How do I fix this error message: “name of datatype (String) is used as a function”?

module MString {
  export provides String
  datatype String = String(s: string)
module MX {
  import opened MString
  const S := String("asd")

The names visible in module MX are the datatype String but not its constructor, since the dataype is only imported as provides. Consequently, the only option for the name String in module MX is that datatype, which then causes the error message.

The fix to this is to declare the export as export reveals String. If String is meant to be opaque (only provided) then you cannot construct elements of it; if it is meant to be transparent, then you must reveal it.