This is a work-in-progress java maven library to bring some of our beloved functional programming concepts to Java. Feel free to contribute. :)

Currently, the major features of Functjonal are:

• Simple Algebraic Structures: Functjonal provides interfaces for semigroups and monoids. These structures operate on a specific domain. For example, a Semigroup<A> is a binary composition operator for values of type A. By convention, any type that is a semigroup or monoid should provide a respective implementation as a public static final field with the name of the structure in capslock. For example:

  import Monoid;
  
  public record PassedHours(int hours, int minutes) {
    public static final Monoid<PassedHours> MONOID = Monoid.From(
      () -> new PassedHours(0, 0),
      (a, b) -> new PassedHours(
          a.hours() + b.hours() + ((a.minutes() + b.minutes()) / 60),
          (a.minutes() + b.minutes()) % 60
      )
    );
  }

  Then, you can use the monoid as such:

  List<PassedHours> hours = ...; // let's say you have a list of passed hours
  Monoid<PassedHours> m = PassedHours.MONOID;
  PassedHours result = hours.stream().reduce(m.neutral(), m::append);

  As java does not feature type classes and inheritance is insufficient (as for instance getting the neutral element from a monoid would require an instance of the monoid's data type), we decided to adopt this convention as it is also flexible to define multiple monoids over the same datatype (e.g., (0, +) and (1, *) for int). This convention also allows us to define monoids for higher-kinded types (which cannot be expressed in java). For example, Products are monoidal when their arguments are monoidal.
• Results, also known as Either from Haskell. A Result<S, F> is a sum type that either has a value of type S (indicating success) or a value of type F (indicating failure).
• Product as a simple pair.
• Unit as an explicit representation for void.
• Lazy evaluation. A Lazy<A> lazily encapsulates a value of type A that can be accessed with lazy.run(). The first, time run is invoked, the value will be computed and cached. Subsequent calls of run, directly return the cached value. In the background, Lazy<A> is wraps a Supplier<A>.

Other features include non-empty lists as well as further quality-of-life utilities for lists, maps, and iterators.