____ __
| _ \ _ _ _ __ ___ / _|_ _ _ __
| |_) | | | | '__/ _ \ |_| | | | '_ \
| __/| |_| | | | __/ _| |_| | | | |
|_| \__,_|_| \___|_| \__,_|_| |_|
This module was developed as the core of the zeromock project. It defines all the basic classes and interfaces used in the rest of the project.
Initially the module only held a few basic interfaces and it has grown to become an entire functional programming library (well, a humble one), and now is an independent library.
Working in this library helped me to learn and understand some important concepts of functional programming, and over time I implemented higher kinded types and type classes in Java. I don't know if this will be helpful for somebody, but the work is here to everyone want to use it.
Finally, I have to say thanks to vavr library author, this library is largely inspired in his work, and also to Scala standard library authors. I don't want to forget some of the projects I've used as reference: Arrow and cats for type classes implementation, fs2 for stream processing, and ZIO to implement my own version in Java. Their awesome work help me a lot.
This project is not ready to be used in production, I use it to learn functional programming concepts by my self, but, if you want to use it, use it at your own risk. Anyway if you think is useful for you, go ahead, also any feedback and PR are very welcome.
In this project I have implemented some patterns of functional programming that need Higher Kinded Types. In Java there are not such thing, but it can be simulated using a especial codification of types.
In Scala we can define a higher kinded typed just like this Monad[F[_]] but in Java it can be codified
like this Monad<F>. Then we can define a type using a special codification like this:
interface SomeType<T> extends SomeTypeOf<T> { }
// Boilerplate
interface SomeTypeOf<T> implements Kind<SomeType<?>, T> {
// this is a safe cast
static SomeType<T> toSomeType(Kind<SomeType<?>, ? extends T> hkt) {
return (SomeType<T>) hkt;
}
}It can be triky but, in the end is easy to work with. By the way, I tried to hide this details to the user of the library. Except with type classes because is the only way to implement them correctly.
So, there are interfaces to encode kinds of 1, 2 and 3 types. It can be defined types for 4, 5 or more types, but it wasn't necessary to implement the library.
In order to simplify working with higher kinded types, in the last version I've included an annotation processor to generate all this boilerplate code:
@HigherKind
interface SomeType<T> extends SomeTypeOf<T> { }With this annotation, all the above code, is generated automatically.
Is an alternative to Optional of Java standard library. It can contains two values, a some or a none
Option<String> some = Option.some("Hello world");
Option<String> none = Option.none();Is an implementation of scala Try in Java. It can contains two values, a success or a failure.
Try<String> success = Try.success("Hello world");
Try<String> failure = Try.failure(new RuntimeException("Error"));Is an implementation of scala Either in Java.
Either<Integer, String> right = Either.right("Hello world");
Either<Integer, String> left = Either.left(100);This type represents two different states, valid or invalid, an also it allows to combine several
validations using map2 to map5 methods.
Validation<String, String> name = Validation.valid("John Smith");
Validation<String, String> email = Validation.valid("[email protected]");
// Person has a constructor with two String parameters, name and email.
Valdation<Sequence<String>, Person> person = Validation.map2(name, email, Person::new);A object with a phantom parameter:
Const<String, Integer> constInt = Const.of("Hello world!");
Const<String, Float> constFloat = constInt.retag();
assertEquals("Hello world!", constFloat.value());This is an experimental implementation of Future. Computations are executed in another thread inmediatelly.
Future<String> future = Future.success("Hello world!");
Future<String> result = future.flatMap(string -> Future.run(string::toUpperCase));
assertEquals(Try.success("HELLO WORLD!"), result.await());These classes allow to hold some values together, as tuples. There are tuples from 1 to 5.
Tuple1<String> tuple1 = Tuple.of("Hello world");
Tuple2<String, Integer> tuple2 = Tuple.of("John Smith", 100);Java doesn't define immutable collections, so I have implemented some of them.
Is the equivalent to java Collection interface. It defines all the common methods.
It represents a linked list. It has a head and a tail.
It represents a set of elements. This elements cannot be duplicated.
It represents an array. You can access to the elements by its position in the array.
This class represents a hash map.
This class represents a binary tree.
This class represents a binary tree map.
Also I have implemented some Monads that allows to combine some operations.
Is the traditional State Modad from FP languages, like Haskel or Scala. It allows to combine operations over a state. The state should be a immutable class. It recives an state and generates a tuple with the new state and an intermediate result.
