[Haskell-cafe] Combining pool and Conduit

[Michael Snoyman]

Because ConduitM is based on continuations, you can't safely acquire
resources inside of it. In order to make safe allocations, you need to use
ResourceT. I wrote a blog post two months ago with a lot more information
on ResourceT:

https://www.fpcomplete.com/blog/2017/06/understanding-resourcet

Because of this, you cannot use `withResource`. The error message you're
getting is confusing, but this is where it's coming from: because of its
continuations, ConduitM cannot provide a MonadBaseControl instance.
Instead, you'll need to use the `takeResource` and either `destroyResource`
or `putResource` functions, plus something like `bracketP` or `allocate`.

On Mon, Aug 7, 2017 at 3:57 PM, Pieter Laeremans <pieter at laeremans.org>
wrote:

> Hello,
>
> I would like to combine a "Connection Pool" with the conduit library.
>
> I have an application were a stream of results from a web api needs to
> be analysed and stored in a database.
> Since the analysis is composed of different parts, I combine some parts
> together ( .|)
>
> Some of those parts need a database connection to do their work.
> Initially I created a Connection and passed it around to each part that
> needed it, this is sub-optimal. So I wanted to try the "resource-pool"
> library to address this problem.
> (https://hackage.haskell.org/package/resource-pool)
>
> So instead of a Connection I pass around a (Pool Connection) But I run
> into problems , and I don't know how to fix them.
>
> Here is a simple snippet of code that demonstrates the problem:
>
>
> -------------------------------------------
> {-# LANGUAGE RankNTypes #-}
>
> module Lib where
>
> import Data.Pool
> import Database.HDBC
> import Database.HDBC.ODBC
> import Data.Conduit
> import Conduit
>
> createConnectionPool :: Pool Connection
> createConnectionPool = undefined
>
> processFoo :: Pool Connection -> Consumer String IO ()
> processFoo pool = withResource pool $ \conn -> do
>                                                  mapM_C print
>
> -------------------------
>
> I can not get this to compile:
>
> src/Lib.hs:15:19: error:
>     • No instance for (MonadBaseControl IO (ConduitM String o IO))
>         arising from a use of ‘withResource’
>     • In the expression: withResource pool
>       In the expression:
>         withResource pool $ \ conn -> do { mapM_C print }
>       In an equation for ‘processFoo’:
>           processFoo pool = withResource pool $ \ conn -> do { mapM_C
>           print }
>
> But I have no Idea how to tackle this problem.  Any suggestions ?
>
> Thanks,
>
> Pieter
>
> --
>   Pieter Laeremans
>   pieter at laeremans.org
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