Why laziness is important and how to implement and use it in a strictly evaluated language like Java

1. λazy
by Mario Fusco
Red Hat – Principal Software Engineer
@mariofusco

2. Lazy Evaluation
Lazy evaluation (or call-by-name) is an evaluation
strategy which delays the evaluation of an expression
until its value is needed
I know what to do.
Wake me up when
you really need it

3. Strictness vs. Laziness
Strictness is a property of functions (or methods in Java).
A strict function always evaluates its arguments as soon
as they’re passed to it.
Conversely a lazy function may choose not to evaluate one
or more of its arguments and in general it will evaluate
them only when they’re actually needed.
To recap,
strictness is about doing things,
laziness is about noting things to do.

4. Java is a strict language ...
… with some notable (and unavoidable) exceptions
✔
Boolean operators || and &&
✔
Ternary operator ? :
✔
if ... else
✔
for/while loops
✔
Java 8 streams
Q: Can exist a totally strict language?

5. Java is a strict language ...
… with some notable (and unavoidable) exceptions
✔
Boolean operators || and &&
✔
Ternary operator ? :
✔
if ... else
✔
for/while loops
✔
Java 8 streams
Q: Can exist a totally strict language?
A: It’s hard if not impossible to imagine how it could work
boolean isAdult = person != null && person.getAge() >= 18;

6. Turning Java into a lazy language
<T> T ternary(boolean pred, T first, T second) {
if (pred) {
return first;
} else {
return second;
}
}
String val1() {
return "first";
}
String val2() {
return "second";
}
String result1 = bool ? val1() : val2();
String result2 = ternary(bool, val1(), val2());

7. Turning Java into a lazy language
<T> T ternary(boolean pred, Supplier<T> first, Supplier<T> second) {
if (pred) {
return first.get();
} else {
return second.get();
}
}
String val1() {
return "first";
}
String val2() {
return "second";
}
String result1 = bool ? val1() : val2();
String result2 = ternary(bool, () -> val1(), () -> val2());

8. A simple practical example: logging
// pre-Java 8 style optimization
if (logger.isTraceEnabled()) {
logger.trace("Some long-running operation returned {}",
expensiveOperation());
}

9. A simple practical example: logging
// pre-Java 8 style optimization
if (logger.isTraceEnabled()) {
logger.trace("Some long-running operation returned {}",
expensiveOperation());
}
// Java 8 style optimization using laziness
logger.trace("Some long-running operation returned {}",
() -> expensiveOperation());
* from Apache Log4J 2 docs
no need to explicitly check the log level:
the lambda expression is not evaluated
if the TRACE level is not enabled *

10. Laziness: the ultimate performance
optimization technique
Performance optimization pro tip:
before trying to inline/optimize/parallelize a piece of code,
ask yourself if you could avoid to run it at all.
Laziness is
probably the
only form of
performance
optimization
which is (almost)
never premature
There is nothing so useless as doing efficiently
something that should not be done at all

11. The case of Java 8 Streams
IntStream.iterate( 1, i -> i+1 )
.map( i -> i * 2 )
.filter( i -> i > 5 )
.findFirst();

12. The case of Java 8 Streams
IntStream.iterate( 1, i -> i+1 )
.map( i -> i * 2 )
.filter( i -> i > 5 )
.findFirst();
Thank to laziness the Stream
can be potentially infinite

13. The case of Java 8 Streams
IntStream.iterate( 1, i -> i+1 )
.map( i -> i * 2 )
.filter( i -> i > 5 )
.findFirst();
Thank to laziness the Stream
can be potentially infinite
Intermediate operations are lazy:
they don’t perform any action until
a terminal operation is reached

14. The case of Java 8 Streams
IntStream.iterate( 1, i -> i+1 )
.map( i -> i * 2 )
.filter( i -> i > 5 )
.findFirst();
Thank to laziness the Stream
can be potentially infinite
Intermediate operations are lazy:
they don’t perform any action until
a terminal operation is reached
Only the terminal operation
triggers the pipeline of
computations
A Stream is not a data structure.
It is the lazy specification of a how
to manipulate a set of data.

