Creating a Conduit streaming source using postgresql-simple

Question

postgresql-simple provides functions for streaming requests, for example

fold 
  :: (FromRow row, ToRow params)
  => Connection -> Query -> params -> a -> (a -> row -> IO a) -> IO a

I want to create a channel source that makes full use of streaming.

mySource :: (FromRow row, Monad m) => Source m row

Unfortunately, since it IOappears in a contravariant position (I think?) In fold, I really struggle with types. The following types are checked, but add up the entire stream to get the values.

getConduit :: Connection -> IO (C.ConduitM () Event IO ())
getConduit conn = fold_ conn queryEventRecord CL.sourceNull foo
  where
    foo :: C.ConduitM () Event IO () -> Event -> IO (C.ConduitM () Event IO ())
    foo cond evt = pure (cond >> C.yield evt)

Any pointers on how to implement this would be greatly appreciated! Thank!

Answer 1

One (not so good) way to do this for

• create a new one TMChanto get the strings
• foreach_,
• , stm-conduit,

,

import Conduit
import Database.PostgreSQL.Simple (foreach_)
import Data.Conduit.TMChan (sourceTMChan)
import Control.Concurrent.STM.TMChan (newTMChanIO, writeTMChan, atomically)

mySource :: (FromRow row, MonadIO m) => Connection -> Query -> IO (Source m row)
mySource connection query = do
  chan <- newTMChanIO
  forEach_ connection query (atomically . writeTMChan chan)
  pure (sourceTMChan chan)

forEach_ :: (MonadIO m, FromRow r) => Connection -> Query -> (r -> m ()) -> m (), ...

Answer 2

Alec , . mkPgSource - , .

import Database.PostgreSQL.Simple
import Database.PostgreSQL.Simple.FromRow
import Database.PostgreSQL.Simple.ToRow
import Control.Monad.IO.Class (MonadIO)
import Data.Conduit.TMChan (sourceTMChan)
import Control.Concurrent.STM.TMChan (newTMChanIO, writeTMChan, 
closeTMChan, TMChan)
import GHC.Conc (atomically, forkIO)
import Conduit

--closes the channel after action is done to terminate the source
mkPgSource :: (MonadIO m, FromRow r) => ((r -> IO ()) -> IO ()) -> IO (Source m r)
mkPgSource action = do
  chan <- newTMChanIO
  _ <- forkIO $ do action $ atomically . (writeTMChan chan)
               atomically $ closeTMChan chan
  pure $ sourceTMChan chan

sourceQuery :: (ToRow params, FromRow r, MonadIO m) =>
     Connection -> Query -> params -> IO (Source m r)
sourceQuery conn q params = mkPgSource $ forEach conn q params

sourceQuery_ :: (FromRow r, MonadIO m) => Connection -> Query -> IO 
(Source m r)
sourceQuery_ conn q = mkPgSource $ forEach_ conn q