Simplification of types, easier interop with other libraries

[snoyberg]

I've been working on a simplified approach to setting up types and classes in persistent. In this process, I realized that the current setup of the mongoDB package will make such a rewrite impossible, since mongoDB forces usage of the Action transformer without exposing its internals.

I've worked on a fairly substantial overhaul of the types, which doesn't really change the inner operations or the user-facing API in a significant way. Some of the ideas behind this are described in my not-yet-published blog post on the subject:

https://www.fpcomplete.com/tutorial-preview/2896/z9jwaCGLzb

I'm not sure if all of the changes I've made here will be acceptable to you for merging, but I'd like to discuss two changes in particular:

• Remove ErrorT from the transformer stack.
• Instead of using an abstract Action, use a ReaderT Context type. Alternatively, if you want to keep the Action type, exposing its constructor so that persistent could treat it as a ReaderT.

Pinging @gregwebs.

[gregwebs]

looks good to me. Append a ?w=q to avoid looking at all the indentation changes

[gregwebs]

sorry, I meant ?w=1

[snoyberg]

Bumping this issue. persistent 2.0 will require some form of different API to make a mongoDB backend, I'd like to move ahead on figuring out how we can make that happen.

[gregwebs]

@superbobry @knsd @selectel can you take a look?

[si14]

@superbobry @knsd please take a look, +1

[superbobry]

Michael, I like the proposal, so +1 for merging this.

Me and @knsd are working on the new MongoDB driver, so now is the right time to discuss the bad parts in the existing API.

[gregwebs]

I would like to help test the new driver. The only bad part for me is the delay mentioned in #18. And the only thing I really want to add is #14 an understanding of how to auto reconnect.

[knsd]

Hello @snoyberg, sorry for delay. I agree with your ideas, and I've just merged your PR, unfortunately it contains some backward incompatible changes, so we need to update documentation and usage examples and release new version after that.

[snoyberg]

Awesome, thanks!