1. #JavalandJavaslang @koenighotze
java
slang
The Monad strikes back

2. #JavalandJavaslang @koenighotze
David Schmitz - koenighotze
Senacor Technologies
Principal Architect
Programmer!
That’s me coding Scala

3. #JavalandJavaslang @koenighotze
What’s in it for you?
Functional programming is hip
How Javaslang helped us
Live Code - OMG!
Because
of
Beamer

4. #JavalandJavaslang @koenighotze
Functors,
applicatives, monads
stay home

5. #JavalandJavaslang @koenighotze
Code that is easier
to reason about

6. #JavalandJavaslang @koenighotze
Side-effects are evil
try {
int i = 1/0;
} catch (Throwable t) {
…
}
Exceptions are goto-statements
:(

7. #JavalandJavaslang @koenighotze
Referential Transparency
Math.random();
Math.max(1, 2);
Math.random();
Math.max(1, 2);
Pure functions are a Good ThingTm
:(

8. #JavalandJavaslang @koenighotze
Thinking in values
Immutability
Performance
Safety

9. #JavalandJavaslang @koenighotze
In a nutshell, think about
“what to code“
not
“how to code”

10. #JavalandJavaslang @koenighotze
Enter java Eight
(a) -> a + 2
list.stream().filter…
Excitement…

11. #JavalandJavaslang @koenighotze
Try filtering all
invalid users from
a list of users

12. #JavalandJavaslang @koenighotze
Excitement?…until
users.stream()
.filter(user -> {
try {
return user.validate();
} catch (Exception ex) {
return false;
}})
.collect(Collectors.toList());

13. #JavalandJavaslang @koenighotze
import static javaslang.API.*;

14. #JavalandJavaslang @koenighotze
What we’ll cover
Immutable collections
Some functional sugar
Pattern matching
Property based testing
Circuit breaker

15. #JavalandJavaslang @koenighotze
Before we get to the details…
Let’s fix that ugly code

16. #JavalandJavaslang @koenighotze
Fixing Things….
users.stream()
.filter(user -> {
try {
return user.validate();
} catch (Exception ex) {
return false;
}})
.collect(Collectors.toList());

17. #JavalandJavaslang @koenighotze
Functional collections
List.ofAll(users)
.filter(user -> {
try {
return user.validate();
} catch (Exception ex) {
return false;
}})
.collect(Collectors.toList());

18. #JavalandJavaslang @koenighotze
No need for Collectors
List.ofAll(users)
.filter(user -> {
try {
return user.validate();
} catch (Exception ex) {
return false;
}})
.collect(Collectors.toList());

19. #JavalandJavaslang @koenighotze
No need for Collectors
List.ofAll(users)
.filter(user -> {
try {
return user.validate();
} catch (Exception ex) {
return false;
}}); .collect(Collectors.toLis
t());

20. #JavalandJavaslang @koenighotze
Wrapping Exceptions
List.ofAll(users)
.filter(user -> {
try {
return user.validate();
} catch (Exception ex) {
return false;
}}); .collect(Collectors.toLis
t());

21. #JavalandJavaslang @koenighotze
Wrapping Exceptions
List.ofAll(users)
.filter(user ->
Try.of(user::validateAddress)
.getOrElse(false)
); } catch (IllegalStateException
ex) { return
); .collect(Collectors.toList();

22. #JavalandJavaslang @koenighotze
List.ofAll(users)
List.filter(user ->
Try.of(user::validateAddress)
.getOrElse(false));

23. #JavalandJavaslang @koenighotze
immutable Collections

24. #JavalandJavaslang @koenighotze
Mutable Collections are Evil
Returning void == Side-Effect!
interface Collection<E> {
…
void clear();
}
interface Collection<E> {
…
void clear();
}

25. #JavalandJavaslang @koenighotze
java.util.Collections
Collections
.unmodifiableList(list);
.add(“💥”)

26. #JavalandJavaslang @koenighotze
java.util.Collections
Collections
.unmodifiableList(list)
.add(“💥”);

27. #JavalandJavaslang @koenighotzePainting by Gustav Courbet
java.lang.UnsupportedOperationException at
java.util.Collections$UnmodifiableCollection
.add(Collections.java:1055)
at java.util.stream.AbstractPipeline.<init>
(AbstractPipeline.java:203)
at java.util.stream.ReferencePipeline.<init>
(ReferencePipeline.java:94)

28. #JavalandJavaslang @koenighotze
Javaslang Collections
https://cdn.infoq.com/statics_s2_20170314-0434/resource/news/2016/11/the-road-to-javaslang-3/en/resources/1infoq-version2.0-library.png

29. #JavalandJavaslang @koenighotze
Functional data structures
Immutable
Referentially transparent
Persistent

30. #JavalandJavaslang @koenighotze
heroes = List.of("Han", "Luke")
Cons@661 “Han” T
Cons@666 “Luke” ()
heroes
Effectively immutable

31. #JavalandJavaslang @koenighotze
Cons@661 “Han” T
Cons@666 “Luke” ()
heroes
Cons@662 “Ben” Tmore
more = heroes.prepend("Ben")
Effectively immutable

32. #JavalandJavaslang @koenighotze
droids =
TreeSet.of(
"C3PO",
"R2D2",
"K2SO"
)
Node@676
Node@679 Node@681“K2SO”
() ()“C3PO”
() ()“R2D2”
Effectively immutable

33. #JavalandJavaslang @koenighotze
droids.add("Chopper");
Effectively immutable

34. #JavalandJavaslang @koenighotze
Node@689
Node@691
() Node@694
() ()“Chopper”
Node@676
Node@679 Node@681“K2SO”
() ()“C3PO”
() ()“R2D2”
Effectively immutable

35. #JavalandJavaslang @koenighotze
old == new
Checking for updates?

