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README.md |
Mon
Wrapper, TypeAlias, Maybe, Result, Tree, Lazy, Either and Combinators for Java.
The documentation below is being slowly migrated to Javadoc.
- Maven Usage
- Wrapper - light-weight type-alias-like
- TypeAlias - type-alias-like monadic wrapper
- Maybe - Maybe, Just or Nothing
- Result - Result, Success or Err
- Tree - generic trees
- Lazy - lazy evaluation
- Either - Either, Left or Right
- Combinators - Before, After or Around
Maven Usage
<dependency>
<groupId>net.kemitix</groupId>
<artifactId>mon</artifactId>
<version>${mon.version}</version>
</dependency>
Wrapper
A simple @FunctionalInterface
that contains a value. Can be used to implement
a form of type-alias in Java.
In Haskell, it is possible to create an alias for a Type, and to then use that alias with the same behaviour as the original, except that the compiler doesn't treat the alias as the same Type and will generate compiler errors if you try to use them together. e.g.:
newtype PhoneNumber = PhoneNumber String
newtype Name = Name String
newtype PhoneBookEntry = PhoneBookEntry (Name, PhoneNumber)
newtype PhoneBook = PhoneBook [PhoneBookEntry]
In Java, we don't have the ability to have that true alias, so Wrapper
simply
wraps the value within a new type. It's as close as I could get to a Haskell
type alias in Java.
The benefits of using Wrapper
in this way are:
- encapsulation of the wrapped type when passing references through code that doesn't need to access the actual value, but only to pass it on
- type-safe parameters where you would otherwise be passing generic
String
s,Integer
s,List
s, or other general classes - less verbose than implementing your own
Example
interface PhoneNumber extends Wrapper<String> {}
PhoneNumber pn = () -> "01234 567890";
String v = pn.value();
Instance Methods
T value()
Returns the value in the Wrapper
.
Skip the Mon import
If the only thing you want is Wrapper
, you can skip importing the mon
dependency by declaring your types like so:
interface PhoneNumber {String value();}
This is functionally identical to the example above using Wrapper
.
TypeAlias
This was a precursor to Wrapper
and should be considered deprecated. It is
also a form of wrapper, but is also a Monadic wrapper, unlike Wrapper
.
Example
class PhoneNumber extends TypeAlias<String> {
private PhoneNumber(final String value) {
super(value);
}
public static PhoneNumber of(final String phoneNumber) {
return new PhoneNumber(phoneNumber);
}
}
PhoneNumber pn = PhoneNumber.of("01234 567890");
String v = pn.getValue();
Instance Methods
final <R> R map(final Function<T, R> f)
Map the TypeAlias
into another value.
StudentId studentId = StudentId.of(123);
String idString = studentId.map(id -> String.valueOf(id));
class StudentId extends TypeAlias<Integer> {
private StudentId(Integer value) {
super(value);
}
static StudentId of(Integer id) {
return new StudentId(id);
}
}
final <R, U extends TypeAlias<R>> U flatMap(final Function<T, U> f)
Map the TypeAlias
into another TypeAlias
.
StudentId studentId = StudentId.of(123);
StudentName studentName = studentId.flatMap(id -> getStudentName(id));
class StudentName extends TypeAlias<String> {
private StudentName(String value) {
super(value);
}
static StudentName of(final String name) {
return new StudentName(name);
}
}
T getValue()
Get the value of the TypeAlias
.
String name = studentName.getValue();
Maybe
Allows specifying that a value may or may not be present. Similar to
Optional
. Maybe
provides additional methods that Optional
doesn't (as of
Java 8): isNothing()
, stream()
, ifNothing()
and match()
. Maybe
does
not have a get()
method.
Maybe
is a Monad.
Unlike Optional
, when a map()
results in a null
, the Maybe
will
continue to be a Just
(i.e. have a value - that value is null
). Optional
would switch to being empty.
Example
import net.kemitix.mon.maybe.Maybe;
import java.util.function.Function;
import java.util.function.Predicate;
class MaybeExample {
public static void main(String[] args) {
Maybe.just(countArgs(args))
.filter(isEven())
.map(validMessage())
.match(
just -> System.out.println(just),
() -> System.out.println("Not an valid value")
);
}
private static Function<Integer, String> validMessage() {
return v -> String.format("Value %d is even", v);
}
private static Predicate<Integer> isEven() {
return v -> v % 2 == 0;
}
private static Integer countArgs(String[] args) {
return args.length;
}
}
In the above example, the number of command line arguments are counted, if
there are an even number of them then a message is created and printed by
the Consumer
parameter in the match
call. If there is an odd number of
arguments, then the filter will return Maybe.nothing()
, meaning that the
nothing
drops straight through the map
and triggers the Runnable
parameter
in the match
call.