State<ImmutableList<String>, Option<String>> read = State.state(list -> Tuple.of(list.tail(), list.head()));
Tuple<ImmutableList<String>, Option<String>> result = read.run(ImmutableList.of("a", "b", "c"));
assertEquals(Tuple.of(ImmutableList.of("b", "c"), Option.some("a")), result);This is an implementation of Reader Monad. It allows to combine operations over a common input. It can be used to inject dependencies.
Reader<ImmutableList<String>, String> read2 = Reader.reader(list -> list.tail().head().orElse(""));
String result = read2.eval(ImmutableList.of("a", "b", "c"));
assertEqual("b", result);It allow to combine operations over a common output.
Writer<ImmutableList<String>, Integer> writer = Writer.<String, Integer>listPure(5)
.flatMap(value -> listWriter("add 5", value + 5))
.flatMap(value -> listWriter("plus 2", value * 2));
assertAll(() -> assertEquals(Integer.valueOf(20), writer.getValue()),
() -> assertEquals(listOf("add 5", "plus 2"), writer.getLog()));This is a experimental implementation of IO Monad in java. Inspired in this work.
IO<Unit> echo = Console.print("write your name")
.andThen(Console.read())
.flatMap(name -> Console.print("Hello " + name))
.andThen(Console.print("end"));
echo.unsafeRunSync();Implements recursion using an iteration and is stack safe.
private Trampoline<Integer> fibLoop(Integer n) {
if (n < 2) {
return Trampoline.done(n);
}
return Trampoline.more(() -> fibLoop(n - 1)).flatMap(x -> fibLoop(n - 2).map(y -> x + y));
}Finally, after hours of hard coding, I managed to implement a Free monad. This is a highly unstable implementation and I have implemented because it can be implemented. Inspired in this work.
Free<IOProgram_, Unit> echo =
IOProgram.write("what's your name?")
.andThen(IOProgram.read())
.flatMap(text -> IOProgram.write("Hello " + text))
.andThen(IOProgram.write("end"));
Kind<IO_, Unit> foldMap = echo.foldMap(IOInstances.monad(), new IOProgramInterperter());
foldMap.fix(toIO()).unsafeRunSync();Similar to Free monad, but allows static analysis without to run the program.
FreeAp<DSL_, Tuple5<Integer, Boolean, Double, String, Unit>> tuple =
applicative.map5(
DSL.readInt(2),
DSL.readBoolean(false),
DSL.readDouble(2.1),
DSL.readString("hola mundo"),
DSL.readUnit(),
Tuple::of
).fix(toFreeAp());
Kind<Id_, Tuple5<Integer, Boolean, Double, String, Unit>> map =
tuple.foldMap(idTransform(), IdInstances.applicative());
assertEquals(Id.of(Tuple.of(2, false, 2.1, "hola mundo", unit())), map.fix(toId()));Monad Transformer for Option type
OptionT<IO_, String> some = OptionT.some(IO.monad(), "abc");
OptionT<IO_, String> map = some.flatMap(value -> OptionT.some(IOInstances.monad(), value.toUpperCase()));
assertEquals("ABC", map.get().fix(toIO()).unsafeRunSync());Monad Transformer for Either type
EitherT<IO_, Nothing, String> right = EitherT.right(IO.monad(), "abc");
EitherT<IO_, Nothing, String> map = right.flatMap(value -> EitherT.right(IOInstances.monad(), value.toUpperCase()));
assertEquals("ABC", map.get().fix(toIO()).unsafeRunSync());Monad Transformer for State type
StateT<IO_, ImmutableList<String>, Unit> state =
pure("a").flatMap(append("b")).flatMap(append("c")).flatMap(end());
IO<Tuple2<ImmutableList<String>, Unit>> result = state.run(ImmutableList.empty()).fix(toIO());
assertEquals(Tuple.of(listOf("a", "b", "c"), unit()), result.unsafeRunSync());Monad Transformer for Writer type
WriterT<Id_, Sequence<String>, Integer> writer =
WriterT.<Id_, Sequence<String>, Integer>pure(monoid, monad, 5)
.flatMap(value -> lift(monoid, monad, Tuple.of(listOf("add 5"), value + 5)))
.flatMap(value -> lift(monoid, monad, Tuple.of(listOf("plus 2"), value * 2)));
assertAll(() -> assertEquals(Id.of(Integer.valueOf(20)), writer.getValue()),
() -> assertEquals(Id.of(listOf("add 5", "plus 2")), writer.getLog()));Also I implemented the Kleisli composition for functions that returns monadic values like Option, Try or Either.
