Scala lift from List[A] to List[Option[A]]

Question

In scala, how can i create a general purpose function to perform the following operation:

f: List[A] ==> List[Option[A]]

@evan058 `Some(_)` will give you `Some(null)` when some element of `in` is `null` which I doubt is what OP wants. — 2rs2ts, Jan 06 '17 at 16:13
Yeah I guess I don't know what OP wants. @OP, what do you want? What values in your input should be `None`? Any of them? — evan.oman, Jan 06 '17 at 16:16
actually i do not want any of them to be evaluated to `None`. I want this functionality to integrate with other parts, so that the user of the library does not need to worry with the Option framework — raul ferreira, Jan 06 '17 at 16:19
I can't imagine a sane reason why you would ever want `Some(null)`... — 2rs2ts, Jan 06 '17 at 16:37

score 6 · Accepted Answer · answered Jan 06 '17 at 16:11

6

Isn't it as simple as _.map(Option.apply)?

answered Jan 06 '17 at 16:11

2rs2ts

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of course it is! ahh it's amazing how you can easily waste a day with something so much simple. thanks! – raul ferreira Jan 06 '17 at 16:15
So `Option.apply` knows that `null` should be `None`. What else would it map to `None`? – evan.oman Jan 06 '17 at 16:21
1

Ha, that is the only value: `def apply[A](x: A): Option[A] = if (x == null) None else Some(x)` ([src](https://github.com/scala/scala/blob/v2.12.1/src/library/scala/Option.scala#L25)) – evan.oman Jan 06 '17 at 16:28

score 3 · Answer 2 · answered Jan 06 '17 at 16:24

If you want all elements to be Some(...) (like you mentioned in your comment) use something like this:

scala> def f[A](in: List[A]): List[Option[A]] = in.map(Some(_))
f: [A](in: List[A])List[Option[A]]

scala> f(0 to 5 toList)
warning: there was one feature warning; re-run with -feature for details
res4: List[Option[Int]] = List(Some(0), Some(1), Some(2), Some(3), Some(4), Some(5))

scala> f(List("a", "b", null))
res5: List[Option[String]] = List(Some(a), Some(b), Some(null))

@2rs2ts's answer would give you:

scala> def f_2rs2ts[A](in: List[A]): List[Option[A]] = in.map(Option.apply)
f_2rs2ts: [A](in: List[A])List[Option[A]]

scala> f_2rs2ts(List("a", "b", null))
res6: List[Option[String]] = List(Some(a), Some(b), None)

Scala lift from List[A] to List[Option[A]]

2 Answers2