Application example for state and other MTL monks?

Question

Looking at the documents for Control.Applicative , I notice that they have instance declarations for certain monads (like IO , Maybe and especially ST ), but there are no instances for MTAD monads like State and RWS . Instead, there seems to be a certain type of WrappedMonad that I assume covers all other cases.

So here are my questions:

• Why are there no Applicative instances for monads MTL? The best answer that I could find on my own was a three-year article where someone implemented these instances and were briefly ignored.

• What is the deal with WrappedMonad ? I found a post on Reddit that explains this briefly, but I think I'm confused about how to use it.

Ultimately, I would like to use State in an applicative style (as I recommended ), but if I have to mutate my code with WrappedMonad data WrappedMonad , then this doesn't seem to be a win. I could also completely ignore WrappedMonad and define the Applicative instance in the same way as I did for IO , ST , etc .: in terms of return and ap ... But this seems to be silly.

Answer 1

This documentation is only created based on the contents of the base package, which does not contain State - therefore, it does not contain Applicative instances for it. If you look at the MTL documentation , you will see the Applicative instance listed.

Alternatively, you can open GHCi, import Control.Monad.State and Control.Applicative and use :i Applicative for the list of Applicative instances or :i StateT for the list of classes for which StateT has instances ( State is an alias of type StateT with fixed one from its parameters). Or it will show that there really is such an instance.

Answer 2

Applicative instances for monad mtl cannot be specified in the base documents because there is no mtl at this point. The good news is that mtl has applicative StateT instances , for example , since mtl-2.0.0.0.