Kleisli<Try_, String, Integer> toInt = Kleisli.lift(Try.monad(), Integer::parseInt);
Kleisli<Try_, Integer, Double> half = Kleisli.lift(Try.monad(), i -> i / 2.);
Kind<Try_, Double> result = toInt.compose(half).run("123");
assertEquals(Try.success(61.5), result);An experimental version of a Stream like scala fs2 project.
StreamOf<IO_> streamOfIO = Stream.ofIO();
IO<String> readFile = streamOfIO.eval(IO.of(() -> reader(file)))
.flatMap(reader -> streamOfIO.iterate(() -> Option.of(() -> readLine(reader))))
.takeWhile(Option::isPresent)
.map(Option::get)
.foldLeft("", (a, b) -> a + "\n" + b)
.fix(toIO())
.recoverWith(UncheckedIOException.class, cons("--- file not found ---"));
String content = readFile.unsafeRunSync();An experimental version of PureIO similar to ZIO.
PureIO<Console, Throwable, Unit> echoProgram =
Console.println("what's your name?")
.andThen(Console.readln())
.flatMap(name -> Console.println("Hello " + name));
interface Console {
<R extends Console> Console.Service<R> console();
static PureIO<Console, Throwable, String> readln() {
return PureIO.accessM(env -> env.console().readln());
}
static PureIO<Console, Throwable, Unit> println(String text) {
return PureIO.accessM(env -> env.console().println(text));
}
interface Service<R extends Console> {
PureIO<R, Throwable, String> readln();
PureIO<R, Throwable, Unit> println(String text);
}
}Additionally, there are aliases for some PureIO special cases:
UIO<T> => PureIO<?, Nothing, T>
EIO<E, T> => PureIO<?, E, T>
Task<T> => PureIO<?, Throwable, T>
RIO<R, T> => PureIO<R, Throwable, T>
URIO<T> => PureIO<R, Nothing, T>
Also, I have implemented a version of delimited control monad based in this project. With this monad, you can implement algebraic effects. Example:
// the effect
interface Amb {
Control<Boolean> flip();
}
// the program, if true then 2 else 3
Control<Integer> program(Amb amb) {
return amb.flip().map(x -> x ? 2 : 3);
}
@Test
void test() {
Control<ImmutableList<Integer>> handled = ambList(this::program);
assertEquals(listOf(2, 3), handled.run());
}
<R> Control<ImmutableList<R>> ambList(Function1<Amb, Control<R>> program) {
return new AmbList<R>().apply(amb -> program.apply(amb).map(ImmutableList::of));
}
final class AmbList<R> implements Handler<ImmutableList<R>, Amb>, Amb {
@Override
public Amb effect() { return this; }
@Override
public Control<Boolean> flip() {
return use(resume ->
resume.apply(true) // first flip return true
.flatMap(ts -> resume.apply(false) // second flip return false
.map(ts::appendAll)));
}
}With higher kinded types simulation we can implement typeclases.
Invariant -- Contravariant
\
SemigroupK Functor -- Comonad
| / \
MonoidK _ Applicative Traverse -- Foldable
| / | \
Alternative Selective ApplicativeError
| |
MonadWriter Monad |
\________________| |
/ / \ /
MonadState MonadReader MonadError_____
\ \
MonadThrow Bracket
\ /
Defer -- MonadDefer -- Timer
|
Async
|
Concurrent
public interface Functor<F extends > extends Invariant<F> {
<T, R> Kind<F, R> map(Kind<F, T> value, Function1<T, R> map);
}public interface Applicative<F> extends Functor<F> {
<T> Kind<F, T> pure(T value);
<T, R> Kind<F, R> ap(Kind<F, T> value, Kind<F, Function1<T, R>> apply);
@Override
default <T, R> Kind<F, R> map(Kind<F, T> value, Function1<T, R> map) {
return ap(value, pure(map));
}
}public interface Selective<F> extends Applicative<F> {
<A, B> Kind<F, B> select(Kind<F, Either<A, B>> value, Kind<F, Function1<A, B>> apply);
default <A, B, C> Kind<F, C> branch(Kind<F, Either<A, B>> value,
Kind<F, Function1<A, C>> applyA,
Kind<F, Function1<B, C>> applyB) {
Kind<F, Either<A, Either<B, C>>> abc = map(value, either -> either.map(Either::left));
Kind<F, Function1<A, Either<B, C>>> fabc = map(applyA, fb -> fb.andThen(Either::right));
return select(select(abc, fabc), applyB);
}
}public interface Monad<F> extends Selective<F> {
<T, R> Kind<F, R> flatMap(Kind<F, T> value, Function1<T, ? extends Kind<F, R>> map);
@Override
default <T, R> Kind<F, R> map(Kind<F, T> value, Function1<T, R> map) {
return flatMap(value, map.andThen(this::pure));
}
@Override
default <T, R> Kind<F, R> ap(Kind<F, T> value, Kind<F, Function1<T, R>> apply) {
return flatMap(apply, map -> map(value, map));
}
@Override
default <A, B> Kind<F, B> select(Kind<F, Either<A, B>> value, Kind<F, Function1<A, B>> apply) {
return flatMap(value, either -> either.fold(a -> map(apply, map -> map.apply(a)), this::<B>pure));
}
}It represents a binary operation over a type.
