then(Function<T, Result<R>> nextFn) {
        return (Result<R>) this;
    }
}

public class M {
    final static BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

    public static Result<Integer> readAndAdd(int prevNum) {
        try {
            System.out.print("Enter a number: ");
            return new Success<>(prevNum + Integer.parseInt(br.readLine()));
        } catch (Throwable e) {
            return new Failure<>(e);
        }
    }

    public static void main(String... args) {
        final var result = new Success<Integer>(0)
            .then(M::readAndAdd)
            .then(M::readAndAdd)
            .then(M::readAndAdd);

        System.out.println(result);
    }
}