The Scala Option class and how lift uses it - CircleShare Blog - The Diigo Meta page

• 22 Jul 10
  

  Esfand S

  scala java option null list top comprehension

  • If you come from an imperative (Java, Ruby) background, you'll probably recognize the following code:

      

    x = someOperation
    if !x.nil?
      y = someOtherOperation
      if !y.nil?
        doSomething(x,y)
        return "it worked"
      end
    end
    return "it failed"
  • In Java, null is a non-object. It has no methods. It is the exception to the statically typed rule (null has no class, but any reference of any class can be set to null.) Invoking a method on null has one and only one result: an exception is thrown. null is often returned from methods as a flag indicating that the method ran successfully, but yielded no meaningful value. For example, CardHolder.findByCreditCardNumber("2222222222")

  • Scala does something different.

    There's an abstract class, called Option. Options are strongly typed. They are declared Option[T]. This means an Option can be of any type, but once its type is defined, it does not change. There are two subclasses of Option: Some and None. None is a singleton (like nil). Some is a container around the actual answer.

  • So, you might have a method that looks like:

     

    def findUser(name: String): Option[User] = {
      val query = buildQuery(name)
      val resultSet = performQuery(query)
      val retVal = if (resultSet.next) Some(createUser(resultSet))
                   else None
      resultSet.close
      retVal
    }

    Some, you've got a findUser method that returns either Some(User) or None. So far, it doesn't look a lot different than our example above. So, to confuse everyone, I'm going to talk about collections for a minute.

  • rich list operations. Rather than creating a counter and pulling list (array) elements out one by one, you write a little function and pass that function to the list. The list calls the function with each element and returns a new list with the values returned from each call. It's easier to see it in code:

    scala> List(1,2,3).map(x => x * 2)
    line0: scala.List[scala.Int] = List(2,4,6)
  • Option implements map, flatMap, and filter (the methods necessary for the Scala compiler to use in the 'for' comprehension.
  • when I first encountered the phrase "'for' comprehension", I got scared. I've been doing programming for years and never heard of a "comprenhension" let alone a 'for' one. Turns out, that there's nothing fancy going on, but "'for' comprehension" is just a term of art for the above construct.

  5 more annotations...

• 21 Jun 10
  

  realbot

  idioms scala

• 31 May 10
  

  Christian Winkler

  scala option

• 01 Mar 10
  

  Byju Veedu

  Scala

• 16 Sep 09
  

  df nord

  scala option

• 28 Jun 09
  

  scala null option

• 09 Jan 09
  

  timwee

  functional scala monads lift

• 22 Nov 07
  

  Foo Foo

  scala option null nil none

• 17 May 07
  

  Ken Wei

  lift programming-language scala

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