mon/README.org

* Mon

** TypeAlias, Maybe and Result for Java.

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** Maven

#+BEGIN_SRC xml
    <dependency>
        <groupId>net.kemitix</groupId>
        <artifactId>mon</artifactId>
        <version>RELEASE</version>
    </dependency>
#+END_SRC

    The latest version should be shown above with the nexus and maven-central
    badges or can be found on [[https://search.maven.org/#search%7Cga%7C1%7Cg%3A%22net.kemitix%22%20AND%20a%3A%22mon%22][Maven Central]].


** TypeAlias

   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 and use them together. e.g.:

   #+BEGIN_SRC haskell
   type PhoneNumber = String
   type Name = String
   type PhoneBook = [(Name,PhoneNumber)]
   #+END_SRC

   In Java we don't have the ability to have that true alias, so TypeAlias is
   more of a type-wrapper. It's as close as I could get to a Haskell type alias
   in Java.

   The benefits of using TypeAlias 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 Strings,
     Integers, Lists, or other general classes
   - equality and hashcode
   - less verbose than implementing your own

   *TypeAlias Example:*

   #+BEGIN_SRC java
   class PhoneNumber extends TypeAlias<String> {
       private PhoneNumber(final String value) {
           super(value);
       }
       public static PhoneNumber of(final String phoneNumber) {
           return new PhoneNumber(phoneNumber);
       }
   }
   #+END_SRC

   *Roll your own:*

   #+BEGIN_SRC java
   class PhoneNumber {
       private final String value;
       private PhoneNumber(final String value) {
           this.value = value;
       }
       public static PhoneNumber of(final String phoneNumber) {
           return new PhoneNumber(phoneNumber);
       }
       @Override
       public boolean equals(Object o) {
           if (this == o) return true;
           if (o == null || getClass() != o.getClass()) return false;
           PhoneNumber that = (PhoneNumber) o;
           return Objects.equals(value, that.value);
       }
       @Override
       public int hashCode() {
           return Objects.hash(value);
       }
       public String getValue() {
           return value;
       }
   }
   #+END_SRC

   *Lombok:*

   Although, if you are using Lombok, that can be equally terse, both it and
   TypeAlias<String> coming in at 8 lines each, compared to 24 for rolling your
   own:

   #+BEGIN_SRC java
   @Value
   @RequiredArgsConstructor(access = AccessLevel.PRIVATE)
   class PhoneNumber {
       private final String value;
       public static PhoneNumber of(final String phoneNumber) {
           return new PhoneNumber(phoneNumber);
       }
   }
   #+END_SRC

*** =TypeAlias= *can* be a Monad

    #+BEGIN_SRC java
    package net.kemitix.mon;

    import org.assertj.core.api.WithAssertions;
    import org.junit.Test;

    import java.util.function.Function;

    public class TypeAliasMonadTest implements WithAssertions {

        private final int v = 1;
        private final Function<Integer, AnAlias<Integer>> f = i -> a(i * 2);
        private final Function<Integer, AnAlias<Integer>> g = i -> a(i + 6);

        private static AnAlias<Integer> a(Integer v) {
            return AnAlias.of(v);
        }

        @Test
        public void leftIdentity() {
            assertThat(
                a(v).flatMap(f)
            ).isEqualTo(
                f.apply(v)
            );
        }

        @Test
        public void rightIdentity_inline() {
            // java isn't able to properly infer the correct types when used in-line
            assertThat(
                a(v).<Integer, AnAlias<Integer>>flatMap(x -> a(x))
            ).isEqualTo(
                a(v)
            );
        }

        @Test
        public void rightIdentity_explicitValue() {
            final AnAlias<Integer> integerAnAlias = a(v).flatMap(x -> a(x));
            assertThat(
                integerAnAlias
            ).isEqualTo(
                a(v)
            );
        }

        @Test
        public void associativity() {
            assertThat(
                a(v).flatMap(f).flatMap(g)
            ).isEqualTo(
                a(v).flatMap(x -> f.apply(x).flatMap(g))
            );
        }

        static class AnAlias<T> extends TypeAlias<T> {
            private AnAlias(T value) {
                super(value);
            }

            static <T> AnAlias<T> of(T value) {
                return new AnAlias<>(value);
            }
        }
    }
    #+END_SRC


*** Instance Methods

**** =final <R> R map(final Function<T, R> f)=

     Map the TypeAlias into another value.

     #+BEGIN_SRC java
     final StudentId studentId = StudentId.of(123);
     final 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);
         }
     }
     #+END_SRC


**** =final <R, U extends TypeAlias<R>> U flatMap(final Function<T, U> f)=

     Map the TypeAlias into another TypeAlias.

