CODEWITHSUNDEEP
haskellconduit
Cubic
Cubic

Reputation: 15673

Missing instance MonadIO for ConduitM?

According to the documentation there is a MonadIO instance for ConduitM if the underlying Monad has a MonadIO instance (MonadIO m => MonadIO (ConduitM i o m)).

Why then does this:

yield (1::Int) $$ await >>= (liftIO.print) :: IO ()

fail with this:

No instance for (MonadIO (ConduitM Int Data.Void.Void IO)) arising from a use of ‘liftIO’ In the first argument of ‘(.)’, namely ‘liftIO’ In the second argument of ‘(>>=)’, namely ‘(liftIO . print)’ In the second argument of ‘($$)’, namely ‘await >>= (liftIO . print)’

What am I missing here?

Just checked, apparently I have both transformers 0.22 and transformers 0.3, that might be the cause.

Upvotes: 0

Views: 259

Answers (1)

Cubic
Cubic

Reputation: 15673

The problem was that conduit depends on transformers-0.2.2.1 (for some reason) and I had both that and transformers-0.3.0.0 - hiding transformers-0.3.0.0 fixed this for now (if I had used cabal it probably would've worked too). In effect, this was the same problem as here.

Upvotes: 2

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