The cats root package contains all the trait signatures of most Scala type classes.
Cats type classes are implemented using the approach from the
Type classes as objects and implicits article.
For each type class, cats provides three pieces:
- Its signature: a trait that is polymorphic on a type parameter.
Type class traits inherit from other type classes to indicate that any implementation of the lower type class (e.g. Applicative)
can also serve as an instance for the higuer type class (e.g. Functor).
- Type class 'instances, which are classes and objects that implement one or more type class signatures for some specific types.
Type class instances for several data types from the Java or Scala standard libraries are declared in the subpackage cats.instances.
- Syntax extensions, each of which provides the methods of the type class defines as extension methods
(which in Scala 2 are encoded as implicit classes) for values of any type F; given that an instance of the type class
for the receiver type (this) is in the implicit scope.
Symtax extensions are declared in the cats.syntax package.
- A set of laws, that are also generic on the type of the class, and are only defined on the operations of the type class.
The purpose of these laws is to declare some algebraic relations (equations) between Scala expressions involving the operations
of the type class, and test (but not verify) that implemented instances satisfy those equations.
Laws are defined in the cats-laws package.
Although most of cats type classes are declared in this package, some are declared in other packages:
- type classes that operate on base types (kind *), and their implementations for standard library types,
are contained in cats.kernel, which is a different SBT project. However, they are re-exported from this package.
- type classes of kind F[_, _], such as cats.arrow.Profunctor" or cats.arrow.Arrow, which are relevant for
Functional Reactive Programming or optics, are declared in the cats.arrow package.
- Also, those type classes that abstract over (pure or impure) functional runtime effects are declared
in the cats-effect library.
- Some type classes for which no laws can be provided are left out of the main road, in a small and dirty alley.
These are the alleycats.