463 // Option type

   464 //=============

   465 

   466 //in Java if something unusually happens, you return null or something

   467 //

   468 //in Scala you use Options instead

   469 //   - if the value is present, you use Some(value)

   470 //   - if no value is present, you use None

   471 

   472 

   473 List(7,2,3,4,5,6).find(_ < 4)

   474 List(5,6,7,8,9).find(_ < 4)

   475 

   476 

   477 

   478 // error handling with Options (no exceptions)

   479 //

   480 //  Try(something).getOrElse(what_to_do_in_case_of_an_exception)

   481 //

   482 import scala.util._

   483 import io.Source

   484 

   485 val my_url = "https://nms.kcl.ac.uk/christian.urban/"

   486 

   487 Source.fromURL(my_url).mkString

   488 

   489 Try(Source.fromURL(my_url).mkString).getOrElse("")

   490 

   491 Try(Some(Source.fromURL(my_url).mkString)).getOrElse(None)

   492 

   493 

   494 // the same for files

   495 Try(Some(Source.fromFile("text.txt").mkString)).getOrElse(None)

   496 

   497 

   498 

   499 // how to implement a function for reading something from files...

   500 

   501 def get_contents(name: String) : List[String] = 

   502   Source.fromFile(name).getLines.toList

   503 

   504 get_contents("test.txt")

   505 

   506 // slightly better - return Nil

   507 def get_contents(name: String) : List[String] = 

   508   Try(Source.fromFile(name).getLines.toList).getOrElse(List())

   509 

   510 get_contents("text.txt")

   511 

   512 // much better - you record in the type that things can go wrong 

   513 def get_contents(name: String) : Option[List[String]] = 

   514   Try(Some(Source.fromFile(name).getLines.toList)).getOrElse(None)

   515 

   516 get_contents("text.txt")

   517 get_contents("test.txt")

   518