Presentation on handling non-existence of data in Java et. al. (e.g. the problem with pesky nulls) and an introduction to the Option monad in Scala as a "solution" to this problem. I presented this talk June, 28th 2013 at CPH Scala Group meeting, and a week later, July 3rd, at the "Scala User Group Århus" meetup. In this short introduction, I try to frame the problem, i.e. the large amounts of error-prone null-checking code we usually have to write in Java, and Introduce the Option monad (Some/None) in Scala, as a solution. I explain the basics of what the Option class provides, and various ways of using it, ranging from basic level isEmtpy, over pattern-matching to more advanced fully functional "collection-style" (e.g. map, flatMap) operations and finally by using the for-comprehension. Also includes links to relevant resources for further reading on the last slide.

1. Some(slides)
val reasonsToUseNull = None
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2. Who am I?
Java (& Scala) Developer at Schantz A/S
Polyglot curious, Coursera junkie
Interested in HCI and Usability
https://github.com/JKrag
@jankrag
• Geek, builder and flyer of kites, reptile & cat breeder, Rubik's puzzle fan
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3. Oh we wish...
val customer = Customers.findById(1234)
customer.getAccount(FUNSTUFF).getLastInterest.getAmount
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4. Oh we wish...
val customer = Customers.findById(1234)
customer.getAccount(FUNSTUFF).getLastInterest.getAmount
NullPointers !
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5. Classic solutions (java)
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6. Classic solutions (java)
Nested if’s
if(customer != null {
! if(customer.getAccount(FUNSTUFF) != null) {
! ! if(customer.getAccount(FUNSTUFF).getLastInterest != null) {
! ! ! return customer.getAccount(FUNSTUFF).getLastInterest.getAmount
! ! }
! }
}
return null;
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7. Classic solutions (java)
Nested if’s
if(customer != null {
! if(customer.getAccount(FUNSTUFF) != null) {
! ! if(customer.getAccount(FUNSTUFF).getLastInterest != null) {
! ! ! return customer.getAccount(FUNSTUFF).getLastInterest.getAmount
! ! }
! }
}
return null; UGLY
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8. Classic solutions (java)
Nested if’s
if(customer != null {
! if(customer.getAccount(FUNSTUFF) != null) {
! ! if(customer.getAccount(FUNSTUFF).getLastInterest != null) {
! ! ! return customer.getAccount(FUNSTUFF).getLastInterest.getAmount
! ! }
! }
}
return null;
Early returns
if (customer == null) return null;
if (customer.getAccount(FUNSTUFF) == null) return null;
if (customer.getAccount(FUNSTUFF).getLastInterest == null) return null;
return customer.getAccount(FUNSTUFF).getLastInterest.getAmount
UGLY
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9. Classic solutions (java)
Nested if’s
if(customer != null {
! if(customer.getAccount(FUNSTUFF) != null) {
! ! if(customer.getAccount(FUNSTUFF).getLastInterest != null) {
! ! ! return customer.getAccount(FUNSTUFF).getLastInterest.getAmount
! ! }
! }
}
return null;
Early returns
if (customer == null) return null;
if (customer.getAccount(FUNSTUFF) == null) return null;
if (customer.getAccount(FUNSTUFF).getLastInterest == null) return null;
return customer.getAccount(FUNSTUFF).getLastInterest.getAmount
UGLY
Still UGLY!
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10. Same in Scala
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11. Same in Scala
val customer = Customers.findById(1234)
if (customer != null) {
! val account = customer.account(FUNSTUFF);
! if (account != null) {
! ! val interest = account.getLastInterest
! ! if (interest != null)
! ! ! interest.amount
! ! else
! ! ! null
! } else
! ! null
} else
! null
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12. Same in Scala
val customer = Customers.findById(1234)
if (customer != null) {
! val account = customer.account(FUNSTUFF);
! if (account != null) {
! ! val interest = account.getLastInterest
! ! if (interest != null)
! ! ! interest.amount
! ! else
! ! ! null
! } else
! ! null
} else
! null
Even in Scala,
Still UGLY!
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13. Same in Scala
val customer = Customers.findById(1234)
if (customer != null) {
! val account = customer.account(FUNSTUFF);
! if (account != null) {
! ! val interest = account.getLastInterest
! ! if (interest != null)
! ! ! interest.amount
! ! else
! ! ! null
! } else
! ! null
} else
! null
Even in Scala,
Still UGLY!
...and
errorprone
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14. non-existence
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15. non-existence
Java
null, null, null, null :-(
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16. non-existence
Java
null, null, null, null :-(
Groovy (et al.)
Safe navigation operator
def amount = customer?.account?.interest?.amount
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17. non-existence
Java
null, null, null, null :-(
Groovy (et al.)
