/Я language (β)/Glossary/Kleisli/

Kleisli is a category assosiated to any monad. But since we don't operate on monads at all, in Я Kleisli is associated to any functor instead - that's why we say these are lax Kleisli.

Since almost all parametric types are functors, you can associate Kleisli to a shape of morphism that looks like this:

Related article that explains lax Kleisli usage in operators: Building an intuition over lax Kleisli morphisms.