The document discusses type-driven development in Scala. It covers various techniques for preventing bugs using static types, including value classes, Option types, dependent types, the IO monad, and validation. Advanced concepts from libraries like Scalaz and Shapeless are also mentioned, such as path dependent types, sized types, and using numbers as types. While static types add complexity, the document argues they can help catch bugs earlier by allowing compilers to validate code correctness.

1. Type driven development
Maciek Próchniak

2. Whoami?

3. Maciek Próchniak
Algebraic topology
hocolim
Group cohomology
Monads
GWT
TouK
Camel
OSGi
CQRS
Scala

4. How to prevent bugs?
● Tests
● Defensive programming
● Tests
● Contracts
● Tests

5. Guerilla defensive programming

6. How to prevent bugs?
In the end, having static types is all about
preventing bugs
by allowing the über-smart compiler
to humiliate you on a regular basis,
making sure you’re doing the right thing
before it’s too late.
(http://danielwestheide.com)

7. What bugs?
“Only 1% of issues in Python system are
TypeErrors, AttributeErrors, NameErrors”

8. What about
● NullPointerException
● (uncaught) IOException
● ArrayOutOfBoundsException
● ItemNotFoundException

9. Typesafe stronghold
... strings and nulls shall not overcome
http://en.wikipedia.org/wiki/File:Cit%C3%A9_de_Carcassonne,
_woman_on_wall.jpg

10. Security layers
main/servlet
parsing
validation
business “logic”
collections, utils
io

11. Scalaz
If you are thinking about using
Scalaz, stop now while you still have
your sanity!
And don’t get into arguments with the
people who suggest that you use it – it is
pointless.

12. Equal?
long userId = 14
User user = getUser(...)
userId.equals(user)

13. Equal?
val userId : Long = 14
val user : User = getUser(...)
import scalaz.syntax.equal._
userId === user

14. Names are important
BigDecimal balance;
BigDecimal price;
balance.compareTo(price) > 0

15. Names are important
double log(double a)
double log(positiveDouble)
double log(double positive)
double log(PositiveDouble a)

16. Tagged types
@Nullable
@Email
@Valid
@Notempty
private String mail

17. Oh, really?
@PUT
@Path("/fulfilment/address")
@ApiOperation(value = "Set shipping method for cart", response =
CartDTO.class)
@ApiResponses(value = {
@ApiResponse(code = 400, message = "Method cannot be used due to
bussines rules"),
@ApiResponse(code = 404, message = "Unknown fulfilment option"),
@ApiResponse(response = CartDTO.class, code = 200, message =
"Customer cart after selecting fulfilment")})
public CartDTO setFulfillmentAddress(
@ApiParam(value = "New shipping method", required = true) @Valid
AddressDTO addressDTO
) throws PricingException, FulfillmentPriceException {

18. Tagged types
Double @@ Meter
sealed trait Meter
sealed trait Liter

19. Tagged types
val length = Tag[Double,Meter](10)
val capacity = Tag[Double,Liter](10)
length : Double @@ Meter
capacity : Double @@ Liter
length + 15 : Double

20. Value classes

21. Java??
public class LengthInMeters(int value) {
private final int length;
public LengthInMeters(int value) {
this.length = value;
}
public int getValue() {
return length;
}
@Override
public int hashCode() {
return 31 * length;;
}
@Override public boolean equals(Object o) {
if (o == this)
return true;
if (!(o instanceof LengthInMeters))
return false;
LengthInMeters c = (LengthInMeters) o;
return Integer.compare(c.length, length) == 0;
}
}

22. Value classes
case class CustomerId(id:String)
extends AnyVal {
def load:Customer
}
def loadFriends
(cid:CustomerId, bid:BranchId)
val cid:CustomerId = parseId(cidString)

23. Value classes
implicit class CustomerId(
val id:String) extends AnyVal {
def load:Customer
}
“12345”.load : Customer

24. What about
● NullPointerException
● (uncaught) IOException
● ArrayOutOfBoundsException
● ItemNotFoundException

25. Option[A]
val maybeResult : Option[Result]
= Option(unsafeResult)
maybeResult.map(_.name):Option[String]
maybeResult
.getOrElse(defaultResult) : Result
maybeResult.get

26. Option[String]
val str = Option(“str”)
val none = Option(null)
val none2 = Option.empty[String]
val fake = Some(null)

27. Syntax sugar IS important
val maybe = Option.empty[String]
maybe.map(_.toUpperCase)

28. Syntax sugar IS important
Optional<String> maybeName =
Optional.absent();
maybeName.transform(
new Function<String, Object>() {
@Override
public Object apply(String s) {
return s.toUpperCase();
}
});

29. Syntax sugar IS important
String maybe = null;
if (maybe != null) {
maybe = maybe.toUpperCase();
}

30. What about
● NullPointerException
● (uncaught) IOException
● ArrayOutOfBoundsException
● ItemNotFoundException

31. IO...
https://www.flickr.com/photos/oddsock/100761143/

32. Why not exceptions?
● They are invisible in the source code.
● They create too many possible exit points
A better alternative is to have your functions
return error values (...), no matter how verbose
it might be
Joel Spolsky

33. IO[Result]
val fileNameIO : IO[String] =
IO { System.getProperty("file.config") }
val config : IO[String] = for {
fileName <- fileNameIO
fileReader = new BufferedReader(
new FileReader(fileName))
line <-safeReadLn(fileReader)
} yield line
val data : String = config.unsafePerformIO