@FunctionalInterface
public interface Semigroup<T> {
T combine(T t1, T t2);
}There are instances for strings and integers.
Extends Semigroup adding a zero operation that represent an identity.
public interface Monoid<T> extends Semigroup<T> {
T zero();
}There are instances for strings and integers.
It represents a Semigroup but defined for a kind, like a List, so it extends a regular Semigroup.
The same like SemigroupK but for a Monoid.
public interface Invariant<F> {
<A, B> Kind<F, B> imap(Kind<F, A> value, Function1<A, B> map, Function1<B, A> comap);
}public interface Contravariant<F> extends Invariant<F> {
<A, B> Kind<F, B> contramap(Kind<F, A> value, Function1<B, A> map);
}public interface Bifunctor<F> {
<A, B, C, D> Kind<Kind<F, C>, D> bimap(Kind<Kind<F, A>, B> value, Function1<A, C> leftMap, Function1<B, D> rightMap);
}public interface Profunctor<F> {
<A, B, C, D> Kind<Kind<F, C>, D> dimap(Kind<Kind<F, A>, B> value, Function1<C, A> contramap, Function1<B, D> map);
}public interface ApplicativeError<F, E> extends Applicative<F> {
<A> Kind<F, A> raiseError(E error);
<A> Kind<F, A> handleErrorWith(Kind<F, A> value, Function1<E, ? extends Kind<F, A>> handler);
}public interface MonadError<F, E> extends ApplicativeError<F, E>, Monad<F> {
default <A> Kind<F, A> ensure(Kind<F, A> value, Producer<E> error, Matcher1<A> matcher) {
return flatMap(value, a -> matcher.match(a) ? pure(a) : raiseError(error.get()));
}
}public interface MonadThrow<F> extends MonadError<F, Throwable> {
}public interface MonadReader<F, R> extends Monad<F> {
Kind<F, R> ask();
default <A> Kind<F, A> reader(Function1<R, A> mapper) {
return map(ask(), mapper);
}
}public interface MonadState<F, S> extends Monad<F> {
Kind<F, S> get();
Kind<F, Unit> set(S state);
default Kind<F, Unit> modify(Operator1<S> mapper) {
return flatMap(get(), s -> set(mapper.apply(s)));
}
default <A> Kind<F, A> inspect(Function1<S, A> mapper) {
return map(get(), mapper);
}
default <A> Kind<F, A> state(Function1<S, Tuple2<S, A>> mapper) {
return flatMap(get(), s -> mapper.apply(s).applyTo((s1, a) -> map(set(s1), x -> a)));
}
}public interface MonadWriter<F, W> extends Monad<F> {
<A> Kind<F, A> writer(Tuple2<W, A> value);
<A> Kind<F, Tuple2<W, A>> listen(Kind<F, A> value);
<A> Kind<F, A> pass(Kind<F, Tuple2<Operator1<W>, A>> value);
default Kind<F, Unit> tell(W writer) {
return writer(Tuple.of(writer, unit()));
}
}public interface Comonad<F> extends Functor<F> {
<A, B> Kind<F, B> coflatMap(Kind<F, A> value, Function1<Kind<F, A>, B> map);
<A> A extract(Kind<F, A> value);
default <A> Kind<F, Kind<F, A>> coflatten(Kind<F, A> value) {
return coflatMap(value, identity());
}
}public interface Foldable<F> {
<A, B> B foldLeft(Kind<F, A> value, B initial, Function2<B, A, B> mapper);
<A, B> Eval<B> foldRight(Kind<F, A> value, Eval<B> initial, Function2<A, Eval<B>, Eval<B>> mapper);
}public interface Traverse<F> extends Functor<F>, Foldable<F> {
<G, T, R> Kind<G, Kind<F, R>> traverse(Applicative<G> applicative, Kind<F, T> value,
Function1<T, ? extends Kind<G, R>> mapper);
}public interface Semigroupal<F> {
<A, B> Kind<F, Tuple2<A, B>> product(Kind<F, A> fa, Kind<F, B> fb);
}public interface Defer<F> {
<A> Kind<F, A> defer(Producer<Kind<F, A>> defer);
}public interface Bracket<F, E> extends MonadError<F, E> {
<A, B> Kind<F, B> bracket(Kind<F, A> acquire, Function1<A, ? extends Kind<F, B>> use, Consumer1<A> release);