36. #JavalandJavaslang @koenighotze
Streams are glorified iterators
Stream<String> jdk
= Stream.of("a", “B");
jdk.map(String::toUpperCase);
jdk.map(String::toLowerCase);

37. #JavalandJavaslang @koenighotzePainting by Gustav Courbet
java.lang.IllegalStateException:
stream has already been operated
upon or closed
at java.util.stream.AbstractPipeline.<init>
(AbstractPipeline.java:203)
at java.util.stream.ReferencePipeline.<init>
(ReferencePipeline.java:94)

38. #JavalandJavaslang @koenighotze
Stream<String> slang =
Stream.of("a", "B");
slang.map(String::toUpperCase);
// “A”,“B”
slang.map(String::toLowerCase);
// “a”,“b”
Javaslang streams

39. #JavalandJavaslang @koenighotze
Javaslang Streams
Stream<String> slang =
Stream.of("a", "B");
slang.map(String::toUpperCase)
.take(2);
slang.map(String::toLowerCase)
.take(2);

40. #JavalandJavaslang @koenighotze
Javaslang Streams
Stream<String> slang =
Stream.of("a", "B");
slang.map(String::toUpperCase)
.take(2);// “A”,“B”
slang.map(String::toLowerCase)
.take(2);// “a”,“b”

41. #JavalandJavaslang @koenighotze
Better performance
Less unexpected behaviour

42. #Devoxx #Javaslang @koenighotze
functional Sugar

43. #JavalandJavaslang @koenighotze
find a user and, if
an address is
available, fetch
the user’s street

44. #JavalandJavaslang @koenighotze
Cascading Pile of Shame
User user = repo.findOne("id");
if (user != null) {
Address address = user.getAddress();
if (null != address) {
return address.getStreet();
}
}

45. #JavalandJavaslang @koenighotze
optional?
Optional<User> opt =
Optional.ofNullable(user);
if (optional.isPresent()) {
User user = optional.get();
}

46. #JavalandJavaslang @koenighotze
And now, young
coder...
you will die.

47. #JavalandJavaslang @koenighotze

48. #JavalandJavaslang @koenighotze
map or flatmap
There is no
isPresent()

49. #JavalandJavaslang @koenighotze
optional or option?
Optional

50. #JavalandJavaslang @koenighotze
optional or option?
Option
Some None

51. #JavalandJavaslang @koenighotze
optional or option?
Option
Some None
Value
Iterable

52. #JavalandJavaslang @koenighotze
optional or option?
Option
Some None
Value
Iterable

53. #JavalandJavaslang @koenighotze
optional or option?
Option
Some None
Value
Iterable
Serializable

54. #JavalandJavaslang @koenighotze
An option is like a Gift Box
NoneSome

55. #JavalandJavaslang @koenighotze
Map opens the Gift Box
Map
Some Some
( )->

56. #JavalandJavaslang @koenighotze
nothing from nothing
( )->
Map
None None

57. #JavalandJavaslang @koenighotze
Fixing the Pile of Shame
User user = repo.findOne("id");
if (user != null) {
Address address = user.getAddress();
if (null != address) {
return address.getStreet();
}
}

58. #JavalandJavaslang @koenighotze
option all the Things
User user = repo.findOne("id");
if (user != null) {
Address address = user.getAddress();
if (null != address) {
return address.getStreet();
}
}

59. #JavalandJavaslang @koenighotze
option all the Things
Option<User> user = repo.findOne("id");
if (user != null) {
Address address = user.getAddress();
if (null != address) {
return address.getStreet();
}
}

60. #JavalandJavaslang @koenighotze
option all the Things
Option<User> user = repo.findOne("id");
if (user != null) {
Address address = user.getAddress();
if (null != address) {
return address.getStreet();
}
}

61. #JavalandJavaslang @koenighotze
option all the Things
Option<User> user = repo.findOne("id");
user.flatMap(User::getAddress)
Address address = user.getAddress();
if (null != address) {
return address.getStreet();
}
}
Option<User> user = repo.findOne("id");
user.flatMap(User::getAddress)
Address address = user.getAddress();
if (null != address) {
return address.getStreet();
}
} Option<Address> getAddress()

62. #JavalandJavaslang @koenighotze
option all the Things
Option<User> user = repo.findOne("id");
user.flatMap(User::getAddress)
Address address = user.getAddress();
if (null != address) {
return address.getStreet();
}
}

