Short introduction to Javaslang's features and API power

1. JΛVΛSLΛNGFunctional Sugar For Java

2. slang (noun ˈslaŋ)
1. language peculiar to a
particular group
2. a non-standard vocabulary
composed of words and senses
characterized primarily by
connotations of extreme
informality

3. Functional Progamming?
Side-Effects
Referential Transparency
Thinking in Values

4. Side-Effects
Exceptions are non-local goto-statements
try {
int i = 1/0;
} catch (Throwable t) {
…
} :(

5. Referential Transparency
Math.random();
Math.max(1, 2);
Pure functions are a Good ThingTm
Side-effect :(

6. Thinking in Values
Because Rich Hickey is always right

7. A taste of JΛVΛSLΛNG!
Immutable Collections
The Try Monad
Structural Decomposition

8. Immutable Collections
Image by Douglas Muth, https://flic.kr/p/acFwxG

9. Mutable Collections
Returning void is a code smell!
interface Collection<E> {
…
void clear();
} Side-effect :(

10. Functional Data Structures
Immutable
Persistent
Referential transparent
(and fully integrated with java.util.*)

11. List<~> t = List.of(withName(“F95”));
List<~> t2 = t.append(withName(“FCK”));
List
F95 /t

12. List<~> t = List.of(withName(“F95”));
List<~> t2 = t.append(withName(“FCK”));
List
F95 /
FCK
t
t2

13. List<~> t = List.of(withName(“F95”));
List<~> t2 = t.append(withName(“FCK”));
List
F95 /
FCK
t
t2

14. The Try Monad

15. Classic Exception Handling
try {
URL url = new URL(“http://…”);
…
}
catch (MalformedURLException e)
{ … }
catch (IOException e) { … }

16. Exception Handling as a Monad
Try.of(() -> new URL(“http://…”))
.flatMap(
u->Try.of(u::openConnection)
)
.flatMap(
c->Try.of(c::getInputStream)
)…

17. Recovering from failure
Try.of(() -> new URL(“http://…”))
…
.recover(
t-> singletonList(t.getMessage())
)
.getOrElse(emptyList());

18. Structural Decomposition
Image by Arend, https://flic.kr/p/pkBe4g

19. Handling a HTTP response the classic way
if (OK.equals(res.getStatusCode()))
{
return exchange.getBody();
}
else {
return emptyList();
}

20. …now the JΛVΛSLΛNG way
Match(res.getStatusCode())
.of(
Case($OK),
res.getBody()),
Case(API.<HttpStatus>$(),
emptyList())
);

21. Guard Clauses
Match(Option.of("bla")).of(
Case(Some(
$(v -> v.length()>4)),
identity()),
Case(Some($()), "other")
)

22. An example from the trenches
Image by Shawn Carpenter, https://flic.kr/p/7FMFjC

23. A taste, classic stuff
public void reset(IModel<?> model) {
MyLadenModel<?, ?> myLadenModel;
if (model instanceof IResettableWrapModel) {
IResettableWrapModel<?> res = (IResettableWrapModel<?>) model;
myLadenModel = (AbstractKontenLadenModel<?, ?>)
res.getWrappedModel();
} else if (model instanceof AbstractLadenModel) {
myLadenModel = (AbstractLadenModel<?, ?>) model;
} else {
throw new IllegalArgumentException(“Peng!”);
}
kontenLadenModel.reset();
}

24. JΛVΛSLΛNG to the rescue
public void reset(IModel<?> model) {
MyLadenModel<?, ?> myLadenModel =
Match(model).of(
Case($(instanceOf(IResettableWrapModel.class)),
res->res.getWrappedModel()),
Case($(instanceOf(AbstractLadenModel.class)),
identity())
);
myLadenModel.reset();
}

25. We can’t cover everything
Collections Seq, Set, Tree…
Functional Sugar Lift, Memoize, Currying
Values Option, Lazy, Future
Extensions Circuit Breaker, Property Test

26. But wait…what are the drawbacks?
Java’s baroque type-system
Functional Programming…use Clojure
#Collection-lib > #Logging Impl

27. Do you want to know more?
Javaslang Homepage
http://www.javaslang.io/javaslang-docs/
Mario Fusco, From object oriented to functional domain modeling
http://bit.ly/28Nj1Ut

28. THANK YOU!
<david.schmitz@senacor.com> @koenighotze