}public interface MonadDefer<F> extends MonadThrow<F>, Bracket<F, Throwable>, Defer<F>, Timer<F> {
default <A> Kind<F, A> later(Producer<A> later) {
return defer(() -> Try.of(later::get).fold(this::raiseError, this::pure));
}
}public interface Async<F> extends MonadDefer<F> {
<A> Kind<F, A> async(Consumer1<Consumer1<Try<A>>> consumer);
}public interface Timer<F> {
Kind<F, Unit> sleep(Duration duration);
}It represents a natural transformation between two different kinds.
public interface FunctionK<F, G> {
<T> Kind<G, T> apply(Kind<F, T> from);
} __Iso__
/ \
Lens Prism
\ /
Optional
An Iso is an optic which converts elements of some type into elements of other type without loss.
In other words, it's isomorphic.
Point point = new Point(1, 2);
Iso<Point, Tuple2<Integer, Integer>> pointToTuple =
Iso.of(p -> Tuple.of(p.x, p.y),
t -> new Point(t.get1(), t.get2()));
assertEquals(point, pointToTuple.set(pointToTuple.get(point)));A Lens is an optic used to zoom inside a structure. In other words, it's an abstraction of a setter and a getter
but with immutable objects.
Lens<Employee, String> nameLens = Lens.of(Employee::getName, Employee::withName);
Employee pepe = new Employee("pepe");
assertEquals("pepe", nameLens.get(pepe));
assertEquals("paco", nameLens.get(nameLens.set(pepe, "paco")));We can compose Lenses to get deeper inside. For example, if we add an attribute of type Address into Employee.
We can create a lens to access the city name of the address of the employee.
Lens<Employee, Address> addressLens = Lens.of(Employee::getAddress, Employee::withAddress);
Lens<Address, String> cityLens = Lens.of(Address::getCity, Address::withCity);
Lens<Employee, String> cityAddressLens = addressLens.compose(cityLens);
Employee pepe = new Employee("pepe", new Address("Madrid"));
assertEquals("Madrid", cityAddressLens.get(pepe));A Prism is a lossless invertible optic that can see into a structure and optionally find a value.
Function1<String, Option<Integer>> parseInt = ...; // is a method that only returns a value when the string can be parsed
Prism<String, Integer> stringToInteger = Prism.of(parseInt, String::valueOf);
assertEquals(Option.some(5), stringToInteger.getOption("5"));
assertEquals(Option.none(), stringToInteger.getOption("a"));
assertEquals("5", stringToInteger.reverseGet(5));An Optional is an optic that allows to see into a structure and getting, setting like a Lens an optional find a value like a Prism.
Optional<Employee, Address> addressOptional = Optional.of(
Employee::withAddress, employee -> Option.of(employee::getAddress)
);
Address madrid = new Address("Madrid");
Employee pepe = new Employee("pepe", null);
assertEquals(Option.none(), addressOptional.getOption(pepe));
assertEquals(Option.some(madrid), addressOptional.getOption(addressOptional.set(pepe, madrid)));| Optional | Prism | Lens | Iso | |
|---|---|---|---|---|
| Optional | Optional | Optional | Optional | Optional |
| Prism | Optional | Prism | Optional | Prism |
| Lens | Optional | Optional | Lens | Lens |
| Iso | Optional | Prism | Lens | Iso |
This class helps to create readable equals methods. An example:
@Override
public boolean equals(Object obj) {
return Equal.<Data>of()
.comparing(Data::getId)
.comparing(Data::getValue)
.applyTo(this, obj);
}
purefun is released under MIT license
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