Remove Rpc monad

[kolmodin]

The Rpc monad is defined as

type Rpc a = ExceptT RpcError IO a

which is problematic.
It doesn't compose well. Catching errors in the monad gives a false security that exceptions have been caught, since there are also IO exceptions and pure exceptions.
Currently no other feature are needed from the monad except for exceptions (no pun intended).

I'd like to remove the Rpc monad and just throw IO exceptions, and document properly what method will throw, and on best practices.

[erikd]

It doesn't compose well.

I'd like to see examples of where you see that, because in the code we have at work ExceptT composes very well. It does so because we have a library of ExceptT combinators to help with the composition.

Catching errors in the monad gives a false security that exceptions have been caught, since there are also IO exceptions and pure exceptions.

The fact that all your exceptions aren't being caught isn't a good argument for doing away with ExceptT. Instead, you should be improving your code to make sure all exceptions are caught and converted to RpcErrors.

For an example of some of our code calling createProcess and trapping exceptions in ExceptT, see:
https://github.com/ambiata/mafia/blob/master/src/Mafia/Cabal/Process.hs#L41-L60

I'd like to remove the Rpc monad and just throw IO exceptions

That would be reasonable if there is only one or two layers of software. I've been spending time chasing exceptions in some of our code. Our code is called mismi, which sits on top of amazonka, which sits on top of http-conduit which sits on top of http-client-tls which sits on top of tls which sits on top of network. The problem I've been chasing is that (very) occasionally we have been getting an exception that bubbles up to the top and kills a complex multi-threaded program.

[kolmodin]

Discussion started here; https://plus.google.com/+ErikdeCastroLopo/posts/TbRiXuNucED
Suggestion to try tryEitherT https://github.com/ambiata/x/blob/3a37cf0ace1c7223f3467c0c000fac9085c602ef/x-eithert/src/X/Control/Monad/Trans/Either.hs#L236-L238

[kolmodin]

@erikd The ExceptT library I've used doesn't have all your fancy combinators. It looks much more composable with a proper library, indeed.

ambiata-x-eithert and its Ambiata dependencies are not on hackage. How is it intended to be used?

[erikd]

We use our libraries as git submodules using mafia, a thin wrapper around cabal.

Now that you seem convinced that relying more heavily on ExceptT is a good idea, I can point you to the errors package which has most of what you need.

[kolmodin]

I guess I need to define an alternative liftIO that will catch IO exceptions and put them into RpcError.

[erikd]

What about handleEitherT from the errors package?

[kolmodin]

Yes, though maybe it'd be nice to provide some utility that converts into RpcError.

Thanks, @erikd, looks like I can continue to use ExceptT :)

[erikd]

I'm writing a blog post about this stuff. Will let you and Sean read it before I post it publically.

[kolmodin]

@erikd Great, please show some common idioms, that'd be useful.

In the case of gRPC, RpcError may have to contain all other exceptions. Need to look into how to best model that.

[erikd]

If you can wait another couple of hours, the code for my blog post will probably be ready :). Its spring where you are right? If the weather is good, you should go an enjoy it :).

[erikd]

First pass at the code for my blog post is at https://github.com/erikd/exceptT-demo

You would mainly be interested in this file.