15. Things you can’t do without laziness
There are several algorithms that can’t be
(reasonably) implemented without laziness.
For example let’s consider the following:
1. Take the list of positive integers.
2. Filter the primes.
3. Return the list of the first ten results.

16. Wait! I can achieve the same with a
strict algorithm
Yes, but how?
1. Take the first integer.
2. Check whether it’s a prime.
3. If it is, store it in a list.
4. Check whether the list has ten elements.
5. If it has ten elements, return it as the result.
6. If not, increment the integer by 1.
7. Go to line 2.

17. Wait! I can achieve the same with a
strict algorithm
Yes, but how?
1. Take the first integer.
2. Check whether it’s a prime.
3. If it is, store it in a list.
4. Check whether the list has ten elements.
5. If it has ten elements, return it as the result.
6. If not, increment the integer by 1.
7. Go to line 2.
Sure, doable …
but what a mess!

18. Laziness lets us separate the description of an
expression from the evaluation of that expression
Laziness is an enabler for separation of concerns

19. List<String> errors =
Files.lines(Paths.get(fileName))
.filter(l -> l.startsWith("ERROR"))
.limit(40)
.collect(toList());
Separation of Concerns
List<String> errors = new ArrayList<>();
int errorCount = 0;
File file = new File(fileName);
String line = file.readLine();
while (errorCount < 40 && line != null) {
if (line.startsWith("ERROR")) {
errors.add(line);
errorCount++;
}
line = file.readLine();
}

20. Cool! Now I know:
I will use a Stream
also for prime
numbers

21. Cool! Now I know:
I will use a Stream
also for prime
numbers
Let’s give this a try ...

22. Creating a Stream of prime numbers
public IntStream primes(int n) {
return IntStream.iterate(2, i -> i + 1)
.filter(this::isPrime)
.limit(n);
}
public boolean isPrime(int candidate) {
return IntStream.rangeClosed(2, (int)Math.sqrt(candidate))
.noneMatch(i -> candidate % i == 0);
}

23. Creating a Stream of prime numbers
public IntStream primes(int n) {
return IntStream.iterate(2, i -> i + 1)
.filter(this::isPrime)
.limit(n);
}
public boolean isPrime(int candidate) {
return IntStream.rangeClosed(2, (int)Math.sqrt(candidate))
.noneMatch(i -> candidate % i == 0);
}
It iterates through every number every time to see if it can be
exactly divided by a candidate number, but it would be enough
to only test numbers that have been already classified as prime
Inefficient

24. Recursively creating a Stream of primes
static Intstream numbers() {
return IntStream.iterate(2, n -> n + 1);
}
static int head(IntStream numbers) {
return numbers.findFirst().getAsInt();
}
static IntStream tail(IntStream numbers) {
return numbers.skip(1);
}
static IntStream primes(IntStream numbers) {
int head = head(numbers());
return IntStream.concat(
IntStream.of(head),
primes(tail(numbers).filter(n -> n % head != 0))
);
}
Cannot invoke 2
terminal operations
on the same Streams
Problems?
No lazy evaluation
in Java leads to an
endless recursion

25. Lazy evaluation in Scala
def numbers(n: Int): Stream[Int] = n #:: numbers(n+1)
def primes(numbers: Stream[Int]): Stream[Int] =
numbers.head #::
primes(numbers.tail filter (n -> n % numbers.head != 0))
lazy concatenation
In Scala the #:: method (lazy concatenation)
returns immediately and the elements are
evaluated only when needed

26. interface HeadTailList<T> {
T head();
HeadTailList<T> tail();
boolean isEmpty();
HeadTailList<T> filter(Predicate<T> p);
}
Implementing a lazy list in Java
class LazyList<T> implements HeadTailList<T> {
private final T head;
private final Supplier<HeadTailList<T>> tail;
public LazyList(T head,
Supplier<HeadTailList<T>> tail) {
this.head = head;
this.tail = tail;
}
public T head() { return head; }
public HeadTailList<T> tail() { return tail.get(); }
public boolean isEmpty() { return false; }
}

