for-comprehension: type mismatch - required Option[?]

Question

I am relatively new to using Scala in a project and stumbled over a problem I just don't understand.

This is my code. I annotated types for clarity:

val fcs = for {
      fc: Call <- funCalls
      ann: List[CapabilityArg] <- context.annotationOption(Annotations.CapabilityArguments, fc)
      arg: CapabilityArg <- ann
} yield CapabilityInfo(
      "CapabilityArgument", arg.id.name, new LSPRange, new LSPRange
)

which errors with:

type mismatch;
 found   : List[LSPServer.this.CapabilityInfo]
 required: Option[?]

If if remove the last line in the for-comprehension, the error disappears. Shouldn't that line be okay, since I only map over a List[CapabilityArg]?

Thank you in advance!

Answer 1

The problem is that you are mixing List and Option in the for. The result collection type of a for is the collection type of the first line, which I assume is Option. The last line in the for will generate multiple values but Option can only hold one value.

This is probably what you want:

val fcs = for {
  fc: Call <- funCalls
  ann: List[CapabilityArg] <- context.annotationOption(Annotations.CapabilityArguments, fc)
} yield {
  ann.map(arg => CapabilityInfo(
    "CapabilityArgument", arg.id.name, new LSPRange, new LSPRange
  ))
}

This will return Option[List[CapabilityInfo]]