Alleycats

Overview

Alleycats is a module of the Cats project that exists to support types which are not entirely savory or law-abiding but which may prove useful or necessary in some situations.

In some cases, a type class instance can almost (but not quite) obey the required laws (e.g. a Monad instance for scala.util.Try). In other cases, type classes which lack laws or constraints may still be useful in some cases (e.g. Empty[_], a type class which provides some notion of "emptiness").

Rather than argue about whether to permit these types in Cats proper, we provide a (slightly disreputable) home for them here.

Type classes

Alleycats introduces several new type classes. Here is an overview of the instances introduced.

Empty[A], Zero[A], and One[A]

A commonly-requested type class is one that encodes the idea of a set having an identity element. Normally this would be done by defining a Monoid[A] instance, but in some cases the idea of emptiness is independent of a particular associative operation.

In this case, Empty[A] may be used in place of Monoid[A]. It provides access to the identity element (via the .empty method), and can also provide .isEmpty and .nonEmpty if an Eq[A] is available.

The two other type classes, Zero[A] and One[A], are similar, except they correspond to AdditiveMonoid[A] and MultiplicativeMonoid[A] (found in the algebra.ring package). Their methods are called zero and one, respectively.

While none of these type classes have their own laws, they are required not to violate the monoid laws when applicable. This means that if Empty[A] and Semigroup[A] are both available, that Empty[A].empty must function as an identity element for Semigroup[A].combine. In fact, together these instances can be viewed as a Monoid[A] (and there is an implicit method to that effect).

The same rules apply to Zero[A] and One[A] and their respective associative operations.

Pure[F[_]] and Extract[F[_]]

The Pure[F] type class represents the pure method of Applicative[F] separated from its map and ap methods. Like the previous type classes, if Pure[F] and Apply[F] are both available they are required to be consistent (and should provide a valid Applicative[F] instance).

Similarly, Extract[F] represents the extract method of Comonad[F] without coflatMap and map methods. When Extract[F] and CoflatMap[F] are available, they should provide a valid Comonad[F] instance.

EmptyK[F[_]]

Finally, EmptyK[F] generalizes the empty[A] method from MonoidK[F]. The pattern here is the same as before -- SemigroupK[F] and EmptyK[F] should provide a valid MonoidK[F] instance.

Instances

Alleycats also provides some "disreputable" type class instances.

Set[_] instances

Scala's Set[_] takes advantage of the universal availability of .hashCode and .equals. This makes it difficult to use parametricity to reason about sets, and casts some doubt on their use with functors and monads.

Alleycats provides Monad[Set] and Traverse[Set]. You can import these instances via import alleycats.std.set._.

Try[_] instances

Scala's Try[_] is intended to replace the need for try { ... } catch { ... } syntax in Scala programs, to ease error-handling, and to transport exceptions as data. Due to the way Try transparently catches exceptions in .map and .flatMap, some people are skeptical that Try fulfills the necessary functor/monad laws.

Alleycats provides a Monad[Try]. You can import this instance via import alleycats.std.try._.

Iterable[_] instances

Scala's collection.Iterable[_] offers no guarantees that it's immutable, since it abstracts over the mutable and immutable variants. However it's the type used to represent a Maps values, and its often desirable to treat the values of a map as a Foldable collection. Alleycats provides a Foldable[Iterable], eg:

import cats.syntax.all._
import alleycats.std.iterable._

// Result "AppleOrange"
Map(1 -> "Apple", 2 -> "Orange").values.combineAll

Contributing

This module's goal is to be very liberal about accepting type class instances, but to only provide instances which are absent from Cats proper. Law-abiding instances will end up in Cats, and everything else will end up here.