Static Constructors
static <T> Maybe<T> maybe(T value)
Create a Maybe for the value that may or may not be present.
Where the value is null
, that is taken as not being present.
Maybe<Integer> just = Maybe.maybe(1);
Maybe<Integer> nothing = Maybe.maybe(null);
static <T> Maybe<T> just(T value)
Create a Maybe
for the value that is present.
The value
must not be null
or a NullPointerException
will be thrown.
Maybe<Integer> just = Maybe.just(1);
static <T> Maybe<T> nothing()
Create a Maybe
for a lack of a value.
Maybe<Integer> nothing = Maybe.nothing();
static <T> Maybe<T> findFirst(Stream<T> stream)
Creates a Maybe
from the first item in the stream, or nothing if the stream
is empty.
Maybe<Integer> just3 = Maybe.findFirst(Stream.of(3, 4, 2, 4));
Maybe<Integer> nothing = Maybe.findFirst(Stream.empty());
Instance Methods
Maybe<T> filter(Predicate<T> predicate)
Filter a Maybe by the predicate, replacing with Nothing when it fails.
Maybe<Integer> maybe = Maybe.maybe(getValue())
.filter(v -> v % 2 == 0);
<R> Maybe<R> map(Function<T,R> f)
Applies the function to the value within the Maybe
, returning the result
within another Maybe
.
Maybe<Integer> maybe = Maybe.maybe(getValue())
.map(v -> v * 100);
<R> Maybe<R> flatMap(Function<T,Maybe<R>> f)
Applies the function to the value within the Maybe
, resulting in another
Maybe
, then flattens the resulting Maybe<Maybe<T>>
into Maybe<T>
.
Maybe<Integer> maybe = Maybe.maybe(getValue())
.flatMap(v -> Maybe.maybe(getValueFor(v)));
void match(Consumer<T> just, Runnable nothing)
Matches the Maybe
, either to just
or nothing
, and performs either the
Consumer
, for a Just
value, or the Runnable
for a Nothing
value.
Maybe.maybe(getValue())
.match(
just -> workWithValue(just),
() -> nothingToWorkWith()
);
<R> R matchValue(Function<T, R> justMatcher, Supplier<R> nothingMatcher)
Matches the Maybe
, either Just
or Nothing
, and performs either the
Function
, for a Just
value, or the Supplier
for a Nothing
value,
returning the result.
String value = Maybe.maybe(getValue())
.matchValue(
just -> Integer.toString(just),
() -> "nothing"
);
T orElse(T otherValue)
A value to use when the Maybe
is Nothing
.
Integer value = Maybe.maybe(getValue())
.orElse(1);
T orElseGet(Supplier<T> otherValueSupplier)
Provide a value to use when the Maybe
is Nothing
.
Integer value = Maybe.maybe(getValue())
.orElseGet(() -> getDefaultValue());
T or(Supplier<Maybe<T> alternative)
Provide an alternative Maybe
to use when the Maybe
is Nothing
.
Maybe<Integer> value = Maybe.maybe(getValue())
.or(() -> Maybe.just(defaultValue));
void orElseThrow(Supplier<Exception> error)
Throw the exception if the Maybe
is Nothing
.
Integer value = Maybe.maybe(getValue())
.orElseThrow(() -> new RuntimeException("error"));
Maybe<T> peek(Consumer<T> consumer)
Provide the value within the Maybe
, if it exists, to the Consumer
, and
returns the original Maybe
.
Maybe<Integer> maybe = Maybe.maybe(getValue())
.peek(v -> v.foo());
void ifNothing(Runnable runnable)
Run the Runnable
if the Maybe
is Nothing
, otherwise do nothing.
Maybe.maybe(getValue())
.ifNothing(() -> doSomething());
Stream<T> stream()
Converts the Maybe
into either a single value stream or an empty stream.
Stream<Integer> stream = Maybe.maybe(getValue())
.stream();
boolean isJust()
Checks if the Maybe
is a Just
.
boolean isJust = Maybe.maybe(getValue())
.isJust();
boolean isNothing()
Checks if the Maybe
is Nothing
.
boolean isNothing = Maybe.maybe(getValue())
.isNothing();
Optional<T> toOptional()
Convert the Maybe
to an Optional
.
Optional<Integer> optional = Maybe.maybe(getValue())
.toOptional();
Tree
A Generalised tree, where each node may or may not have an item, and may have any number of sub-trees. Leaf nodes are Trees with zero sub-trees.