     #+BEGIN_SRC java
     final StudentId studentId = StudentId.of(123);
     final 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);
         }
     }
     #+END_SRC


**** =T getValue()=

     Get the value of the TypeAlias.

     #+BEGIN_SRC java
     final String name = studentName.getValue();
     #+END_SRC


** Maybe

   Allows specifying that a value may or may not be present. Similar to
   =Optional=. =Maybe= provides additional methods that =Optional= doesn't:
   =isNothing()=, =stream()=, =ifNothing()= and =match()=. =Maybe= does not
   have a =get()= method.

   Unlike =Optional=, when a =map()= results in a =null=, the =Maybe= will
   continue to be a =Just=. =Optional= would switch to being empty. [[http://blog.vavr.io/the-agonizing-death-of-an-astronaut/][vavi.io
   follows the same behaviour as =Maybe=]].

   #+BEGIN_SRC java
   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;
       }
   }
   #+END_SRC

   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.

*** =Maybe= is a Monad:

  #+BEGIN_SRC java
  package net.kemitix.mon;

  import net.kemitix.mon.maybe.Maybe;
  import org.assertj.core.api.WithAssertions;
  import org.junit.Test;

  import java.util.function.Function;

  public class MaybeMonadTest implements WithAssertions {

      private final int v = 1;
      private final Function<Integer, Maybe<Integer>> f = i -> m(i * 2);
      private final Function<Integer, Maybe<Integer>> g = i -> m(i + 6);

      private static Maybe<Integer> m(int value) {
          return Maybe.maybe(value);
      }

      @Test
      public void leftIdentity() {
          assertThat(
                  m(v).flatMap(f)
          ).isEqualTo(
                  f.apply(v)
          );
      }

      @Test
      public void rightIdentity() {
          assertThat(
                  m(v).flatMap(x -> m(x))
          ).isEqualTo(
                  m(v)
          );
      }

      @Test
      public void associativity() {
          assertThat(
                  m(v).flatMap(f).flatMap(g)
          ).isEqualTo(
                  m(v).flatMap(x -> f.apply(x).flatMap(g))
          );
      }

  }
  #+END_SRC


*** 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.

     #+BEGIN_SRC java
     final Maybe<Integer> just = Maybe.maybe(1);
     final Maybe<Integer> nothing = Maybe.maybe(null);
     #+END_SRC


**** =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.
     If you can't prove that the value won't be =null= you should use
     =Maybe.maybe(value)= instead.

     #+BEGIN_SRC java
     final Maybe<Integer> just = Maybe.just(1);
     #+END_SRC


**** =static <T> Maybe<T> nothing()=

     Create a Maybe for a lack of a value.

     #+BEGIN_SRC java
     final Maybe<Integer> nothing = Maybe.nothing();
     #+END_SRC


*** Instance Methods

**** =Maybe<T> filter(Predicate<T> predicate)=

     Filter a Maybe by the predicate, replacing with Nothing when it fails.

     #+BEGIN_SRC java
     final Maybe<Integer> maybe = Maybe.maybe(getValue())
                                       .filter(v -> v % 2 == 0);
     #+END_SRC


**** =<R> Maybe<R> map(Function<T,R> f)=

     Applies the function to the value within the Maybe, returning the result within another Maybe.

     #+BEGIN_SRC java
     final Maybe<Integer> maybe = Maybe.maybe(getValue())
                                       .map(v -> v * 100);
     #+END_SRC


**** =<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>=.

     Monad binder maps the Maybe into another Maybe using the binder method f

     #+BEGIN_SRC java
     final Maybe<Integer> maybe = Maybe.maybe(getValue())
                                       .flatMap(v -> Maybe.maybe(getValueFor(v)));
     #+END_SRC


**** =void match(Consumer<T> just, Runnable nothing)=

     Matches the Maybe, either just or nothing, and performs either the Consumer, for Just, or Runnable for nothing.

     #+BEGIN_SRC java
     Maybe.maybe(getValue())
          .match(
              just -> workWithValue(just),
                () -> nothingToWorkWith()
          );
     #+END_SRC


**** =T orElse(T otherValue)=

     A value to use when Maybe is Nothing.

     #+BEGIN_SRC java
     final Integer value = Maybe.maybe(getValue())
                                .orElse(1);
     #+END_SRC


**** =T orElseGet(Supplier<T> otherValueSupplier)=

     Provide a value to use when Maybe is Nothing.

