MonadBaseControl IO instance for Handlers

[Dmitry Dzhus]

How are you gentlemen?

I'd like to switch to using Pool from resource-pool [1] package in my
snaplet [2] to handle database connection pool. This requires my
Handlers to have MonadBaseControl IO instance, whereas Snap provides
MonadCatchIO. Seems like using resource-pool-catchio fork [3] could do
the trick, but is there any chance to see MonadBaseControl support in
Snap?

[1]: http://hackage.haskell.org/package/resource-pool
[2]: https://github.com/dzhus/snap-redis-crud/blob/master/src/RedisDB.hs
[3]: http://hackage.haskell.org/package/resource-pool-catchio

[Gregory Collins]

I think one of Jurriën or Jasper was working on it, but it's actually not easy to do; there are little wrinkles in our MonadCatchIO instance which MonadBaseControl doesn't support. You're better off just lifting to IO and running your database code there.

G

-- 

Gregory Collins <gr...@gregorycollins.net>

[Jurriën Stutterheim]

I've made an attempt to port Snap to monad-control, but it broke the entire framework in non-obvious ways. It is possible to define a `MonadControlIO` instance for `Handler`, though (monad-control-0.2.x). That would allow you to use the original resource-pool. I didn't get around to writing instances for the latest version of monad-control though (I decided to switch back to `MonadCatchIO` instead). Here's the code I had for `MonadControlIO`:

#if MIN_VERSION_monad_control(0,3,0)

#else
instance MonadControlIO (Handler b v) where
liftControlIO f = liftBase (f return)
#endif

Jurriën