tl;dr Cats MTL 1.6.0 introduces a brand new lightweight syntax for managing user-defined error types in the Cats ecosystem without requiring complex monad transformers. One of the most famous and longstanding limitations of the Cats Effect IO type (and the Cats generic typeclasses) is the fact that the only available error channel is Throwable. This stands in contrast to bifunctor or polyfunctor techniques, which add a typed error channel within the monad itself. You can see this easily in ty...
