Question

While trying to play with Options in scala I have come across this peculiar problem.

I started off with creating a List[Option[Int]] as follows:

scala> List(Some(1),Some(2),None,Some(3))
res0: List[Option[Int]] = List(Some(1), Some(2), None, Some(3))

Then I tried to map an addition to 1 over the entries of the list in res0 as follows:

scala> res0 map (_ + 1)

This gave me the error:

<console>:9: error: type mismatch;
 found   : Int(1)
 required: String
              res0 map (_ + 1)
                            ^

Then I tried flatMapping an addition over the entries as follows:

scala> res0 flatMap (_ + 1)

This gave me the same error:

<console>:9: error: type mismatch;
 found   : Int(1)
 required: String
              res0 flatMap (_ + 1)
                                ^

But something like res0.flatMap(r => r) works just fine with a result of:

res9: List[Int] = List(1, 2, 3)

Can anybody tell me why adding the entry to 1 would fail for both map and flatMap?

Answer 1

The first two things you tried failed because you are trying to add an Option to an Int, and that’s not possible.

The weird error message happens because Scala assumes, since Option doesn’t have a + method, that you are trying String concatenation, but you’d have to either add an Option to a String, or a String to an Option, and you are doing neither, hence the error message.

In the last case, you are not trying to add anything, you are simply returning Option as is, hence no error message.