Safe navigation operator
def amount = customer?.account?.interest?.amount
Ceylon, Kotlin etc.
both nullable and null-safe types...
String name = null; //compile error: null is not an instance of String
String? name = null; //OK
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18. non-existence
Java
null, null, null, null :-(
Groovy (et al.)
Safe navigation operator
def amount = customer?.account?.interest?.amount
Ceylon, Kotlin etc.
both nullable and null-safe types...
String name = null; //compile error: null is not an instance of String
String? name = null; //OK
Others (e.g. Clojure): ‘nil’ type - close but...!
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19. non-existence
Java
null, null, null, null :-(
Groovy (et al.)
Safe navigation operator
def amount = customer?.account?.interest?.amount
Ceylon, Kotlin etc.
both nullable and null-safe types...
String name = null; //compile error: null is not an instance of String
String? name = null; //OK
Others (e.g. Clojure): ‘nil’ type - close but...!
Scala ....
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20. non-existence
Java
null, null, null, null :-(
Groovy (et al.)
Safe navigation operator
def amount = customer?.account?.interest?.amount
Ceylon, Kotlin etc.
both nullable and null-safe types...
String name = null; //compile error: null is not an instance of String
String? name = null; //OK
Others (e.g. Clojure): ‘nil’ type - close but...!
Scala .... patience...
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21. We need something
like:
Container
Empty
container
Important: Same ‘shape’ outside
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22. Let me present:
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23. Let me present:
Option monad
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24. Let me present:
Option monadSHHH
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25. Scala’s Option type:
Some(2) None
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26. Option - concept
sealed trait Option[A]
case class Some[A](a: A) extends Option[A]
case class None[A] extends Option[A]
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27. Advantages
• Values that may or may not exist now
stated in type system
• Clearly shows possible non-existence
• Compiler forces you to deal with it
• You won’t accidentally rely on value
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28. Option - in RL
sealed abstract class Option[A] extends Product
case class Some[+A](a: A) extends Option[A]
case object None extends Option[Nothing]
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29. Option - in RL
sealed abstract class Option[A] extends Product {
def isEmpty: Boolean
def get: A
...
}
final case class Some[+A](x: A) extends Option[A] {
def isEmpty = false
def get = x
}
case object None extends Option[Nothing] {
def isEmpty = true
def get = throw new NoSuchElementException("None.get")
}
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30. WAT?
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31. Creating Options
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32. Creating Options
• Direct: !
val o = Some(3)
//> o : Option[Int] = Some(3)
val n = None
//> n : None.type = None
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33. BUT NEVER: val aaargh = Some(null)
Creating Options
• Direct: !
val o = Some(3)
//> o : Option[Int] = Some(3)
val n = None
//> n : None.type = None
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34. BUT NEVER: val aaargh = Some(null)
Creating Options
• Direct: !
val o = Some(3)
//> o : Option[Int] = Some(3)
val n = None
//> n : None.type = None
• Factory method on companion object: !
val o = Option(3)
//> o : Option[Int] = Some(3)
val nn = Option(null)
//> nn : Option[Null] = None
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35. val schroedingersBox : Option[Cat] =
! if(random.nextBoolean) then
Some(Garfield)
else
None
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36. Many mays to use
• isDefined
• isEmpty
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37. Many mays to use
• isDefined
• isEmpty
if (customer.isDefined)
! customer.account;
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38. Many mays to use
• isDefined
• isEmpty
if (customer.isDefined)
! customer.account;
Much more type-safe and null-safe
than original null-based java-flavour,
but code just as ugly
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39. get?
three.get
! //> res10: Int = 3
nope.get
! //> java.util.NoSuchElementException: None.get
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40. get?
three.get
! //> res10: Int = 3
nope.get
! //> java.util.NoSuchElementException: None.get
$> Yay. We can still write the other
ugly version with Exception
handling :-)
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41. Apprentice level:
Pattern matching
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42. Apprentice level:
Pattern matching
val foo = request.param("foo") match
{
! case Some(foo) => foo
! case None => "Default foo"
}
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43. Apprentice level:
Pattern matching
val foo = request.param("foo") match
{
! case Some(foo) => foo
! case None => "Default foo"
}
Sometimes useful, but...
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44. Apprentice level:
Pattern matching
val foo = request.param("foo") match
{
! case Some(foo) => foo
! case None => "Default foo"
}
Sometimes useful, but...
at some point a Jedi you must become
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45. What we really want is
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46. What we really want is
... to do stuff with our values
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47. What we really want is
... to do stuff with our values
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48. But...
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49. We want...?
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50. Padawan level:
functional
• Treat Option as a (very small) collection
• “Biased” towards Some
• map, flatMap etc.
