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scalafunctional-programmingmaxscala-option

How is for comprehension returning None on comparison?

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
Solution

  • 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