     #+BEGIN_SRC java
     final Integer value = Maybe.maybe(getValue())
                                .orElseGet(() -> getDefaultValue());
     #+END_SRC


**** =T or(Supplier<Maybe<T> alternative)=

     Provide an alternative Maybe to use when Maybe is Nothing.

     #+BEGIN_SRC java
     final Maybe<Integer> value = Maybe.maybe(getValue())
                                       .or(() -> Maybe.just(defaultValue));
     #+END_SRC


**** =void orElseThrow(Supplier<Exception> error)=

     Throw the exception if the Maybe is a Nothing.

     #+BEGIN_SRC java
     final Integer value = Maybe.maybe(getValue())
                                .orElseThrow(() -> new RuntimeException("error"));
     #+END_SRC


**** =Maybe<T> peek(Consumer<T> consumer)=

     Provide the value within the Maybe, if it exists, to the Consumer, and returns this Maybe. Conceptually equivalent to the idea of =ifPresent(...)=.

     #+BEGIN_SRC java
     final Maybe<Integer> maybe = Maybe.maybe(getValue())
                                       .peek(v -> v.foo());
     #+END_SRC


**** =void ifNothing(Runnable runnable)=

     Run the runnable if the Maybe is a Nothing, otherwise do nothing.

     #+BEGIN_SRC java
     Maybe.maybe(getValue())
          .ifNothing(() -> doSomething());
     #+END_SRC


**** =Stream<T> stream()=

     Converts the Maybe into either a single value stream or an empty stream.

     #+BEGIN_SRC java
     final Stream<Integer> stream = Maybe.maybe(getValue())
                                         .stream();
     #+END_SRC


**** =boolean isJust()=

     Checks if the Maybe is a Just.

     #+BEGIN_SRC java
     final boolean isJust = Maybe.maybe(getValue())
                                 .isJust();
     #+END_SRC


**** =boolean isNothing()=

     Checks if the Maybe is Nothing.

     #+BEGIN_SRC java
     final boolean isNothing = Maybe.maybe(getValue())
                                    .isNothing();
     #+END_SRC


**** =Optional<T> toOptional()=

     Convert the Maybe to an Optional.

     #+BEGIN_SRC java
     final Optional<Integer> optional = Maybe.maybe(getValue())
                                             .toOptional();
     #+END_SRC


** Result

   Allows handling error conditions without the need to catch exceptions.

   When a =Result= is returned from a method it will contain one of two values.
   Either the actual result, or an error in the form of an =Exception=. The
   exception is returned within the =Result= and is not thrown.

   #+BEGIN_SRC java
   import net.kemitix.mon.result.Result;

   import java.io.IOException;

   class ResultExample implements Runnable {

       public static void main(final String[] args) {
           new ResultExample().run();
       }

       @Override
       public void run() {
           Result.of(() -> callRiskyMethod())
                 .flatMap(state -> doSomething(state))
                 .match(
                     success -> System.out.println(success),
                     error -> error.printStackTrace()
                 );
       }

       private String callRiskyMethod() throws IOException {
           return "I'm fine";
       }

       private Result<String> doSomething(final String state) {
           return Result.of(() -> state + ", it's all good.");
       }

   }
   #+END_SRC

   In the above example the string ="I'm fine"= is returned by
   =callRiskyMethod()= within a successful =Result=. The =.flatMap()= call,
   unwraps that =Result= and, as it is a success, passes the contents to
   =doSomething()=, which in turn returns a =Result= that the =.flatMap()= call
   returns. =match()= is called on the =Result= and, being a success, will call
   the success =Consumer=.

   Had =callRiskyMethod()= thrown an exception it would have been caught by the
   =Result.of()= method which would have then been an error =Result=. An error
   Result would have ignored the =flatMap= and skipped to the =match()= when it
   would have called the error =Consumer=.

*** =Result= is a Monad

    #+BEGIN_SRC java
    package net.kemitix.mon;

    import net.kemitix.mon.result.Result;
    import org.assertj.core.api.WithAssertions;
    import org.junit.Test;

    import java.util.function.Function;

    public class ResultMonadTest implements WithAssertions {

        private final int v = 1;
        private final Function<Integer, Result<Integer>> f = i -> r(i * 2);
        private final Function<Integer, Result<Integer>> g = i -> r(i + 6);

        private static Result<Integer> r(int v) {
            return Result.ok(v);
        }

        @Test
        public void leftIdentity() {
            assertThat(
                    r(v).flatMap(f)
            ).isEqualTo(
                    f.apply(v)
            );
        }

        @Test
        public void rightIdentity() {
            assertThat(
                    r(v).flatMap(x -> r(x))
            ).isEqualTo(
                    r(v)
            );
        }

        @Test
        public void associativity() {
            assertThat(
                    r(v).flatMap(f).flatMap(g)
            ).isEqualTo(
                    r(v).flatMap(x -> f.apply(x).flatMap(g))
            );
        }

    }
    #+END_SRC


*** Static Constructors

**** =static <T> Result<T> of(Callable<T> callable)=

     Create a Result for a output of the Callable.

