Wrap Application in custom monad for Raw endpoint

[maksbotan]

Currently handler Raw endpoint for is just an Application: a value.
This value has no access to the context of an arbitrary monad that may
be used instead of the default Handler.

This commit changes handler type to m Application, allowing the user
to do arbitrary actions in m prior to generating an Application.

This is useful to do custom logic behind servants back while still
having access to the environment of user's monad.

[maksbotan]

This is a breaking change, so maybe it makes sense to introduce a new combinator, like RawM, to have this behavior. However, I think the new behavior is "more right" and the users just have to migrate, the change is not that big (add return before your application).

[fisx]

I like it, but as you can tell we're a bit behind with processing PRs. Would you be interested in joining the maintainer team? :-)

If not, we'll get to this eventually. I agree with the breaking change, any other opinions?

[maksbotan]

Well, maintaining such an important library is a great responsibility...

Can I help with current problems? I'd love to see uverbs finally in master, for example.

[maksbotan]

Actually I was able to generalize this even further, please take a look.

[amesgen]

JFYI: There is https://github.com/cdepillabout/servant-rawm, which solves this exact problem.

I think it is a very good idea to integrate it/sth similiar in this repo!

In particular, it will make it easier to close cdepillabout/servant-rawm#7, as servant-auth can provide the required instance out of the box (without having to write a servant-auth-rawm compat package).

[maksbotan]

Oops, perhaps I should've looked for prior art :)

However, newest version of this PR is actually better than that package, as it allows you to run m-based effects depending on the request. I've found it useful in my app.

[maksbotan]

@fisx so how about co-maintaining? I could at least help with technical releases for version bounds and stuff.

cc @fizruk.

[maksbotan]

I've just released 0.18.1 to Hackage. I believe this change will have to go into 0.19, as it's kinda breaking. What do you think?

[amesgen]

I agree that the Raw rework should go in 0.19.

One nice feature of servant-rawm is that it allows to customize the HasDocs instance of RawM'. This should not be too hard to add to this PR.