public sealed interface Maybe<T> {
<T2> Maybe<T2> map(Function<T, T2> mapper);
<T2> Maybe<T2> flatMap(Function<T, Maybe<T2>> mapper);
T elseGet(T fallback);
record Just<T>(T value) implements Maybe<T> {
public <T2> Maybe<T2> map(Function<T, T2> mapper) {
return new Just<T2>(mapper.apply(value));
}
public <T2> Maybe<T2> flatMap(Function<T, Maybe<T2>> mapper) {
return mapper.apply(value);
}
public T elseGet(T fallback) {
return value;
}
}
record Empty<T>() implements Maybe<T> {
public <T2> Maybe<T2> map(Function<T, T2> mapper) {
return new Empty<>();
}
public <T2> Maybe<T2> flatMap(Function<T, Maybe<T2>> mapper) {
return new Empty<>();
}
public T elseGet(T fallback) {
return fallback;
}
}
}
public void demo() {
Maybe<String> foo = new Maybe.Empty<String>();
System.out.println(
switch (foo) {
case Maybe.Just(String val) -> "Hello " + val;
case Maybe.Empty() -> "Fine, leave me hanging";
}
);
};