[Haskell-cafe] Combining pool and Conduit

7 Aug 2017

      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@laeremans.org