You can use `Control.Monad.void`:
void :: Functor m => m a -> m ()
void m = fmap (\_ -> ()) m
Another equivalent way to do this is:
(p >> return ())
... where `p` is the `Producer` whose return value you want to swallow.
This actually works for any `Monad`, not just `Producer`.
On Jul 10, 2014 11:52 AM, "Sibi Prabakaran" <[email protected]> wrote:
>
> I have the following piece of code which typechecks perfectly:
>
> import Control.Applicative ((<*))
> import Control.Monad (liftM)
> import Data.Attoparsec.Text
> import Pipes
> import Pipes.Attoparsec (parsed, ParsingError)
> import qualified Pipes.Prelude as P
> import Pipes.Text.IO (fromHandle)
> import qualified System.IO as IO
>
> type LinkID = Int
>
> data Link = Link {
> lid :: LinkID,
> llength :: Int
> }
>
> data Snap = Snap {
> slid :: LinkID,
> slength :: Int
> }
>
> getSnap
> :: Monad m
> => Int
> -> Producer Link m ()
> -> Producer Snap m ()
> -> Producer (LinkID, [Snap]) m ()
> getSnap n lks snp = do
> snp' <- lift (P.toListM snp)
> lks >-> P.filter (\x -> lid x > n)
> >-> P.map (\x -> (lid x, filter (\y -> lid x == slid y) snp'))
>
> drawData :: (LinkID, [Snap]) -> IO ()
> drawData = undefined
>
> linkFile = "some file path"
> snapshotFile = "some file path"
>
> parseLink :: Parser Link
> parseLink = undefined
>
> snapshotParser :: Parser Snap
> snapshotParser = undefined
>
> Note that some parts of the code are defined as undefined for the sake of
> brevity. Now when I include main function it doesn't typecheck:
>
> main = IO.withFile linkFile IO.ReadMode $ \linkHandle ->
> IO.withFile snapshotFile IO.ReadMode $ \snapHandle -> runEffect $
> for (getSnap 2 (parsed (parseLink <* endOfLine) (fromHandle linkHandle))
> (parsed (snapshotParser <* endOfLine) (fromHandle snapHandle))) $
> \d ->
> lift (drawData d)
>
> The error:
>
> Expected type: Producer Link IO ()
> Actual type: Producer
> Link
> IO
> (Either (ParsingError, Producer
> Data.Text.Internal.Text IO ()) ())
> In the return type of a call of `parsed'
> In the second argument of `getSnap', namely
>
> Now, in order to fix this problem, I change getSnap function:
>
> getSnap
> :: Monad m
> => Int
> -> Producer Link m r
> -> Producer Snap m r
> -> Producer (LinkID, [Snap]) m r
> getSnap n lks snp = do
> snp' <- lift (P.toListM snp)
> lks >-> P.filter (\x -> lid x > n)
> >-> P.map (\x -> (lid x, filter (\y -> lid x == slid y) snp'))
>
> Now, it seems P.toListM only operates on Producer a m () so, I want to
> convert Producer a m r to that. What is the recommended way of solving this
> problem ?
>
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