How is for comprehension returning None on comparison?

Question

How is a below printed as None as the max is 3 here

val firstNum: Option[Int] = None
val secondNum: Option[Int] = Some(3)

val a = for {
  f <- firstNum
  s <- secondNum

} yield Math.max(f, s)

println(a)

output

None

Because first `<-` is actually a `flatMap` method, and it is called only if the `Option` is present (its value is `Some`). — insan-e, Feb 03 '21 at 15:51
It's because `option` obey all Monads law including left identity, meaning `None.flatMap(f)` always returns None. — Ivan Kurchenko, Feb 03 '21 at 15:51
okay but, firstNum when flattened, it is still None but the other one, secondNum after flattened, it will be 3.. and then 3 is the max .. — supernatural, Feb 03 '21 at 15:54
After firstNum is flattened and you get None, the evaluation stops there. When you attempt to call flatMap again on None, you still get None, so everything after "f <- firstNum" doesn't matter. — Allen Han, Feb 03 '21 at 17:11

score 4 · Accepted Answer · answered Feb 03 '21 at 15:55

4

As in comment section was mentioned, your are using for-comprehension construction, which under the hood invokes flatMap method, which according to left identity monad law always works like this: None.flatmap(f) == None.

If you you want to find max between two Option[Int] and ignore if any of them absent, try to:


val firstNum: Option[Int] = None
val secondNum: Option[Int] = Some(3)

println(List(firstNum, secondNum).flatten.max)

Scatie: https://scastie.scala-lang.org/UbCy36hHS3iVLKEdqzqUCw

answered Feb 03 '21 at 15:55

Ivan Kurchenko

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okay, one just doubt , so at the end in the yield Math.max(f, s), how are the values passed there, it is not as `Math.max(None, 3)` ? – supernatural Feb 03 '21 at 16:00
@supernatural Excuse me, I'm afraid I'm not following. Math.max does not accept Option, your example won't compile. – Ivan Kurchenko Feb 03 '21 at 16:02
1

@supernatural If you de-sugar `for-yield` it will be : firstNum.flatMap(f => secondNum.map(s => Math.max(f, s))) – Ivan Kurchenko Feb 03 '21 at 16:04
ok, so if s is None then it will only return f – supernatural Feb 03 '21 at 16:07
1

and if f is None, it wont go the first map itself and return None – supernatural Feb 03 '21 at 16:11

Tomer Shetah · Answer 2 · 2021-02-03T16:55:29.263

Just to add on top of the great answer by @IvanKurchenko. .max might throw an exception in case there are no elements in the List. For example:

List[Option[Int]](None).flatten.max

will throw an exception:

java.lang.UnsupportedOperationException: empty.max

Instead, you can use maxOption:

List(Some(3), None).flatten.maxOption

which will provide Some(3), and

List[Option[Int]](None).flatten.max

will provide None.

How is for comprehension returning None on comparison?

2 Answers2