Static Constructors
static <R> Tree<R> leaf(R item)
Create a leaf containing the item. The leaf has no sub-trees.
Tree<String> tree = Tree.leaf("item");
static<R> Tree<R> of(R item, Collection<Tree<R>> subtrees)
Create a tree containing the item and sub-trees.
Tree<String> tree = Tree.of("item", Collections.singletonList(Tree.leaf("leaf"));
static <B> TreeBuilder<B> builder(final Class<B> type)
Create a new TreeBuilder
starting with an empty tree.
TreeBuilder<Integer> builder = Tree.builder(Integer.class);
static <B> TreeBuilder<B> builder(final Tree<B> tree)
Create a new TreeBuilder
for the given tree.
Tree<Integer> tree = ...;
TreeBuilder<Integer> builder = Tree.builder(tree);
Instance Methods
<R> Tree<R> map(Function<T, R> f)
Applies the function to the item within the Tree
and to all sub-trees,
returning a new Tree
.
Tree<UUID> tree = ...;
Tree<String> result = tree.map(UUID::toString);
Maybe<T> item()
Returns the contents of the Tree
node within a Maybe
.
Tree<Item> tree = ...;
Maybe<Item> result = tree.item();
int count()
Returns the total number of items in the Tree
, including sub-trees. Null
items don't count.
Tree<Item> tree = ...;
int result = tree.count();
List<Tree<T> subTrees()
Returns a list of sub-trees within the Tree
.
Tree<Item> tree = ...;
List<Tree<Item>> result = tree.subTrees();
TreeBuilder
A mutable builder for a Tree
. Each TreeBuilder
allows modification of a
single Tree
node. You can use the select(childItem)
method to get a
TreeBuilder
for the subtree that has the given child item.
Example
TreeBuilder<Integer> builder = Tree.builder();
builder.set(12).addChildren(Arrays.asList(1, 3, 5, 7));
TreeBuilder<Integer> builderFor3 = builder.select(3);
builderFor3.addChildren(Arrays.asList(2, 4));
Tree<Integer> tree = builder.build();
Will produce a Tree
like:
Static Constructors
None. The TreeBuilder
is instantiated by Tree.builder()
.
Instance Methods
Tree<T> build()
Create the immutable Tree.
TreeBuilder<Integer> builder = Tree.builder();
Tree<Integer> tree = builder.build();
TreeBuilder<T> item(T item)
Set the current Tree
's item and return the TreeBuilder
.
TreeBuilder<T> add(Tree<T> subtree)
Adds the subtree to the current tree.
TreeBuilder<T> addChild(T childItem)
Add the Child item as a sub-Tree.
TreeBuilder<T> addChildren(List<T> children)
Add all the child items as subTrees.
Maybe<TreeBuilder<T>> select(T childItem)
Create a TreeBuilder
for the subTree of the current Tree
that has the
childItem.
Lazy
A lazily evaluated expression. Using a Supplier
to provide the value, only
evaluates the value when required, and never more than once.
Static Constructors
static <R> Lazy<R> of(Supplier<R> supplier)
Create a new Lazy
value from the Supplier
.
Suppler<UUID> supplier = ...;
Lazy<UUID> lazy = Lazy.of(supplier);
Instance Methods
boolean isEvaluated()
Checks if the value has been evaluated.
Lazy<UUID> lazy = ...;
boolean isEvaluated = lazy.isEvaluated();
T value()
The value, evaluating it if necessary.
Lazy<UUID> lazy = ...;
UUID value = lazy.value();
<R> Lazy<R> map(Function<T, R> f)
Maps the Lazy
instance into a new Lazy
instance using the Function
.
Lazy<UUID> uuidLazy = ...;
Lazy<String> stringLazy = uuidLazy.map(v -> v.toString());
Either
Allows handling a value that can be one of two types, a left value/type, or a right value/type.
Either
is not a Monad.
When an Either
is returned from a method it will contain either a left or a
right.
Where the Either
is used to represent success/failure, the left case is, by
convention, used to indicate the error, and right the success. An alternative
is to use the Result
which more clearly distinguishes success from failure.
Static Constructors
static <L, R> Either<L, R> left(final L l)
Create a new Either
holding a left value.
Either<Integer, String> left = Either.left(getIntegerValue());
static <L, R> Either<L, R> right(final R r)
Create a new Either
holding a right value.
Either<Integer, String> right = Either.right(getStringValue());
Instance Methods
boolean isLeft()
Checks if the Either
holds a left value.
Either<Integer, String> either = Either.left(getIntegerValue());
boolean leftIsLeft = either.isLeft();
boolean rightIsLeft = either.isLeft();
boolean isRight()
Checks if the Either
holds a right value.
