All notable changes to this project will be documented in this file.

The format is based on Keep a Changelog.

• Breaking Change: Sequence now has two more type parameters to aid in inference
• Breaking Change: Traversable#traverse now has three more type parameters to aid in inference
• Bifunctor is now a BoundedBifunctor where both parameter upper bounds are Object
• Peek2 now accepts the more general BoundedBifunctor
• Identity, Compose, and Const functors all have better toString implementations
• Into3-8 now supports functions with parameter variance
• HListLens#tail is now covariant in Tail parameter
• More functions now automatically deforest nested calls (concat cons, cycle, distinct, drop, dropwhile, filter, map, reverse, snoc, take, takewhile, tail)
• Flatten calls Iterator#hasNext less aggressively, allowing for better laziness

• BoundedBifunctor, a Bifunctor super type that offers upper bounds for both parameters
• Try, a Monad representing an expression-like analog of try/catch/finally
• CheckedRunnable, the Runnable counterpart to CheckedSupplier that can throw checked exceptions
• Unit, the lambda analog to Void, except actually inhabited by a singleton instance
• Kleisli, the abstract representation of a Kleisli arrow (Monad#flatMap) as an Fn1
• These, a CoProduct3 of A, B, or Tuple2<A,B>
• Span, for splitting an Iterable into contiguous elements matching a predicate
• MagnetizeBy and Magnetize, for grouping elements by pairwise predicate tests
• Both, for dually applying two functions and producing a Tuple2 of their results
• Lens#both, for dually focusing with two lenses at once
• IfThenElse, an expression form for if statements
• CheckedRunnable and CheckedSupplier conversion and convenience methods

• Breaking Change: Moved Trampoline and RecursiveResult to better package

• Breaking Change: CollectionLens#asSet is now lawful and preserves new incoming values in the update set
• Breaking Change: IterableLens#head is now a Lens.Simple<Iterable<A>, Maybe<A>> and is lawful
• Breaking Change: ListLens#elementAt is now a Lens.Simple<List<X>, Maybe<X>> supporting defensive copies
• Breaking Change: MapLens#valueAt is now a Lens.Simple<Map<K,V>, Maybe<V>> supporting defensive copies
• MapLens#keys now uses defensive copies and does not alter the focused on map
• MapLens#values now uses defensive copies and does not alter the focused on map
• MapLens#inverted now uses defensive copies and does not alter the focused on map
• HListLens#head is now covariant in the tail of both S and T
• Predicate#contraMap is now covariant in its return type
• BiPredicate#contraMap and BiPredicate#diMapL are now both covariant in their return types

• Fn3#fn3 and Fn4#fn4 static factory methods
• Fn5 through Fn8
• Tuple5#into
• Tuple6 through Tuple8
• CoProduct6 through CoProduct8 and Choice6 through Choice8
• CoProduct5#diverge and Choice5#diverge
• Into3 through Into8, for applying a Tuple* to an Fn*
• Times, for successively accumulating a result by iterating a function over a value some number of times
• Slide, for "sliding" a window of some number of elements across an Iterable
• Either#filter overload supporting a function from R to L in the failing predicate case
• CollectionLens#asSet(Function), a proper analog of CollectionLens#asSet() that uses defensive copies
• CollectionLens#asStream(Function), a proper analog of CollectionLens#asStream() that uses defensive copies
• Explicitly calling attention to all unlawful lenses in their documentation
• Peek and Peek2, for "peeking" at the value contained inside any given Functor or Bifunctor with given side-effects
• Trampoline and RecursiveResult for modeling primitive tail-recursive functions that can be trampolined

• Either#toOptional, deprecated in previous release
• Either#fromOptional, deprecated in previous release
• sequence overloads supporting Optional, deprecated in previous release
• OptionalLens, deprecated in previous release
• TraversableIterable, deprecated in previous release
• Traversables, deprecated in previous release

• CollectionLens#asSet() in favor of CollectionLens#asSet(Function)
• CollectionLens#asStream() in favor of CollectionLens#asStream(Function)

• Breaking Change: java.util.Optional replaced with Maybe across the board
• Profunctor#diMap/L/R parameters allow variance
• Either#toOptional no longer allows null values in the right side, and is now in sync with CoProduct#projectB
• Unfoldr allows variance on input

• CoProductN#embed no longer eagerly invokes functions
• PrependAll now only creates O(1) Iterables instead of O(3n + 1)

• Monad arrives. The following Applicatives are now also Monad:
  • Lens
  • Const
  • Tuple*
  • Choice*
  • Identity
  • Either
  • Fn*
  • LambdaIterable
  • Maybe
  • SingletonHList
• Force, for forcing iteration of an Iterable to perform any side-effects
• Snoc, for lazily appending an element to the end of an Iterable
• Coalesce, for folding an Iterable<Either<L, R>> into an Either<Iterable<L>, Iterable<R>>
• And, Or, and Xor all gain BiPredicate<Boolean, Boolean> properties
• LambdaIterable, an adapter Iterable that support lambda types
• Maybe, lambda's analog of java.util.Optional conforming to all the lambda types
• Contravariant, an interface representing functors that map contravariantly over their parameters
• Profunctor extends Contravariant
• Tails, for iterating all the tail element subsequences of an Iterable
• Inits, for iterating all the initial element subsequences of an Iterable
• Init, for iterating all but the last element of an Iterable
• CatMaybes, for unwrapping the present values in an Iterable<Maybe<A>> to produce an Iterable<A>

