cats.tests

Helpers provides new concrete types where we control exactly which type class instances are available. For example, the SL type has:

- Semilattice[SL] - Arbitrary[SL] - Eq[SL]

(All types in Helpers have Arbitrary and Eq instances.)

These are useful when a type constructor (e.g. Function0) can produce many different instances depending on which instances are available for its type parameter.

This data type exists purely for testing.

The problem this type solves is to assist in picking up type class instances that have more general constraints.

For instance, OneAnd[*, F[_]] has a Monad instance if F[_] does too. By extension, it has an Applicative instance, since Applicative is a superclass of Monad.

However, if F[_] doesn't have a Monad instance but does have an Applicative instance (e.g. Validated), you can still get an Applicative instance for OneAnd[*, F[_]]. These two instances are different however, and it is a good idea to test to make sure all "variants" of the instances are lawful.

By providing this data type, we can have implicit search pick up a specific type class instance by asking for it explicitly in a block. Note that ListWrapper has no type class instances in implicit scope, save for ones related to testing (e.g. Eq, Arbitrary, Cogen).

{
 implicit val functor: Functor[ListWrapper] = ListWrapper.functor
 checkAll(..., ...)
}