27. … and its lazy filter
class LazyList<T> implements HeadTailList<T> { ...
public HeadTailList<T> filter(Predicate<T> p) {
return isEmpty() ?
this :
p.test(head()) ?
new LazyList<>(head(), () -> tail().filter(p)) :
tail().filter(p);
}
} 2
3 4
5
6
7
8
9
2
3
5
7

28. Back to generating primes
static HeadTailList<Integer> primes(HeadTailList<Integer> numbers) {
return new LazyList<>(
numbers.head(),
() -> primes(numbers.tail()
.filter(n -> n % numbers.head() != 0)));
}
static LazyList<Integer> from(int n) {
return new LazyList<Integer>(n, () -> from(n+1));
}

29. Back to generating primes
static HeadTailList<Integer> primes(HeadTailList<Integer> numbers) {
return new LazyList<>(
numbers.head(),
() -> primes(numbers.tail()
.filter(n -> n % numbers.head() != 0)));
}
static LazyList<Integer> from(int n) {
return new LazyList<Integer>(n, () -> from(n+1));
}
LazyList<Integer> numbers = from(2);
int two = primes(numbers).head();
int three = primes(numbers).tail().head();
int five = primes(numbers).tail().tail().head();

30. LazyList of primes under the hood
from(2) → 2 () -> from(3)
2 () -> primes( from(3).filter(2) )primes(from(2)) →

31. LazyList of primes under the hood
from(2) → 2 () -> from(3)
2 () -> primes( from(3).filter(2) )
3 () -> from(4).filter(2).filter(3)() -> primes( )
3 () -> primes( from(4).filter(2).filter(3) )
primes(from(2)) →
.tail() →

32. LazyList of primes under the hood
from(2) → 2 () -> from(3)
2 () -> primes( from(3).filter(2) )
3 () -> from(4).filter(2).filter(3)() -> primes( )
3 () -> primes( from(4).filter(2).filter(3) )
5 () -> from(6).filter(2).filter(3).filter(5)() -> primes( )
5 () -> primes( from(6).filter(2).filter(3).filter(5) )
primes(from(2)) →
.tail() →
.tail() →

33. Printing primes
static <T> void printAll(HeadTailList<T> list) {
while (!list.isEmpty()){
System.out.println(list.head());
list = list.tail();
}
}
printAll(primes(from(2)));
iteratively

34. Printing primes
static <T> void printAll(HeadTailList<T> list) {
while (!list.isEmpty()){
System.out.println(list.head());
list = list.tail();
}
}
printAll(primes(from(2)));
static <T> void printAll(HeadTailList<T> list) {
if (list.isEmpty()) return;
System.out.println(list.head());
printAll(list.tail());
}
printAll(primes(from(2)));
iteratively
recursively

35. Iteration vs. Recursion
External Iteration
public int sumAll(int n) {
int result = 0;
for (int i = 0; i <= n; i++) {
result += i;
}
return result;
}
Internal Iteration
public static int sumAll(int n) {
return IntStream.rangeClosed(0, n).sum();
}

36. Iteration vs. Recursion
External Iteration
public int sumAll(int n) {
int result = 0;
for (int i = 0; i <= n; i++) {
result += i;
}
return result;
}
Recursion
public int sumAll(int n) {
return n == 0 ? 0 : n + sumAll(n - 1);
}
Internal Iteration
public static int sumAll(int n) {
return IntStream.rangeClosed(0, n).sum();
}

37. public class PalindromePredicate implements Predicate<String> {
@Override public boolean test(String s) {
return isPalindrome(s, 0, s.length()-1);
}
private boolean isPalindrome(String s, int start, int end) {
while (start < end && !isLetter(s.charAt(start))) start++;
while (start < end && !isLetter(s.charAt(end))) end--;
if (start >= end) return true;
if (toLowerCase(s.charAt(start)) !=
toLowerCase(s.charAt(end))) return false;
return isPalindrome(s, start+1, end-1);
}
}
Another Recursive Example
Tail Rescursive Call

38. What's the problem?
List<String> sentences = asList( "Dammit, I’m mad!",
"Rise to vote, sir!",
"Never odd or even",
"Never odd and even",
"Was it a car or a cat I saw?",
"Was it a car or a dog I saw?",
VERY_LONG_PALINDROME );
sentences.stream()
.filter(new PalindromePredicate())
.forEach(System.out::println);