     If the Callable throws and Exception, then the Result will be an error and
     will contain that exception.

     This will be the main starting point for most Results where the callable
     could throw an =Exception=.

     #+BEGIN_SRC java
     final Result<Integer> okay = Result.of(() -> 1);
     final Result<Integer> error = Result.of(() -> {throw new RuntimeException();});
     #+END_SRC


**** =static <T> Result<T> ok(T value)=

     Create a Result for a success.

     Use this where you have a value that you want to place into the Result context.

     #+BEGIN_SRC java
     final Result<Integer> okay = Result.ok(1);
     #+END_SRC


**** =static <T> Result<T> error(Throwable error)=

     Create a Result for an error.

     #+BEGIN_SRC java
     final Result<Integer> error = Result.error(new RuntimeException());
     #+END_SRC


*** Static Methods

    These static methods provide integration with the =Maybe= class.

     #+BEGIN_SRC java
     #+END_SRC

**** =static <T> Maybe<T> toMaybe(Result<T> result)=

     Creates a =Maybe= from the =Result=, where the =Result= is a success, then
     the =Maybe= will contain the value. However, if the =Result= is an error
     then the =Maybe= will be nothing.

     #+BEGIN_SRC java
     final Result<Integer> result = Result.of(() -> getValue());
     final Maybe<Integer> maybe = Result.toMaybe(result);
     #+END_SRC


**** =static <T> Result<T> fromMaybe(Maybe<T> maybe, Supplier<Throwable> error)=

     Creates a =Result= from the =Maybe=, where the =Result= will be an error
     if the =Maybe= is nothing. Where the =Maybe= is nothing, then the
     =Supplier<Throwable>= will provide the error for the =Result=.

     #+BEGIN_SRC java
     final Maybe<Integer> maybe = Maybe.maybe(getValue());
     final Result<Integer> result = Result.fromMaybe(maybe, () -> new NoSuchFileException("filename"));
     #+END_SRC


**** =static <T> Result<Maybe<T>> invert(Maybe<Result<T>> maybeResult)=

     Swaps the =Result= within a =Maybe=, so that =Result= contains a =Maybe=.

     #+BEGIN_SRC java
     final Maybe<Result<Integer>> maybe = Maybe.maybe(Result.of(() -> getValue()));
     final Result<Maybe<Integer>> result = Result.invert(maybe);
     #+END_SRC


**** =static <T,R> Result<Maybe<R>> flatMapMaybe(Result<Maybe<T>> maybeResult, Function<Maybe<T>,Result<Maybe<R>>> f)=

     Applies the function to the contents of a Maybe within the Result.

     #+BEGIN_SRC java
     final Result<Maybe<Integer>> result = Result.of(() -> Maybe.maybe(getValue()));
     final Result<Maybe<Integer>> maybeResult = Result.flatMapMaybe(result, maybe -> Result.of(() -> maybe.map(v -> v * 2)));
     #+END_SRC


*** Instance Methods

**** =<R> Result<R> map(Function<T,R> f)=

     Applies the function to the value within the Functor, returning the result
     within a Functor.

     #+BEGIN_SRC java
     final Result<String> result = Result.of(() -> getValue())
                                         .map(v -> String.valueOf(v));
     #+END_SRC


**** =<R> Result<R> flatMap(Function<T,Result<R>> f)=

     Returns a new Result consisting of the result of applying the function to
     the contents of the Result.

     #+BEGIN_SRC java
     final Result<String> result = Result.of(() -> getValue())
                                         .flatMap(v -> Result.of(() -> String.valueOf(v)));
     #+END_SRC


**** =<R> Result<R> andThen(Function<T,Callable<R>> f)=

     Maps a Success Result to another Result using a Callable that is able to
     throw a checked exception.

     #+BEGIN_SRC java
     final Result<String> result = Result.of(() -> getValue())
                                         .andThen(v -> () -> {throw new IOException();});
     #+END_SRC


**** =void	match(Consumer<T> onSuccess, Consumer<Throwable> onError)=

     Matches the Result, either success or error, and supplies the appropriate
     Consumer with the value or error.

