Re: [Haskell-cafe] Combining pool and Conduit

No problem On Mon, Aug 7, 2017, 10:02 PM Pieter Laeremans wrote:

Thans a lot!

Pieter Laeremans

On 7 Aug 2017, at 20:15, Michael Snoyman wrote:

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 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@laeremans.org _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.