The document discusses programming practices in Java and Scala, particularly focusing on handling null values and the complications associated with them. It introduces the concept of the 'Option' monad in Scala as a solution to represent values that may or may not exist, emphasizing its advantages in type safety and usability. Various coding examples illustrate the use of Option for managing optional values, alongside comparisons to handling null in Java.

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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