[Haskell-cafe] Combining pool and Conduit

[Michael Snoyman]

No problem

On Mon, Aug 7, 2017, 10:02 PM Pieter Laeremans <pieter at laeremans.org> wrote:

> Thans a lot!
>
>
> Pieter Laeremans
>
> On 7 Aug 2017, at 20:15, Michael Snoyman <michael at snoyman.com> 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 <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
>> _______________________________________________
>> 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.
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/haskell-cafe/attachments/20170807/6d1ae189/attachment.html>