39. What's the problem?
List<String> sentences = asList( "Dammit, I’m mad!",
"Rise to vote, sir!",
"Never odd or even",
"Never odd and even",
"Was it a car or a cat I saw?",
"Was it a car or a dog I saw?",
VERY_LONG_PALINDROME );
sentences.stream()
.filter(new PalindromePredicate())
.forEach(System.out::println);
Exception in thread "main" java.lang.StackOverflowError
at java.lang.Character.getType(Character.java:6924)
at java.lang.Character.isLetter(Character.java:5798)
at java.lang.Character.isLetter(Character.java:5761)
at org.javaz.trampoline.PalindromePredicate.isPalindrome(PalindromePredicate.java:17)
at org.javaz.trampoline.PalindromePredicate.isPalindrome(PalindromePredicate.java:21)
at org.javaz.trampoline.PalindromePredicate.isPalindrome(PalindromePredicate.java:21)
at org.javaz.trampoline.PalindromePredicate.isPalindrome(PalindromePredicate.java:21)
……..

40. Tail Call Optimization
int func_a(int data) {
data = do_this(data);
return do_that(data);
} ... | executing inside func_a()
push EIP | push current instruction pointer on stack
push data | push variable 'data' on the stack
jmp do_this | call do_this() by jumping to its address
... | executing inside do_this()
push EIP | push current instruction pointer on stack
push data | push variable 'data' on the stack
jmp do_that | call do_that() by jumping to its address
... | executing inside do_that()
pop data | prepare to return value of 'data'
pop EIP | return to do_this()
pop data | prepare to return value of 'data'
pop EIP | return to func_a()
pop data | prepare to return value of 'data'
pop EIP | return to func_a() caller
...
caller

41. Tail Call Optimization
int func_a(int data) {
data = do_this(data);
return do_that(data);
} ... | executing inside func_a()
push EIP | push current instruction pointer on stack
push data | push variable 'data' on the stack
jmp do_this | call do_this() by jumping to its address
... | executing inside do_this()
push EIP | push current instruction pointer on stack
push data | push variable 'data' on the stack
jmp do_that | call do_that() by jumping to its address
... | executing inside do_that()
pop data | prepare to return value of 'data'
pop EIP | return to do_this()
pop data | prepare to return value of 'data'
pop EIP | return to func_a()
pop data | prepare to return value of 'data'
pop EIP | return to func_a() caller
...
caller
avoid putting
instruction
on stack

42. from Recursion to Tail Recursion
Recursion
public int sumAll(int n) {
return n == 0 ?
0 :
n + sumAll(n - 1);
}

43. from Recursion to Tail Recursion
Recursion
public int sumAll(int n) {
return n == 0 ?
0 :
n + sumAll(n - 1);
}
Tail Recursion
public int sumAll(int n) {
return sumAll(n, 0);
}
private int sumAll(int n, int acc) {
return n == 0 ?
acc :
sumAll(n – 1, acc + n);
}

44. Tail Recursion in Scala
def isPalindrome(s: String): Boolean = isPalindrome(s, 0, s.length-1)
@tailrec
def isPalindrome(s: String, start: Int, end: Int): Boolean = {
val pos1 = nextLetter(s, start, end)
val pos2 = prevLetter(s, start, end)
if (pos1 >= pos2) return true
if (toLowerCase(s.charAt(pos1)) !=
toLowerCase(s.charAt(pos2))) return false
isPalindrome(s, pos1+1, pos2-1)
}
@tailrec def nextLetter(s: String, start: Int, end: Int): Int =
if (start > end || isLetter(s.charAt(start))) start
else nextLetter(s, start+1, end)
@tailrec def prevLetter(s: String, start: Int, end: Int): Int =
if (start > end || isLetter(s.charAt(end))) end
else prevLetter(s, start, end-1)

45. Tail Recursion in Java?
Scala (and many other functional languages) automatically
perform tail call optimization at compile time
@tailrec annotation ensures the compiler will optimize a tail
recursive function (i.e. you will get a compilation failure if you
use it on a function that is not really tail recursive)
Java compiler doesn't perform any tail call optimization (and
very likely won't do it in a near future)
How can we overcome this limitation and
have StackOverflowError-free functions also
in Java tail recursive methods?