63. #JavalandJavaslang @koenighotze
option all the Things
Option<User> user = repo.findOne("id");
user.flatMap(User::getAddress)
Address address = user.getAddress();
.map(Address::getStreet)
return address.getStreet();
}
}

64. #JavalandJavaslang @koenighotze
repo.findOne("id")
.flatMap(User::getAddress)
.map(Address::getStreet)
.getOrElse("");

65. #JavalandJavaslang @koenighotze
option.of(value)
.map(nested code)
== understandable story

66. #JavalandJavaslang @koenighotze
working with legacy code

67. #JavalandJavaslang @koenighotze
Nice validation Code
public static String check(String iban){
if (validationMagic(iban)) {
return iban;
}
throw new IllegalArgumentException(“Peng”);
}

68. #JavalandJavaslang @koenighotze
Awesome WTF Code
String iban;
try {
iban = check(“AL47…”);
}
catch (IllegalArgumentException ex) {
iban = "";
}

69. #JavalandJavaslang @koenighotze
From Exceptions to options
String iban = lift(Iban::check)
.apply("AL47...")
.getOrElse("");

70. #JavalandJavaslang @koenighotze
From Exceptions to options
String iban = lift(Iban::check)
.apply("AL47...")
.getOrElse("");

71. #JavalandJavaslang @koenighotze
Wrapping Exceptions with Try
Try.of(() -> stuffToDo())

72. #JavalandJavaslang @koenighotze
Wrapping Exceptions with Try
Failure
Exception
Success
Result
Try.of(() -> stuffToDo())

73. #JavalandJavaslang @koenighotze
Exceptions to options with Try
Try.of(() -> check("AL.."))
.getOrElse("")

74. #JavalandJavaslang @koenighotze
Exceptions to options with Try
Try.of(() -> check("AL.."))
.getOrElse("")

75. #JavalandJavaslang @koenighotze
Lifting and Try-ing reduce
exception handling
clutter and side-effects

76. #JavalandJavaslang @koenighotze
Structural Decomposition

77. #JavalandJavaslang @koenighotze
(?:[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:.[a-z0-9!#$%&'*+/=?
^_`{|}~-]+)*|"(?:[x01-x08x0bx0cx0e-x1fx21x23-
x5bx5d-x7f]|[x01-x09x0bx0cx0e-x7f])*")@(?:(?:
[a-z0-9](?:[a-z0-9-]*[a-z0-9])?.)+[a-z0-9](?:[a-
z0-9-]*[a-z0-9])?|[(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9]
[0-9]?).){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?|[a-
z0-9-]*[a-z0-9]:(?:[x01-x08x0bx0cx0e-x1fx21-
x5ax53-x7f]|[x01-x09x0bx0cx0e-x7f])+)])
Pattern Matching Basics

78. #JavalandJavaslang @koenighotze
Pattern Matching Basics
Match(expression)
.of(cases)

79. #JavalandJavaslang @koenighotze
Cases map patterns to function
Case(pattern, function)

80. #JavalandJavaslang @koenighotze
Example Patterns
$() wildcard pattern
$(“foo”) equals pattern
isIn(“a”, “b”) conditional pattern

81. #JavalandJavaslang @koenighotze
Classic HTTP Handling
if (OK.equals(res.getStatusCode()))
{
return res.getBody();
}
return emptyList();

82. #JavalandJavaslang @koenighotze
HTTP Handling Fixed
*Note: some type details missing
Match(res.getStatusCode())
.of(
Case($(OK), res.getBody()),
Case($(), emptyList())
);

83. #JavalandJavaslang @koenighotze
Match(res.getStatusCode())
.of(
Case($(OK), res.getBody()),
Case($(), emptyList())
);
HTTP Handling Fixed
*Note: some type details missing
OK or anything else

84. #JavalandJavaslang @koenighotze
HTTP Handling Fixed
*Note: some type details missing
Match(res.getStatusCode())
.of(
Case($(OK), res.getBody()),
Case($(), emptyList())
);

85. #JavalandJavaslang @koenighotze
HTTP Handling Fixed
*Note: some type details missing
Match(res.getStatusCode())
.of(
Case($(OK), res.getBody()),
Case($(), emptyList())
);

86. #JavalandJavaslang @koenighotze
HTTP Handling Fixed
Match(res.getStatusCode())
.of(
Case($(OK), res.getBody()),
Case($(), emptyList())
);
*Note: some type details missing

87. #JavalandJavaslang @koenighotze
Matching a Try
public Try<…> fetchFromUrl(…) {
…
}

88. #JavalandJavaslang @koenighotze
Matching a Try
Match(fetchFromUrl(“…”))
.of(
Case(Success($()), identity()),
Case(Failure($()), emptyList())
);

89. #JavalandJavaslang @koenighotze
Matching a Try
Match(fetchFromUrl(“…”))
.of(
Case(Success($()), identity()),
Case(Failure($()), emptyList())
);

90. #JavalandJavaslang @koenighotze
Matching a Try
Match(fetchFromUrl(“…”))
.of(
Case(Success($()), identity()),
Case(Failure($()), emptyList())
);

91.