• and compose to your desire when the
option contains a value
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51. map
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52. map
! val three = Some(3)
! ! ! ! > three : Option[Int] = Some(3)
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53. map
! val three = Some(3)
! ! ! ! > three : Option[Int] = Some(3)
! val res = three.map(_ + 3)
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54. map
! val three = Some(3)
! ! ! ! > three : Option[Int] = Some(3)
! val res = three.map(_ + 3)
> res: Option[Int] = Some(6)
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55. map
option.map(foo(_))
equivalent to:
option match {
case None => None
case Some(x) => Some(foo(x))
}
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56. Examples
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57. def sqr(i:Int) = {i*i}
Examples
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58. def sqr(i:Int) = {i*i}
val three = Option(3)
Examples
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59. def sqr(i:Int) = {i*i}
val three = Option(3)
three.map(i => sqr(i))
Examples
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60. def sqr(i:Int) = {i*i}
val three = Option(3)
three.map(i => sqr(i))
//> res4: Option[Int] = Some(9)
Examples
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61. def sqr(i:Int) = {i*i}
val three = Option(3)
three.map(i => sqr(i))
//> res4: Option[Int] = Some(9)
three.map(sqr(_))
Examples
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62. def sqr(i:Int) = {i*i}
val three = Option(3)
three.map(i => sqr(i))
//> res4: Option[Int] = Some(9)
three.map(sqr(_))
//> res5: Option[Int] = Some(9)
Examples
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63. def sqr(i:Int) = {i*i}
val three = Option(3)
three.map(i => sqr(i))
//> res4: Option[Int] = Some(9)
three.map(sqr(_))
//> res5: Option[Int] = Some(9)
three.map(sqr)
Examples
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64. def sqr(i:Int) = {i*i}
val three = Option(3)
three.map(i => sqr(i))
//> res4: Option[Int] = Some(9)
three.map(sqr(_))
//> res5: Option[Int] = Some(9)
three.map(sqr)
//> res6: Option[Int] = Some(9)
Examples
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65. flatMap
option.flatMap(foo(_))
is equivalent to:
option match {
case None => None
case Some(x) => foo(x)
}
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66. three.flatMap(x => Some(x.toString))
Option[java.lang.String] = Some(3)
nah.flatMap(x => Some(x.toString))
Option[java.lang.String] = None
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67. Side effects:
foreach
option.foreach(foo(_))
is equivalent to:
option match {
case None => {}
case Some(x) => foo(x)
}
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68. three.foreach(println(_))
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69. val userOpt = UserDao.findById(userId)
userOpt.foreach(user => println(user.name))
or, even shorter:
userOpt.foreach(println)
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70. Working with lists
val o1 = Option(1)! ! //> o1 : Option[Int] = Some(1)
val o2 = Option(2) //> o2 : Option[Int] = Some(2)
val o3 = Option(3) //> o3 : Option[Int] = Some(3)
val l = List(o1, nope, o2, nah, o3)
! //> l : List[Option[Int]]
= List(Some(1), None, Some(2), None, Some(3))
!
l.map(_.map(sqr))
! ! //> res8: List[Option[Int]]
= List(Some(1), None, Some(4), None, Some(9))
l.flatMap(_.map(sqr))
! ! //> res9: List[Int] = List(1, 4, 9)
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71. Jedi level:
for comprehesions
val ageOpt = for {
! user <- UserDao.findById(userId)
! age <- user.ageOpt
} yield age
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72. Jedi mind tricks
//we have a ‘User’ with mandatory name, but optional age
case class User(val name:String , val age:Option[Int])
def prettyPrint(user: User) =
! List(Option(user.name), user.age).flatten.mkString(", ")
val foo = User("Foo", Some(42))
val bar = User("Bar", None)
prettyPrint(foo) //prints "Foo, 42"
prettyPrint(bar) //prints "Bar"
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73. val userOpt =
UserDao.findById(userId) OrElse Some(UserDao.create)
or:
val user =
UserDao.findById(userId) getOrElse UserDao.create
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74. other option options
def filter(p: A => Boolean): Option[A]
def exists(p: A => Boolean): Boolean
fold
collect
iterator
toList
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75. Resources
References, Thanks, Resources and further
reading
Attributions:
Thanks to Adit Bhargava for a great blogpost on monads in Haskel and for letting me
use his cartoon drawings:
http://adit.io/posts/2013-04-17-functors,_applicatives,_and_monads_in_pictures.html
For broadening my mind on higher-order use of Options: http://blog.tmorris.net/posts/
scalaoption-cheat-sheet/
Further reading
http://marakana.com/static/courseware/scala/presentation/comprehending-monads.html
http://blog.xebia.com/2011/06/02/scala-options-the-slick-way/
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