Unlocking Java 8 Monads with the Magic of Promises

Unlocking the Magic of Monads
with Java 8
Oleg Šelajev
@shelajev
ZeroTurnaround

What do we want to achieve?
have fun while learning stuﬀ
understand the concept of Monad
get generic constructs for Monads
solve a real-world problem
ignore lack of “ad hoc” polymorphism
and other slightly relevant facts

Java 8: lambda recap
@FunctionalInterface 
public interface Function<T, R> {
R apply(T t);
}

Function<String, Integer> f =
Integer::valueOf;

String prefix = “JavaDay Kiyv: ";
 
Function<String, Integer> f = (str) -> { 
System.out.println(prefix + str); 
return str.hashCode(); 
};

Problem statement
object.methodCall(arg, () -> {
// callback  
// callback
// …
}); 
}); 
});

Problem statement
object.interact("option1", () -> { 
object.doThing("fast", () -> { 
if(wasSuccessful()) { 
object.celebrate(100, TimeUnit.SECONDS, () -> { 
System.out.println("Back to work"); 
}); 
} 
else { 
object.beSad(":(", () -> { 
System.out.println("Still back to work"); 
}); 
}); 
});

Wishful thinking
object.interact("option1") 
.then((o) -> o.doThing("fast")) 
.then((o) -> o.celebrate(100,
SECONDS,
() ->
{ System.out.println("Back to work”); })) 
.or((o) -> o.beSad(":("));

Type: async result
java.util.concurrent.Future<V> 
boolean isDone();
V get() …
V get(long timeout, TimeUnit unit)

Type: async result
java.util.concurrent.Future<V> 
boolean isDone();
V get() …
V get(long timeout, TimeUnit unit)
Can we do
better?

Oh my…
a monad in X is just a
monoid in the
category of
endofunctors of X,
with product ×
replaced by
composition of
endofunctors and unit
set by the identity
endofunctor.

It is known
Every cook can
understand,
compile and
use monads…
V. Lenin (1923)

Monads: intuition
wrapping things
chaining functions on those things
monad is a type

Wrapping: return / pure
Take instance of “a”, return: “m a”
Constructor / Factory method

Pure in Java
public interface Monad<V> { 
Monad<V> pure(V value); 
}

Chaining: bind / (>>=)
take:
monad: “m a”
function: “a => m b”
return: monad “m b”

Bind in Java
Monad<V> pure(V v); 
<R> Monad<R> bind(Function<V, Monad<R>> f); 
}

Hacking time
Promise<V> - result of async computation
Kinda like Future<V>
supports chaining functions: bind

Imagined Promise<V>
existing Future operations
+
finish:
p.invoke(V v);
add callback:
p.onRedeem(Action<Promise<V>> callback);

Promise<V>: pure
public <V> Promise<V> pure(final V v) { 
Promise<V> p = new Promise<>(); 
p.invoke(v); 
return p; 
}

Promise<V>: bind
public <R> Promise<R> bind(final Function<V, Promise<R>>
function) { 
Promise<R> result = new Promise<>(); 
this.onRedeem(callback -> { 
V v = callback.get(); 
Promise<R> applicationResult = function.apply(v); 
applicationResult.onRedeem(c -> { 
R r = c.get(); 
result.invoke(r); 
}); 
return result; 
}

Promise<V>: get
public V get() throws InterruptedException,
ExecutionException { 
if (exception != null) { 
throw new ExecutionException(exception); 
} 
return result; 
}

Example
Promise<String> p = Async.submit(() -> { 
return "hello world"; 
});
 
Promise<Integer> result = p.bind(string ->
Promise.pure(Integer.valueOf(string.hashCode())));
 
System.out.println("HashCode = " + result.get());

Checkpoint
Promise - represents async computation
Handling values AND exceptions
Chaining of functions

Wait, is that it?
Monad vs. Instance of monad

Typeclass? Higher functions?
Common API
Generic functions
over all monads

Greatness
Common operations for Monads
sequence, zip
Limited under parametrised
polymorphism in Java

Greatness
Common operations for Monads
sequence, zip
Sequence of promises => joined asynch ops
Sequence of elephants => a chain of them

Sequence
Monad<List<V>> sequence(Monad<V>... monads);

Monad in Java
Monad<V> pure(V v); 
<R> Monad<R> bind(Function<V, Monad<R> f);
 
V get(); 
}

One does not simply call
oneself a monad!

Laws (don’t be scared)
return a >>= f ≡ f a
m >>= return ≡ m
(m >>= f) >>= g ≡ m >>= (x -> f x >>= g)

Left identity
pure(v).bind(f) ≡ f.apply(v)

Right identity
m.bind(m::pure) ≡ m

Associativity
m.bind(f).bind(g) ≡ m.bind(
(v) -> f.apply(v).bind(g))

Some platforms have it easy
Referential transparency
Partial application
≡ is easy

Mortal platforms
No referential transparency
f.apply(v) != f.apply(v)
equals() + hashcode()

Deﬁning ≡ for Java
Side eﬀects are similar
m.get() observes the same values
values or exceptions

Promise: left identity
Function<Integer, Promise<Boolean>> f =
(x) -> { 
return submit(() -> x % 2 == 0); 
}; 
Integer val = new Integer(100);
 
assertEquals(Promise.pure(val).bind(f).get(),
f.apply(val).get());

Promise: right identity
Integer val = new Integer(100000); 
Promise<Integer> p =
Promise.pure(val).bind(Promise::pure);
 
assertEquals(val, p.get()); 
assertEquals(identityHashCode(val), 
identityHashCode(p.get()));

Quality software
java.util.concurrent.CompletableFuture
thenApply(Function / Consumer / etc)
thenApplyAsync(Function / etc)
Async => FJP.common()

Completable Future
https://vimeo.com/131394616

Optional pure
static <T> Optional<T> of(T value) { 
return new Optional<>(value); 
}
static <T> Optional<T> ofNullable(T value) { 
return value == null ?
empty() :
of(value); 
}

Optional bind
public Optional flatMap(Function<T,
Optional> mapper) { 
Objects.requireNonNull(mapper); 
if (!isPresent()) 
return empty(); 
else { 
return Objects.requireNonNull(mapper.apply(value)); 
} 
}

Optional bind
public Optional map(Function<T, U> mapper) { 
Objects.requireNonNull(mapper); 
if (!isPresent()) 
return empty(); 
else { 
return
Optional.ofNullable(mapper.apply(value)); 
} 
}

oleg@zeroturnaround.com
@shelajev
github.com/shelajev/promises
Contact me

Minority report
Alternative deﬁnition of Monads:
fmap :: (a -> b) -> f a -> f b
join :: m (m a) -> m a

Exercises
Implement (>>=) in terms of fmap and
join.
Now implement join and fmap in terms
of (>>=)and return.

Unlocking Java 8 Monads with the Magic of Promises

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