How come I can't flatMap from Option to List in Scala?

Question

When I try

Some(1).flatMap(_ => List(2))

I get

 error: type mismatch;
 found   : List[Int]
 required: Option[?]

But doing Some(1).map(_ => List(2)).flatten works. How come I get a compile error in the first case?

Another short fix is to use `Some(1).flatMap(_ => Some(2)).toList`, as `Option.flatMap` must return an `Option` — Beryllium, Jun 19 '13 at 14:31

score 5 · Answer 1 · answered Jun 19 '13 at 21:35

5

If you are sure the list has a zero or one elements you can do this:

option.flatMap(_ => list.headOption)

answered Jun 19 '13 at 21:35

EECOLOR

11,034
3
38
72

score 4 · Answer 2 · answered Jun 19 '13 at 15:16

You can't do this, because the type system forbids this. The type of Option.flatMap is

     final def flatMap[B](f: (A) ⇒ Option[B]): Option[B]

So your function must return an Option type, not a List type.

If you want to convert to a different type, you have to use .map(...).flatten

score 3 · Answer 3 · answered Jun 19 '13 at 14:41

3

Not to say it doesn't work on other Scala versions, but it doesn't work for me:

scala> Some(1).map(_ => List(2)).flatten
<console>:8: error: Cannot prove that List[Int] <:< Option[B].
              Some(1).map(_ => List(2)).flatten
                                        ^

More importantly, what would be the result of Some("abc").flatMap(s => s.toList), or its equivalent Some(List('a','b','c')).flatten?

answered Jun 19 '13 at 14:41

Daniel C. Sobral

284,820
82
479
670

I get `Iterable[Int] = List(2)` using scala 2.9.2 – Garrett Hall Jun 19 '13 at 14:54

How come I can't flatMap from Option to List in Scala?

3 Answers3