• Fn1#then(Function<? super B, ? extends C>), deprecated in previous release
• Fn1#adapt(Function<A, B> function), deprecated in previous release
• Fn2#adapt(BiFunction<A, B, C> biFunction), deprecated in previous release

• Traversables and all methods therein, in favor of either LambdaIterable or Maybe
• TraversableOptional in favor of Maybe
• TraversableIterable in favor of LambdaIterable
• Sequence overloads supporting Optional in favor of converting Optional to Maybe and then sequencing
• Either#toOptional and Either#fromOptional in favor of its Maybe counterparts

• Loosening variance on Fn2#fn2 and Fn1#fn1

• ConcatenatingIterator bug where deeply nested xs skip elements

• Fn1#then in favor of Fn1#andThen (redundant)
• Fn1#adapt in favor of Fn1#fn1 (rename)
• Fn2#adapt in favor of Fn2#fn2 (rename)

• Fn1#andThen overload to support composition with Bifunction
• Fn1#compose overload to support composition with Bifunction and Fn2
• LiftA2 to lift and apply a Bifunction to two Applicatives
• Flatten to lazily flatten nested Iterable<Iterable<A>>s to Iterable<A>
• Replicate, short-hand composition of take and repeat
• Distinct to produce an Iterable of distinct values in another Iterable
• Sort and SortBy for eagerly, monolithically sorting Iterables and producing Lists
• IterableLens, general lenses over Iterable
• Xor, a monoid representing logical exclusive-or

• Removing need for various suppressed unchecked warnings in ChoiceN types
• HList abstract super type loses both unnecessary parameters

• ClassCastException BiPredicate.flip

• Uncons, for destructuring an Iterable into its head and tail
• Compose semigroup and monoid formed over CompletableFuture
• Monoid and Semigroup both preserve type specificity through flip calls

• Loosening visibility on Traversables methods to public

• Functor, Bifunctor, and Profunctor (as well as all instances) get a unification parameter
• Identity supports value equality
• Const supports value equality
• partition now only requires iterables of CoProudct2
• CoProductNs receive a unification parameter, which trickles down to Either and Choices
• Concat now represents a monoid for Iterable; previous Concat semigroup and monoid renamed to more appropriate AddAll
• Lens is now an instance of Profunctor

• Either#invert is pulled up into CoProduct2 and additionally specialized for Choice2
• CoProductN#embed
• not, used for negating predicate functions
• empty, used to test if an Iterable is empty
• groupBy, for folding an Iterable into a Map given a key function
• Applicative arrives; all functors gain applicative properties
• Traversable arrives; SingletonHList, Tuple*, Choice*, Either, Identity, and Const gain traversable properties
• TraversableOptional and TraversableIterable for adapting Optional and Iterable, respectively, to Traversable
• sequence for wrapping a traversable in an applicative during traversal
• Compose, an applicative functor that represents type-level functor composition

• CoProductN.[a-e]() static factory methods moved to equivalent ChoiceN class. Coproduct interfaces now solely represent methods, no longer have anonymous implementations, and no longer require a Functor constraint

• ChoiceN types, representing concrete coproduct implementations that are also Functor and BiFunctor
• toMap, last, cons, prependAll, intersperse
• Tuple2/3/4#into, for applying the values in a tuple as positional arguments to a function.
• First and Last monoids over Optional
• And and Or monoids over Boolean

• semigroups and monoids moved under fn2 package

• CoProductN#project, to project disjoint union types into tuples of Optional values
• CoProductN#converge, to drop the magnitude of a coproduct down by one type
• toCollection and size

• Fn1/2#adapt to switch between lambda and java.util.function types more easily
• eq, head, find, and tail
• BiPredicate
• Monoid#foldMap
• HMap#toMap to go from a heterogeneous map to a java.util.Map

• Semigroup and Monoid
• Either#invert
• partition
• Generalized coproducts implemented as CoProduct2 through CoProduct5
• Either is now a CoProduct2

• Lens static factory method renaming

• Heterogeneous list indexes arrive via Index

• Independent Lens parameter mapping via mapS, mapT, mapA, and mapB

• Initial lens support with Lens and SimpleLens types and view, set, and over functions
• Const and Identity functors
• Either#toOptional
• HMap#remove and HMap#removeAll

• All function input values become java.util.function types, and all function output values remain lambda types, for better compatibility

• Profunctor inheritance hierarchy
• Renaming Identity to Id
• Monadic/Dyadic/TriadicFunction is now Fn1/2/3

• HList specializations support random access lookup

• Tuples moved under HList as specialized subtypes

• Either#peek
• HMap, heterogeneous maps
• Tuple2 is now also a Map.Entry

• Better interoperability between lambda and java.util.function types

• scanLeft
• HList, heterogeneous lists
• Added up to Tuple5
• Either, specialized coproduct with success/failure semantics

• Initial implementation of first-class curried functions
• all, any, cartesianProduct, cycle, drop, dropWhile, filter, foldLeft, foldRight, inGroupsOf, map, partial2, partial3, reduceLeft, reduceRight, repeat, take, takeWhile, unfoldr
• Monadic/Dyadic/TriadicFunction, Predicate, Tuple2, Tuple3
• Functor, BiFunctor, ProFunctor