34. Security layers
main/servlet
parsing
validation
business “logic”
collections, utils
io

35. Mixing stuff
IO[State[Map, String]]
State[Map, IO[String,]]

36. Mixing stuff
https://www.flickr.com/photos/oddsock/100761143/

37. What about
● NullPointerException
● (uncaught) IOException
● ArrayOutOfBoundsException
● ItemNotFoundException

38. Empty list?
val list : List[String]
= Nil
list.head
import scalaz.NonEmptyList
val nel : NonEmptyList[String]
= NonEmptyList(“a”,List(“b”))
nel.head

39. Dependent types?
val list : List[String]
val listOfSize5 : List[String,2+3]
def sum[A,L1:Nat,L2:Nat]
(l1 : List[A,L1], l2:List[A,L2])
: List[A,L1+L2]

40. Is it a dream?

41. Path dependent types
class Family {
case class Child(name : String)
def quarell(child1 : Child,
child2 : Child) {
//blaaah
}
}

42. Path dependent types
val kowalscy = new Family()
val nowakowie = new Family()
val pawelK = kowalscy.Child("Pawel")
val pawelN = nowakowie.Child("Pawel")
kowalscy.quarell(pawelK, pawelN)

43. Path dependent types
def hide(family:Family)
(child:family.Child) {
}

44. Shapeless
"maybe it's bug in compiler, but we'll use it and
assume it won't be fixed"
https://github.com/milessabin/shapeless
shapeless is an exploration of generic
(aka polytypic) programming in Scala

45. Sized[Seq[A],N]
trait Sized[T, N] {
???
}
N = ???

46. Part I - numbers as types
trait Nat
class _0 extends Nat
case class Succ[P<:Nat]() extends Nat
val _2 : Succ[Succ[_0]] =
Succ(Succ(_0))

47. Sized[Seq[A],N]
type SSeq[A,M<:Nat] = Sized[Seq[A],M]
Sized("maciek","prochniak")
: SSeq[String,_2]
def sizedZip[A,B,M<:Nat]
(l1:SSeq[A,M],l2:SSeq[B,M])
: SSeq[(A,B),M] = ???

48. Part II - witness
def myFun[N<:Nat](a:SIter[String,N])
(implicit b:All[N]) {}
trait All[N<:Nat] {}
implicit object AllZero
extends All[_0]
implicit def allSucc[N<:Nat]
(implicit a:All[N])
= new All[Succ[N]] {}

49. Part II - witness
trait Even[K<:Nat] {}
implicit object Even0 extends Even[0_]
implicit def succ[K<:Nat]
(implicit n:Even[K])
:Even[Succ[Succ[K]] =
new Even[Succ[Succ[K]]{}

50. Part II - witness
def evenFun[N<:Nat]
(a:Sized[Iterable[String],N])
(implicit n:Even[N]) {}
evenFun(Sized[Seq](“a”,”b”))
//evenFun(Sized[Seq](“a”,”b”,”c))

51. Ultimate challenge...
isSum(_8 :: _4 :: HNil, _12)
//isSum(_8 :: _4 :: HNil, _10)
trait Summed[H <:HList, S<:Nat]
def isSum[L<:HList,S<:Nat](l:L,s:S)
(implicit sumProof: Summed[L,S]) {}

52. Ultimate code...
implicit object NilSummed
extends Summed[HNil, _0]
implicit def pSum
[H<:Nat,T<:HList,PS<:Nat,S<:Nat]
(implicit partial: Summed[T, PS],
sum: Sum.Aux[H, PS, S])
= new Summed[H :: T, S] {}

53. Security layers
main/servlet
parsing
validation
business “logic”
collections, utils
io

54. What about
● NullPointerException
● (uncaught) IOException
● ArrayOutOfBoundsException
● ItemNotFoundException
● (uncaught) ConstraintViolationException

55. Diagram chasing
https://www.flickr.
com/photos/intvgene/370973576/in/photolist-yMkw9-
7DzC2S-cFnmdQ-4zTfBU-4wuofP-5jGxP9-2auo-

56. Types as documentation
https://www.flickr.
com/photos/videolux/2389320345/in/photolist-4D8TGn-
41Hk4j-f5j58F-58cFd2-6jtGaU-8M9Rct-dKjTu-bFuM9p-

57. Are we there yet?

58. Do you do all this?
https://www.flickr.
com/photos/drgaz/277221424/in/photolist-quQgJ-
5KKwCZ

59. Do you do all this?
● IDE support
● library maturity
● verbosity
● is it worth it?

60. Generic vs concrete code
https://www.flickr.com/photos/oddsock/100761143/

61. Generic vs concrete code
Object
Collection
Controller
Domain
https://www.flickr.com/photos/oddsock/100761143/

62. Is your business logic logical?
k4j-f5j58F-58cFd2-6jtGaU-8M9Rct-dKjTu-bFuM9p-
288vk7-6MDA6U-9rpM2p-7uFTQX-by46VA-jtP7tW-
d1yszw-4BxoMU-4Bt4Q8-4Bt8Eg-9qdvDe-9uQehi-

63. Value classes
Option
Dependent types
IOMonad
Validation
Scala Scalaz
Shapeless

64. Dzięki
http://mproch.blogspot.com
mpr@touk.pl
http://github.com/mproch