46. Trampolines to the rescue
A trampoline is an iteration applying a list of functions.
Each function returns the next function for the loop to run.
Func1
return
apply
Func2
return
apply
Func3
return
apply
FuncN
apply
…
result
return

47. Implementing the TailCall …
@FunctionalInterface
public interface TailCall<T> extends Supplier<TailCall<T>> {
default boolean isComplete() { return false; }
default T result() { throw new UnsupportedOperationException(); }
default T invoke() {
return Stream.iterate(this, TailCall::get)
.filter(TailCall::isComplete)
.findFirst()
.get()
.result();
}
static <T> TailCall<T> done(T result) {
return new TerminalCall<T>(result);
}
}

48. … and the terminal TailCall
public class TerminalCall<T> implements TailCall<T> {
private final T result;
public TerminalCall( T result ) {
this.result = result;
}
@Override
public boolean isComplete() { return true; }
@Override
public T result() { return result; }
@Override
public TailCall<T> get() {
throw new UnsupportedOperationException();
}
}

49. Using the Trampoline
public class PalindromePredicate implements Predicate<String> {
@Override public boolean test(String s) {
return isPalindrome(s, 0, s.length()-1).invoke();
}
private TailCall<Boolean> isPalindrome(String s, int start,
int end) {
while (start < end && !isLetter(s.charAt(start))) start++;
while (end > start && !isLetter(s.charAt(end))) end--;
if (start >= end) return done(true);
if (toLowerCase(s.charAt(start)) !=
toLowerCase(s.charAt(end))) return done(false);
int newStart = start + 1;
int newEnd = end - 1;
return () -> isPalindrome(s, newStart, newEnd);
}
}

50. What else laziness can do for us?
Avoiding eager dependency injection by
lazily providing arguments to
computation only when they are needed
i.e.
Introducing the Reader Monad

51. What’s wrong with annotation-
based dependency injection?
➢ Eager in nature
➢ “new” keyword is forbidden
➢ All-your-beans-belong-to-us
syndrome
➢ Complicated objects lifecycle
➢ Depending on scope may
not work well with threads
➢ Hard to debug if something
goes wrong
➢ Easy to abuse leading to
broken encapsulation

52. Annotation based
dependency injection
transforms what
should be a compile
time problem into a
runtime one
(often hard to debug)

53. Introducing the Reader monad ...
public class Reader<R, A> {
private final Function<R, A> exec;
public Reader( Function<R, A> exec ) {
this.exec = exec;
}
public <B> Reader<R, B> map(Function<A, B> f) {
return new Reader<>( exec.andThen(f) );
}
public <B> Reader<R, B> flatMap(Function<A, Reader<R, B>> f) {
return new Reader<>( r -> exec.andThen(f).apply(r).apply(r) );
}
public A apply(R r) {
return exec.apply( r );
}
}
The reader monad provides an environment to
wrap an abstract computation without evaluating it

54. The Reader Monad
The Reader monad makes
a lazy computation
explicit
in the type system, while
hiding
the logic to apply it
In other words the reader monad
allows us to treat functions as
values with a context
We can act as if we already know
what the functions will return.

55. @FunctionalInterface
public interface Logger extends Consumer<String> { }
public class Account {
private Logger logger;
private String owner;
private double balance;
public Account open( String owner ) {
this.owner = owner;
logger.accept( "Account opened by " + owner );
return this;
}
public Account credit( double value ) {
balance += value;
logger.accept( "Credited " + value + " to " + owner );
return this;
}
public Account debit( double value ) {
balance -= value;
logger.accept( "Debited " + value + " to " + owner );
return this;
}
public double getBalance() { return balance; }
public void setLogger( Logger logger ) { this.logger = logger; }
}
Usually injected

56. Account account = new Account();
account.open( "Alice" )
.credit( 200.0 )
.credit( 300.0 )
.debit( 400.0 );
The joys of dependency injection