     #+BEGIN_SRC java
     Result.of(() -> getValue())
           .match(
               success -> System.out.println(success),
               error -> System.err.println("error")
           );
     #+END_SRC


**** =Result<T> recover(Function<Throwable,Result<T>> f)=

     Provide a way to attempt to recover from an error state.

     #+BEGIN_SRC java
     final Result<Integer> result = Result.of(() -> getValue())
                                          .recover(e -> Result.of(() -> getSafeValue(e)));
     #+END_SRC


**** =Result<T> peek(Consumer<T> consumer)=

     Provide the value within the Result, if it is a success, to the Consumer,
     and returns this Result.

     #+BEGIN_SRC java
     final Result<Integer> result = Result.of(() -> getValue())
                                          .peek(v -> System.out.println(v));
     #+END_SRC


**** =Result<T> thenWith(Function<T,WithResultContinuation<T>> f)=

     Perform the continuation with the current Result value then return the
     current Result, assuming there was no error in the continuation.

     #+BEGIN_SRC java
      final Result<Integer> result = Result.of(() -> getValue())
                                           .thenWith(v -> () -> System.out.println(v))
                                           .thenWith(v -> () -> {throw new IOException();});
     #+END_SRC


**** =Result<Maybe<T>> maybe(Predicate<T> predicate)=

     Wraps the value within the Result in a Maybe, either a Just if the
     predicate is true, or Nothing.

     #+BEGIN_SRC java
     final Result<Maybe<Integer>> result = Result.of(() -> getValue())
                                                 .maybe(v -> v % 2 == 0);
     #+END_SRC


**** =T orElseThrow()=

     Extracts the successful value from the result, or throws the error
     Throwable.

     #+BEGIN_SRC java
     final Integer result = Result.of(() -> getValue())
                                  .orElseThrow();
     #+END_SRC


**** =void onError(Consumer<Throwable> errorConsumer)=

     A handler for error states.

     #+BEGIN_SRC java
     Result.of(() -> getValue())
           .onError(e -> handleError(e));
     #+END_SRC


**** =boolean isOkay()=

     Checks if the Result is a success.

     #+BEGIN_SRC java
     final boolean isOkay = Result.of(() -> getValue())
                                  .isOkay();
     #+END_SRC


**** =boolean isError()=

       Checks if the Result is an error.

     #+BEGIN_SRC java
     final boolean isError = Result.of(() -> getValue())
                                   .isError();
     #+END_SRC


** Either

   Allows handling a value that can be one of two types, a left value/type or a
   right value/type.

   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.

*** =Either= *is not* a Monad.

*** Static Constructors

**** =static <L, R> Either<L, R> left(final L l)=

     Create a new Either holding a left value.

     #+BEGIN_SRC java
     final Either<Integer, String> left = Either.left(getIntegerValue());
     #+END_SRC


**** =static <L, R> Either<L, R> right(final R r)=

     Create a new Either holding a right value.

     #+BEGIN_SRC java
     final Either<Integer, String> right = Either.right(getStringValue());
     #+END_SRC


*** Instance Methods

**** =boolean isLeft()=

     Checks if the Either holds a left value.

     #+BEGIN_SRC java
     final boolean leftIsLeft = Either.<Integer, String>left(getIntegerValue()).isLeft();
     final boolean rightIsLeft = Either.<Integer, String>right(getStringValue()).isLeft();
     #+END_SRC


**** =boolean isRight()=

     Checks if the Either holds a right value.

     #+BEGIN_SRC java
     final boolean leftIsRight = Either.<Integer, String>left(getIntegerValue()).isRight();
     final boolean rightIsRight = Either.<Integer, String>right(getStringValue()).isRight();
     #+END_SRC


**** =void match(Consumer<L> onLeft, Consumer<R> onRight)=

     Matches the Either, invoking the correct Consumer.

     #+BEGIN_SRC java
     Either.<Integer, String>left(getIntegerValue())
           .match(
               left -> handleIntegerValue(left),
               right -> handleStringValue(right)
           );
     #+END_SRC


**** =<T> Either<T, R> mapLeft(Function<L, T> f)=

     Map the function across the left value.

     #+BEGIN_SRC java
     final Either<Double, String> either = Either.<Integer, String>left(getIntegerValue())
                                                 .mapLeft(i -> i.doubleValue());
     #+END_SRC


**** =<T> Either<L, T> mapRight(Function<R, T> f)=

     Map the function across the right value.

     #+BEGIN_SRC java
     final Either<Integer, String> either = Either.<Integer, String>left(getIntegerValue())
                                                  .mapRight(s -> s + "x");
     #+END_SRC