Either<Integer, String> either = Either.left(getIntegerValue());
boolean leftIsRight = either.isRight();
boolean rightIsRight = either.isRight();
void match(Consumer<L> onLeft, Consumer<R> onRight)
Matches the Either
, invoking the correct Consumer
.
Either<Integer, String> either = Either.left(getIntegerValue());
either.match(
left -> handleIntegerValue(left),
right -> handleStringValue(right)
);
<T> Either<T, R> mapLeft(Function<L, T> f)
Map the Function
across the left value.
Either<Integer, String> either = Either.left(getIntegerValue());
Either<Double, String> either = either.mapLeft(i -> i.doubleValue());
<T> Either<L, T> mapRight(Function<R, T> f)
Map the function across the right value.
Either<Integer, String> either = Either.left(getIntegerValue());
Either<Integer, String> either = either.mapRight(s -> s + "x");
<T> Either<T, R> flatMapLeft(Function<L, Either<T, R>> f)
FlatMap the function across the left value.
Either<Integer, String> either = Either.left(2);
Either<Integer, String> resultLeft = either.flatMapLeft(l -> Either.left(l * 2));
Either<Integer, String> resultRight = either.flatMapLeft(l -> Either.right(l * 2));
<T> Either<T, R> flatMapRight(Function<L, Either<T, R>> f)
FlatMap the function across the right value.
Either<Integer, String> either = Either.right("2");
Either<Integer, String> resultLeft = either.flatMapRight(l -> Either.left(l * 2));
Either<Integer, String> resultRight = either.flatMapRight(l -> Either.right(l * 2));
Optional<L> getLeft()
Returns an Optional
containing the left value, if is a left, otherwise returns
an empty Optional
.
Either<Integer, String> either = Either.right("2");
Optional<Integer> left = either.getLeft();
Optional<R> getRight()
Returns an Optional
containing the right value, if is a right, otherwise
returns an empty Optional
.
Either<Integer, String> either = Either.right("2");
Optional<String> right = either.getRight();
Combinators
Taken from The Bounds of Java Newsletter #3, although the associated article isn't online anymore.
After
Attach a BiConsumer
to a Function
, so that when the Function
is called,
the BiConsumer
is called afterwards, receiving the original argument to the
Function
plus the result.
Example
BiConsumer<BigDecimal, String> after =
(amount, result) ->
System.out.println("Amount was " + amount + ", Result is " + result);
var tax = BigDecimal.valueOf("1.22");
Function<BigDecimal, String> addTax =
amount -> "$" + amount.multiply(tax);
Function<BigDecimal, String> addTaxDecorated =
After.decorate(addTax, after);
var amount = BigDecimal.valueOf("1000");
String result = addTaxDecorated.apply(amount);
static <T, R> Function<T, R> After.decorate(Function<T, R> function, BiConsumer<T, R> after)
Creates a new decorated Function
.
Before
Attach a Consumer
to a Function
, so that when the Function
is called,
the Consumer
is called first, receiving the argument to the Function
.
Example
Consumer<BigDecimal> before =
amount -> System.out.println("Amount is " + amount);
var tax = BigDecimal.valueOf("1.22");
Function<BigDecimal, String> addTax =
amount -> "$" + amount.multiply(tax);
Function<BigDecimal, String> addTaxDecorated =
Before.decorate(before, addTax);
var amount = BigDecimal.valueOf("1000");
String result = addTaxDecorated.apply(amount);
static <T, R> Function<T, R> decorate(Consumer<T> before, Function<T, R> function)
Creates a new decorated Function
.
Around
Attach a BiConsumer
to a Function
, so that when the Function
is called,
the BiConsumer
is called with an Around.Executable
that will invoke the Function
.
The BiConsumer
is responsible for calling execute()
on the Around.Executable
in
order to invoke the Function
.
The BiConsumer
can perform actions before and after calling execute()
on the
Around.Executable
.
Example
BiConsumer<Around.Executable<String>, BigDecimal> around =
(function, amount) -> {
System.out.println("Amount is " + amount);
var result = function.execute(); // INVOKE THE FUNCTION
System.out.println("Result is " + result");
};
var tax = BigDecimal.valueOf("1.22");
Function<BigDecimal, String> addTax =
amount -> "$" + amount.multiply(tax);
Function<BigDecimal, String> addTaxDecorated =
Around.decorate(addTax, around);
var amount = BigDecimal.valueOf("1000");
String result = addTaxDecorated.apply(amount);
static <T, R> Function<T, R> decorate(final Function<T, R> function, final BiConsumer<Executable<R>, T> around)
Creates a new decorated Function
.