How to properly implement this using Vavr?

Question

I would like got your advise on how to properly write this code on a functional way:

private Option<CalcResult> calculate(Integer X, Integer Y) {
    if (X < Y) return Option.none();
    return Option.of( X + Y );
} 

public Option<CalcResult> otherMethod(Obj o) {
    if (o.getAttr()) {
      // getA() & getB() are APIs out of my control and could return a null value
      if (o.getA() != null && o.getB() != null) {
        return calculate(o.getA(), o.getB());
      }
    } 

    return Option.none();
}

Calculate is simple:

private Option<CalcResult> calculate(Integer X, Integer Y) {
    return Option.when(X > Y, () -> X + Y);
}

For otherMethod, This was my first approach:

public Option<CalcResult> otherMethod(Obj o) {
    return Option.when(o.getAttr(), () -> 
      For(Option.of(o.getA()), Option.of(o.getB()))
        .yield(this::calculate)
        .toOption()
        .flatMap(Function.identity())
      ).flatMap(Function.identity()); 
}

But, I feel the code is not as readable as I would expect, compared to first version (double flatMap makes hard to understand, at first sight, why is there)

I tried with this other, which improves the lecture:

public Option<CalcResult> otherMethod(Obj o) {
  return For(
      Option.when(o.getAttr(), o::getAttr()),
      Option.of(o.getA()), 
      Option.of(o.getB()))
    .yield((__, x, y) -> this.calculate(x, y))
    .toOption()
    .flatMap(Function.identity()); 
}

It's way more readable, but I think i'm not correctly using For-comprehension on this case.

What would be your recommendation on this case? am I correctly using vavr's API?

Thanks

score 1 · Answer 1 · answered Jan 14 '19 at 23:00

I would write calculate exactly the way you did (except I would never use upper case for parameters :P).

As for otherMethod, I would use pattern matching. Pattern matching in Vavr is not even close to PM in more functional languages like Haskell, but I think it still correctly represents your intention:

public Option<Integer> otherMethod(Obj o) {
  return Option.when(o.getAttr(), () -> Tuple(Option(o.getA()), Option(o.getB()))) //Option<Tuple2<Option<Integer>, Option<Integer>>>
      .flatMap(ab -> Match(ab).option( // ab is a Tuple2<Option<Integer>, Option<Integer>>
          Case($Tuple2($Some($()), $Some($())), () -> calculate(o.getA(), o.getB())) // Read this as "If the pair (A, B)" has the shape of 2 non-empty options, then calculate with what's inside
          // Result of Match().option() is a Option<Option<Integer>>
      ).flatMap(identity())); // Option<Integer>
}

Alternative, without comments (note we use Match().of() instead of Match().option(), so we HAVE to deal with all possible shapes):

public Option<Integer> otherMethod(Obj o) {
  return Option.when(o.getAttr(), () -> Tuple(Option(o.getA()), Option(o.getB())))
      .flatMap(ab -> Match(ab).of(
          Case($Tuple2($Some($()), $Some($())), () -> calculate(o.getA(), o.getB())),
          Case($(), () -> None())
      ));
}

I replaced CalcResult with Integer since in your example, calculate really returns an Option<Integer>, I let you adapt to your business models.

How to properly implement this using Vavr?

1 Answers1