57. Account account = new Account();
account.open( "Alice" )
.credit( 200.0 )
.credit( 300.0 )
.debit( 400.0 );
Throws NPE if for some
reason the logger
couldn’t be injected
The joys of dependency injection :(
You should never use “new”

58. public static <R, A> Reader<R, A> lift( A obj,
BiConsumer<A, R> injector ) {
return new Reader<>( r -> {
injector.accept( obj, r );
return obj;
} );
}
Lazy injection with the reader monad

59. public static <R, A> Reader<R, A> lift( A obj,
BiConsumer<A, R> injector ) {
return new Reader<>( r -> {
injector.accept( obj, r );
return obj;
} );
}
Lazy injection with the reader monad
Account account = new Account();
Reader<Logger, Account> reader =
lift(account, Account::setLogger )
.map( a -> a.open( "Alice" ) )
.map( a -> a.credit( 200.0 ) )
.map( a -> a.credit( 300.0 ) )
.map( a -> a.debit( 400.0 ) );
reader.apply( System.out::println );
System.out.println(account + " has balance " + account.getBalance());

60. public static <R, A> Reader<R, A> lift( A obj,
BiConsumer<A, R> injector ) {
return new Reader<>( r -> {
injector.accept( obj, r );
return obj;
} );
}
Lazy injection with the reader monad
Account account = new Account();
Reader<Logger, Account> reader =
lift(account, Account::setLogger )
.map( a -> a.open( "Alice" ) )
.map( a -> a.credit( 200.0 ) )
.map( a -> a.credit( 300.0 ) )
.map( a -> a.debit( 400.0 ) );
reader.apply( System.out::println );
System.out.println(account + " has balance " + account.getBalance());

61. Replacing injection with
function application
Account
Function<Logger, Account>
Reader
Function<Logger, Account>
Reader
Function<Logger, Account>
Reader
Function<Logger, Account>
Reader
Function<Logger, Account>
Reader
lift
apply(logger)
account
map(Function<Account, Account>)
map(Function<Account, Account>)

62. Function based injection
Account account = new Account();
Function<Logger, Account> inject = l -> {
account.setLogger( l );
return account;
};
Function<Logger, Account> f = inject
.andThen( a -> a.open( "Alice" ) )
.andThen( a -> a.credit( 200.0 ) )
.andThen( a -> a.credit( 300.0 ) )
.andThen( a -> a.debit( 400.0 ) );
f.apply( System.out::println );
System.out.println(account + " has balance " + account.getBalance());
The reader monad provides a more
structured and powerful approach.
In this simple case a simple function composition
is enough to achieve the same result.

63. public class Account { ...
public MoneyTransfer tranfer( double value ) {
return new MoneyTransfer( this, value );
}
}
public class MoneyTransfer {
private Logger logger;
private final Account account;
private final double amount;
public MoneyTransfer( Account account, double amount ) {
this.account = account;
this.amount = amount;
}
public void setLogger( Logger logger ) { this.logger = logger; }
public MoneyTransfer execute() {
account.debit( amount );
logger.accept( "Transferred " + amount + " from " + account );
return this;
}
}
Injecting into multiple objects

64. Injecting into multiple objects
Account account = new Account();
Reader<Logger, MoneyTransfer> reader =
lift(account, Account::setLogger )
.map( a -> a.open( "Alice" ) )
.map( a -> a.credit( 300.0 ) )
.flatMap( a -> lift( a.tranfer( 200.0 ), MoneyTransfer::setLogger ) )
.map( MoneyTransfer::execute );
reader.apply( System.out::println );
System.out.println(account + " has balance " + account.getBalance());

65. Injecting into multiple objects
Account account = new Account();
Reader<Logger, MoneyTransfer> reader =
lift(account, Account::setLogger )
.map( a -> a.open( "Alice" ) )
.map( a -> a.credit( 300.0 ) )
.flatMap( a -> lift( a.tranfer( 200.0 ), MoneyTransfer::setLogger ) )
.map( MoneyTransfer::execute );
reader.apply( System.out::println );
System.out.println(account + " has balance " + account.getBalance());

66. Mario Fusco
Red Hat – Principal Software Engineer
mario.fusco@gmail.com
twitter: @mariofusco
Q A
Thanks … Questions?