Compiling code from conduit tutorial
Question
I’m trying to understand code from http://www.yesodweb.com/book/conduits. After some fixes (like replacing Resource
with MonadResource
) it still won’t compile:
sumSink :: MonadResource m => Sink Int m Int
sumSink = CL.fold (+) 0
sum' :: [Int] -> Int
sum' input = runST $ runResourceT $ CL.sourceList input $$ sumSink
gives me the following error:
Couldn't match type `GHC.ST.ST s' with `IO'
When using functional dependencies to combine
Control.Monad.Trans.Control.MonadBaseControl
(GHC.ST.ST s) (GHC.ST.ST s),
arising from the dependency `m -> b'
in the instance declaration in `Control.Monad.Trans.Control'
Control.Monad.Trans.Control.MonadBaseControl IO (GHC.ST.ST s),
arising from a use of `runResourceT'
at D:\Works\stablename\test.hs:43:22-33
In the expression: runResourceT
In the second argument of `($)', namely
`runResourceT $ CL.sourceList input $$ sumSink'
I’m using GHC 7.4.1 and conduit-0.4.2.
P.S. Is there any tutorial which isn’t broken / was written with conduit-0.4 in mind?
Solution
Answering my own question:
Monad m
should be used instead ofResource m
.runResourceT
needs to be applied if a function likesinkFile
was used somewhere in the chain, becausesinkFile
requiresMonadResource m
and there is no instance forMonadResource IO
.
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