java11-vavr093-option-workshop

project description

• https://www.vavr.io/vavr-docs/#_option
• https://static.javadoc.io/io.vavr/vavr/0.9.3/io/vavr/control/Option.html
• https://github.com/mtumilowicz/java11-vavr-option
• https://github.com/mtumilowicz/java11-category-theory-optional-is-not-functor
• in the workshop we will try to fix failing tests test/groovy/Workshop
• answers: test/groovy/Answers (same tests as in Workshop but correctly solved)

theory in a nutshell

• similar to Optional, but with bigger, more flexible API
• interface Option<T> extends Value<T>, Serializable
  • interface Value<T> extends Iterable<T>
• Option is isomorphic to singleton list (either has element or not, so it could be treated as collection)
• two implementations:
  • final class Some<T>
  • final class None<T>
• None instance is a singleton
• returning null inside map does not cause Option switch to None (contrary to Optional)

  expect:
  Option.some(2).map(alwaysNull).defined
  Optional.of(2).map(alwaysNull).empty
  

• it is possible to have Some(null), so NPE is possible even on defined Option
• excellent for modelling exists / not exists (Spring Data - findById)
  • not every "exceptional" behaviour could be modelled as exists / not exists

conclusions in a nutshell

• we omit methods that Optional has
• easy conversion Option<T> <-> Optional<T>
  • Option.ofOptional
  • option.toJavaOptional()
• handy conversion List<Option<T>> -> Option<List<T>>
  • Option.sequence(list)
  • if any of the Options are None - returns None
• conditional supplier
  • static Option<T> when(boolean condition, Supplier<? extends T> supplier)
• mapping with the partial function
  • Option<R> collect(PartialFunction<? super T, ? extends R> partialFunction)
  • if function is not defined for the value - returns None
• side effects on None
  • Option<T> onEmpty(Runnable action)
• side effects on Some
  • Option<T> peek(Consumer<? super T> action)
• lazy alternative (in Optional since 11)
  • Option<T> orElse(Supplier<? extends Option<? extends T>> supplier)
• transform function could be nearly always replaced with more expressive and natural map(...).orElse(...)

  given:
  Function<Option<Integer>, String> transformer = { it.isEmpty() ? '' : it.get().toString() }
  expect:
  Option.of(5).transform(transformer) == '5'
  

  same as

  given:
  Function<Option<Integer>, String> transformer = { it.isEmpty() ? '' : it.get().toString() }
  expect:
  transformerFive = Option.of(5)
      .map { it.toString() }
      .orElse('') == '5'
  

• conversion Option<T> <-> List<T>
  • option.toList()
  • list.toOption()
• could be treated as collection
  • boolean contains(T element)
  • boolean exists(Predicate<? super T> predicate)
  • boolean forAll(Predicate<? super T> predicate)
• well written equals
  • Some

    return (obj == this) || (obj instanceof Some && Objects.equals(value, ((Some<?>) obj).value));
    

  • None

    return o == this
    

• map does not breaks monadic laws

  given:
  Function<Integer, Integer> alwaysNull = { null }
  Function<Integer, String> safeToString = { nonNull(it) ? String.valueOf(it) : 'null' }
  Function<Integer, String> composition = alwaysNull.andThen(safeToString)
  
  expect:
  Optional.of(1).map(composition) != Optional.of(1).map(alwaysNull).map(safeToString)
  Optional.of(1).stream().map(composition).findAny() == Optional.of(1).stream()
          .map(alwaysNull)
          .map(safeToString)
          .findAny()
  Option.of(1).map(composition) == Option.of(1).map(alwaysNull).map(safeToString)